**Part 3**

**Material Processing**

248 CO2 Laser – Optimisation and Application

Vincetti L.; Setti V. (2010). Waveguide mechanism in tube lattice fibers. *Optics Express*,

Yeh P.; Yariv A.; Marom E. J. (1978). Theory of Bragg fiber. *Journal of the Optical Society of* 

Vol.18, pp. 23133 - 23146.

*America*, Vol.68, pp. 1196 – 1201.

**9** 

*Poland* 

Joanna Radziejewska

**Application of Laser-Burnishing Treatment** 

**for Improvement of Surface Layer Properties** 

*Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw* 

Laser Beam Machining (LBM) has been successfully applied for improvement of surface layer properties of machine elements. Some of laser treatments are based on melting of surface with a laser beam. Among them is laser hardening, as well as cladding and alloying. The results of broad scope research have shown that surface roughness of elements which underwent the laser melting is too high to apply the process without an additional abrasive machining, even at the optimum parameters of the laser treatment. In most cases after the surface melting with laser beam the tension stresses are observed. That is demonstrated by the cracks in the surface layer and deterioration of its properties (Dietrich Lepski et al., 2009).

A new hybrid treatment was elaborated for laser treated materials. The treatment, combining the laser melting with the burnishing process was performed simultaneously at the laser stage. The aim of the hybrid treatment was to reduce surface roughness formed in the laser process and induce compressive stresses. A surface smoothing effect was the result of plastic deformation of the surface layer in high temperature, while a reduction of the tensile

For many years the surface burnishing has been used as smoothing and strain hardening finishing. The strain hardening, favourable compressive stress and smooth surface is obtained as a result of plastic deformation of the surface layer of elements made of homogenous material, as well as material with surface layer formed in order to obtain operating properties of superior requirements (Shiou & Hsu, 2008; Milad, 2008). The main limitation of the use of burnishing is high hardness and low plasticity of material after alloying and high roughness of surface. Przybylski, 1987, Shiou & Chen, 2003 showed that as a result of burnishing such materials undergo slight deformation due to the process. Strain hardening degree and thickness of plastic zone are small; cracks often form, whereas expected roughness could not be obtained. For this reason, the process of burnishing is not applied in industry as finishing of layers produced by laser beam. The study (Abbas & West, 1991; Arutunian et al., 1989; Meijer, 2004) demonstrated that modification of surface layers of metals by laser beam - such as hardening, alloying or cladding – provide very hard,

After laser remelting the stresses in the surface layer are generated in accordance with a hot stress model and they are mostly large tensile stresses, leading to the formation of micro-

stresses within the surface layer was due to cold work (Radziejewska&Skrzypek 2009).

resistant to abrasive wear, erosion and corrosion layers.

**1. Introduction** 

### **Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties**

Joanna Radziejewska

*Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw Poland* 

#### **1. Introduction**

Laser Beam Machining (LBM) has been successfully applied for improvement of surface layer properties of machine elements. Some of laser treatments are based on melting of surface with a laser beam. Among them is laser hardening, as well as cladding and alloying. The results of broad scope research have shown that surface roughness of elements which underwent the laser melting is too high to apply the process without an additional abrasive machining, even at the optimum parameters of the laser treatment. In most cases after the surface melting with laser beam the tension stresses are observed. That is demonstrated by the cracks in the surface layer and deterioration of its properties (Dietrich Lepski et al., 2009).

A new hybrid treatment was elaborated for laser treated materials. The treatment, combining the laser melting with the burnishing process was performed simultaneously at the laser stage. The aim of the hybrid treatment was to reduce surface roughness formed in the laser process and induce compressive stresses. A surface smoothing effect was the result of plastic deformation of the surface layer in high temperature, while a reduction of the tensile stresses within the surface layer was due to cold work (Radziejewska&Skrzypek 2009).

For many years the surface burnishing has been used as smoothing and strain hardening finishing. The strain hardening, favourable compressive stress and smooth surface is obtained as a result of plastic deformation of the surface layer of elements made of homogenous material, as well as material with surface layer formed in order to obtain operating properties of superior requirements (Shiou & Hsu, 2008; Milad, 2008). The main limitation of the use of burnishing is high hardness and low plasticity of material after alloying and high roughness of surface. Przybylski, 1987, Shiou & Chen, 2003 showed that as a result of burnishing such materials undergo slight deformation due to the process. Strain hardening degree and thickness of plastic zone are small; cracks often form, whereas expected roughness could not be obtained. For this reason, the process of burnishing is not applied in industry as finishing of layers produced by laser beam. The study (Abbas & West, 1991; Arutunian et al., 1989; Meijer, 2004) demonstrated that modification of surface layers of metals by laser beam - such as hardening, alloying or cladding – provide very hard, resistant to abrasive wear, erosion and corrosion layers.

After laser remelting the stresses in the surface layer are generated in accordance with a hot stress model and they are mostly large tensile stresses, leading to the formation of micro-

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 253

proposed hybrid treatment for other materials and coatings. Obtaining improved features of surface layer, such as material microstructure, hardening and the compressive stresses, while reducing the amount of surface roughness, will eliminate additional finishing after the laser improving. New features of surface layer affected the basic functional properties of machine parts. Hybrid treatment improved such properties as surface roughness, contact stiffness and erosive wear mainly required in operating condition. It will extend application of the hybrid treatment on cases in which the surface layers have to meet high durability

The study was performed using the CO2 laser with the maximum power 2.5 kW. The axially-symmetric beam, of the mode close to TEM10, was focused with use of the ZnSe lens. The focal length of it was 5". The set of treatment parameters, such as laser power, feed rate, and diameter of the beam focused on the metal surface, type of shielding gas and speed of the air flow to ensure a sufficient power density to obtain the remelting and optimal results, was selected. Based on previous experience (Radziejewska&Skrzypek, 2009) the ranges of parameters of laser processing were identified in the first stage of the study. The optimization criteria, such as penetration depth, surface roughness and hardness of the

The alloying process was carried out on steel 304. Prior to the alloying a layer of Stellite 6, with a thickness of about 200 µm, was formed on the surface with plasma spraying method. Preliminary studies have shown that in this case, the alloying process takes place preferably at the following parameters: laser power 2 kW, laser beam diameter of 3 mm, feed rate of

On the laser station the burnishing process was carried out simultaneously with the alloying process. The dynamic burnishing process with use of micro-hammers was applied. The technology of micro-hammering was based on a dynamic centrifugal burnishing. For microhammering a high rotational head was developed, providing the possibility of working directly on the laser processing. Processing concept and principle of operation of the head is described in work (Radziejewska et al., 2005). In this study the modified version of the head was applied. Two rows of 8 micro-hammers allow providing greater intensity of the process and the simultaneous treatment at two different temperatures. In order to obtain more uniform deformation of surface material, the oscillation motion of the sample in a direction perpendicular to the direction of feed was introduced. The motion was generated using an oscillating table. The oscillations eliminated the problem of the formation of unfavourable geometric surface structure - the grooves occurring in earlier solution. A constant velocity, of 15 oscillations per second and the amplitude of 2 mm, was used. The small radii of microhammers allow to obtained high surface plastic deformation at low forces. The scheme of the head is shown in Figure 1a while the laser-mechanical treatment presented in Fig. 1b. Figure 1c shows the temperature distribution on the surface along x axis with the selected

requirements.

**2.1 Laser alloying** 

**2. The experiment description** 

resulting surface layer, were taken into consideration.

sample against the laser beam from 150-900 mm/min.

range of temperatures in which the burnishing process was conducted.

**2.2 Laser-mechanical treatment** 

cracks in extreme cases (Grum & Sturm, 2004; Robinson, 1996) Anthony and Cline 1977 proved theoretically that the surface topography is characterized by relatively high asperities and study (Radziejewska, 2006) where waviness and roughness after laser alloying was examined confirmed this. Such a state of surface implies a need for an additional machining in order to improve surface smoothness.

The classical burnishing process applied after laser treatment was proposed in the works (De Hossonand & Noordhuis 1989, Ignatiev et al. 1993). The reduction of surface roughness and tensile stresses was obtained in the case of thin layer produced by laser alloying of titanium. Ignatiev et al. proposed another solution - the application of classical shot-peening process after laser hardening. As a result of shot-peening the change of stresses, from tensile stresses to compressive, in surface layer 70 µm thick was obtained.

The laser heating process is successfully applied to support the mechanical and plastic working of materials which are difficult for machining. Such a hybrid method was applied for cutting and turning of hard ceramic (Tsai & Ou, 2004). The research on local heating with laser beam during turning, milling and grinding of titanium alloys, cast iron and special steel was conducted. The hybrid treatment - laser-assisted burnishing (LAB) - was elaborated by Tian and Shin 2007. The laser heating process was applied for the burnishing of steel. It provided the reduction of the burnishing force, as well as the tool wear. It was shown that LAB can form better surface roughness and higher hardness than conventional burnishing.

In the work (Radziejewska 2007) the new method to modify surface layer, combing the laser melting with the slide burnishing, was proposed. The smoothing of surface was carried out by plastic deformation of surface layer at high temperature, whereas transformation process of stresses, from compressive to tensile stresses, was performed by plastic deformation at low temperatures. All machining operations - LBM, high and low temperature burnishing are performed simultaneously on the laser station, in one pass. Temperature changes while the cooling of material that undergoes the laser beam treatment, are used. It does not extend duration of treatment. It was stated that multiple alloying combined with slide burnishing generated compressive stresses of about – 600 MPa at the surface. Because of the adopted type of burnishing – the slide burnishing and high hardness of material, the relatively small thickness of textured zone, about 30 µm, was obtained. In the case of thick layers it can be insufficient. According to (Przybylski, 1987) high degree of strain hardening of surface is possible to provide using dynamic burnishing.

The current work presents the analysis of the plastic deformation of surface layer when the new type of the laser-burnishing process was applied. The surface layer was generated by laser alloying and dynamic burnishing. Design of new head allows for simultaneous surface burnishing in two different temperatures on the laser station providing high-intensity of the burnishing process. That allowed treatment hard and low plasticity materials. Thus such a technical solution should result in high degree of surface deformation as well as its large depth. The proposed solution is designed for thick layers, above 1mm, mostly produced by LBM.

The aim of this study was to evaluate the influence of the hybrid processing parameters on plastic deformations of surface layer and the analysis of the correlations between treatment parameters and surface layer state. Establishing relationships between process parameters and the state of the surface layer of tested material will determine the application area of the proposed hybrid treatment for other materials and coatings. Obtaining improved features of surface layer, such as material microstructure, hardening and the compressive stresses, while reducing the amount of surface roughness, will eliminate additional finishing after the laser improving. New features of surface layer affected the basic functional properties of machine parts. Hybrid treatment improved such properties as surface roughness, contact stiffness and erosive wear mainly required in operating condition. It will extend application of the hybrid treatment on cases in which the surface layers have to meet high durability requirements.

#### **2. The experiment description**

#### **2.1 Laser alloying**

252 CO2 Laser – Optimisation and Application

cracks in extreme cases (Grum & Sturm, 2004; Robinson, 1996) Anthony and Cline 1977 proved theoretically that the surface topography is characterized by relatively high asperities and study (Radziejewska, 2006) where waviness and roughness after laser alloying was examined confirmed this. Such a state of surface implies a need for an

The classical burnishing process applied after laser treatment was proposed in the works (De Hossonand & Noordhuis 1989, Ignatiev et al. 1993). The reduction of surface roughness and tensile stresses was obtained in the case of thin layer produced by laser alloying of titanium. Ignatiev et al. proposed another solution - the application of classical shot-peening process after laser hardening. As a result of shot-peening the change of stresses, from tensile

The laser heating process is successfully applied to support the mechanical and plastic working of materials which are difficult for machining. Such a hybrid method was applied for cutting and turning of hard ceramic (Tsai & Ou, 2004). The research on local heating with laser beam during turning, milling and grinding of titanium alloys, cast iron and special steel was conducted. The hybrid treatment - laser-assisted burnishing (LAB) - was elaborated by Tian and Shin 2007. The laser heating process was applied for the burnishing of steel. It provided the reduction of the burnishing force, as well as the tool wear. It was shown that LAB

In the work (Radziejewska 2007) the new method to modify surface layer, combing the laser melting with the slide burnishing, was proposed. The smoothing of surface was carried out by plastic deformation of surface layer at high temperature, whereas transformation process of stresses, from compressive to tensile stresses, was performed by plastic deformation at low temperatures. All machining operations - LBM, high and low temperature burnishing are performed simultaneously on the laser station, in one pass. Temperature changes while the cooling of material that undergoes the laser beam treatment, are used. It does not extend duration of treatment. It was stated that multiple alloying combined with slide burnishing generated compressive stresses of about – 600 MPa at the surface. Because of the adopted type of burnishing – the slide burnishing and high hardness of material, the relatively small thickness of textured zone, about 30 µm, was obtained. In the case of thick layers it can be insufficient. According to (Przybylski, 1987) high degree of strain hardening of surface is

The current work presents the analysis of the plastic deformation of surface layer when the new type of the laser-burnishing process was applied. The surface layer was generated by laser alloying and dynamic burnishing. Design of new head allows for simultaneous surface burnishing in two different temperatures on the laser station providing high-intensity of the burnishing process. That allowed treatment hard and low plasticity materials. Thus such a technical solution should result in high degree of surface deformation as well as its large depth. The proposed solution is designed for thick layers, above 1mm, mostly produced by

The aim of this study was to evaluate the influence of the hybrid processing parameters on plastic deformations of surface layer and the analysis of the correlations between treatment parameters and surface layer state. Establishing relationships between process parameters and the state of the surface layer of tested material will determine the application area of the

can form better surface roughness and higher hardness than conventional burnishing.

additional machining in order to improve surface smoothness.

stresses to compressive, in surface layer 70 µm thick was obtained.

possible to provide using dynamic burnishing.

LBM.

The study was performed using the CO2 laser with the maximum power 2.5 kW. The axially-symmetric beam, of the mode close to TEM10, was focused with use of the ZnSe lens. The focal length of it was 5". The set of treatment parameters, such as laser power, feed rate, and diameter of the beam focused on the metal surface, type of shielding gas and speed of the air flow to ensure a sufficient power density to obtain the remelting and optimal results, was selected. Based on previous experience (Radziejewska&Skrzypek, 2009) the ranges of parameters of laser processing were identified in the first stage of the study. The optimization criteria, such as penetration depth, surface roughness and hardness of the resulting surface layer, were taken into consideration.

The alloying process was carried out on steel 304. Prior to the alloying a layer of Stellite 6, with a thickness of about 200 µm, was formed on the surface with plasma spraying method. Preliminary studies have shown that in this case, the alloying process takes place preferably at the following parameters: laser power 2 kW, laser beam diameter of 3 mm, feed rate of sample against the laser beam from 150-900 mm/min.

#### **2.2 Laser-mechanical treatment**

On the laser station the burnishing process was carried out simultaneously with the alloying process. The dynamic burnishing process with use of micro-hammers was applied. The technology of micro-hammering was based on a dynamic centrifugal burnishing. For microhammering a high rotational head was developed, providing the possibility of working directly on the laser processing. Processing concept and principle of operation of the head is described in work (Radziejewska et al., 2005). In this study the modified version of the head was applied. Two rows of 8 micro-hammers allow providing greater intensity of the process and the simultaneous treatment at two different temperatures. In order to obtain more uniform deformation of surface material, the oscillation motion of the sample in a direction perpendicular to the direction of feed was introduced. The motion was generated using an oscillating table. The oscillations eliminated the problem of the formation of unfavourable geometric surface structure - the grooves occurring in earlier solution. A constant velocity, of 15 oscillations per second and the amplitude of 2 mm, was used. The small radii of microhammers allow to obtained high surface plastic deformation at low forces. The scheme of the head is shown in Figure 1a while the laser-mechanical treatment presented in Fig. 1b. Figure 1c shows the temperature distribution on the surface along x axis with the selected range of temperatures in which the burnishing process was conducted.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 255

element and the laser beam axis was determined initially on the basis of calculations of temperature distribution on material surface, when a moving source of heat, which has a Gaussian distribution of energy density, was considered. The distribution in a half-space has an approximate character because of numerous simplified assumptions (Anthony&Cline, 1977). The distribution in a half-space has an approximate character because of following


The calculations were done for parameters of laser treatment, the same as those were applied in preliminary investigations. The results of calculations have been verified by microstructure changes of material. The tools interacted with material within temperature

Study of surface layer state is time-consuming. Therefore determination the effect of treatment conditions on the state of surface layer was based on the theory of planned experiment, which minimizes the number of studies (Filipowski 1996; Montgomery, 1997). An experiment based on a static, determined, multi-factorial, rotatable planned program with repetition PS/DS- λ was carried out. The aim was to find functional relations between

The selection of input variables and ranges of their variation was based on preliminary results of the laser- mechanical process taking their suitability to control the hybrid treatment as an additional criterion. As the measurable, controllable input variables the




As the output factors characterizing the state of surface layer and the effects of hybrid treatment were: change in microhardness compared to the microhardness after laser alloying, thickness of the plastic deformation zones and the ratio of thickness of the plastic deformation zones to thickness of alloyed zone. On the basis of preliminary studies the areas of treatment parameters variability and the intervals of variation of input data were determined. Indications and values of variation ranges of input data are contained in Table 1. The statistical analysis of experimental results included a selection of the regression function, a statistical verification of the approximating function adequacy and verification of significance of the approximating function coefficients. The attempts of approximation using the power function and the first-degree polynomial have been made. The correlation

reflection are constant and temperature independent,


the parameters of the hybrid treatment and the state of the surface layer.

process and temperature in the region of treatment.

simplified assumptions:

range about 200–8500C.

**2.3 Testing methods** 

following quantities were considered:

of the burnishing process,

hammers on the surface,


Fig. 1. A – dynamic burnishing head, b – scheme of the station for laser-mechanic treatment: 1-laser beam, 2-sample, 3-laser path, 4-oscillation table, 5-dynamic burnishing head; c – temperature distribution on the surface, along x axis, with burnishing area.

The treatment with the head is based on cyclical impacts of the burnishing elements onto the machined surface. The micro-hammers are made of bearing steel, and their working part has radius of 1.5 mm. They are placed evenly between body shields of the head and rotary mounted on the axes, providing the swinging motion of the hammers in relation to the head as well as the rotary motion with head. The compact head enables the processing of flat surfaces and curved of small sizes. The head is designed as the smoothing and strengthening treatment of laser modified parts. The head was mounted in a grip of portable grinding tool, which is mounted on the laser treatment station together with the system of the head adjustment. The station enables controlling wide range parameters of the process:


Before basic studies of the preliminary tests were carried out in order determine the optimal position of the tool in relation to the machined surface. The distance between the burnishing element and the laser beam axis was determined initially on the basis of calculations of temperature distribution on material surface, when a moving source of heat, which has a Gaussian distribution of energy density, was considered. The distribution in a half-space has an approximate character because of numerous simplified assumptions (Anthony&Cline, 1977). The distribution in a half-space has an approximate character because of following simplified assumptions:


254 CO2 Laser – Optimisation and Application

4

Fig. 1. A – dynamic burnishing head, b – scheme of the station for laser-mechanic treatment:


**x [cm]**

The treatment with the head is based on cyclical impacts of the burnishing elements onto the machined surface. The micro-hammers are made of bearing steel, and their working part has radius of 1.5 mm. They are placed evenly between body shields of the head and rotary mounted on the axes, providing the swinging motion of the hammers in relation to the head as well as the rotary motion with head. The compact head enables the processing of flat surfaces and curved of small sizes. The head is designed as the smoothing and strengthening treatment of laser modified parts. The head was mounted in a grip of portable grinding tool, which is mounted on the laser treatment station together with the system of the head adjustment. The station enables controlling wide range parameters of the process: • impact forces on the tool surface by controlling the rational speed of the head – *Vrev* and

• temperature of the process zone due to changes of the distance between the impact of

• intensity of the surface hardening by adjusting the feed rate of the sample - *Vf,* and the

Before basic studies of the preliminary tests were carried out in order determine the optimal position of the tool in relation to the machined surface. The distance between the burnishing

1-laser beam, 2-sample, 3-laser path, 4-oscillation table, 5-dynamic burnishing head; c – temperature distribution on the surface, along x axis, with burnishing area.

its distance from the surface undergoing treatment,

micro-hammers and the axis of the laser beam - *X*,

rotational speed of micro-hammers – *Vrev*.

Vf

Vosc

1

5

c

2

3

b

a

**T [C]**


The calculations were done for parameters of laser treatment, the same as those were applied in preliminary investigations. The results of calculations have been verified by microstructure changes of material. The tools interacted with material within temperature range about 200–8500C.

#### **2.3 Testing methods**

Study of surface layer state is time-consuming. Therefore determination the effect of treatment conditions on the state of surface layer was based on the theory of planned experiment, which minimizes the number of studies (Filipowski 1996; Montgomery, 1997). An experiment based on a static, determined, multi-factorial, rotatable planned program with repetition PS/DS- λ was carried out. The aim was to find functional relations between the parameters of the hybrid treatment and the state of the surface layer.

The selection of input variables and ranges of their variation was based on preliminary results of the laser- mechanical process taking their suitability to control the hybrid treatment as an additional criterion. As the measurable, controllable input variables the following quantities were considered:


As the output factors characterizing the state of surface layer and the effects of hybrid treatment were: change in microhardness compared to the microhardness after laser alloying, thickness of the plastic deformation zones and the ratio of thickness of the plastic deformation zones to thickness of alloyed zone. On the basis of preliminary studies the areas of treatment parameters variability and the intervals of variation of input data were determined. Indications and values of variation ranges of input data are contained in Table 1.

The statistical analysis of experimental results included a selection of the regression function, a statistical verification of the approximating function adequacy and verification of significance of the approximating function coefficients. The attempts of approximation using the power function and the first-degree polynomial have been made. The correlation

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 257

The process of contact deformation was carried on the stand using a measuring method of the surface approach proposed by Demkin 1956. The device enables measurement the approach *a* with accuracy 1 µm as a function of applied nominal pressure *q*. Contact is realized between flat and rough surface of the sample, and the smooth and rigid surface of the counter-sample, made of sintered carbide. The counter-samples have three punches, 5mm diameter and nominal area of 58.875 mm2 each. The sample undergoes deformation under the punches. Both samples are placed in a specially constructed device, which is mounted inside the laboratory precision hydraulic press that allows applying the normal pressure up to 1000 MPa. The applied pressure are measured using the compression proving ring, while the approach of samples is measured using the inductive sensor. The results of measurements were recorded in the form of the approach value *a* [µm] for the

The study was carried out to nominal pressure 270 MPa, then the unloading to 0 N. The testing enabled to determine the values of total deformation, denoted as *a1* and *a2,* and plastic deformation, *apl*, as well as the elastic one, *ae*. It also allowed determining the curves of approach - nominal pressure relation. The values of deformation are the averaged values

Due to the large differences in the surface geometric structure, which has a significant influence on the contact stiffness, the surfaces of the samples were subjected to grinding in order to form similar surface topography. Application of the grinding allowed to eliminate the influence of differences in surface roughness on contact stiffness due to the various processes applied previously. It allowed for the analysis of properties of the surface layer, in this case the burnished layer, on the process of contact deformation. After grinding the

The slurry erosive wear test was conducted on test stand equipped with the chamber, the sprinkler head with nozzle of 10 mm diameter, the mixer and a sample holder. The pressure in the power system filled with compressed air was 5 Barr. The erosive tests were performed using 15% aqueous suspension of SiC particles. The size of SiC particles was 42.5 – 46.5 µm. The samples were placed 100 mm from the nozzle. The suspension angle between the sample and surface was 900. Before the test the samples were polished to reach the same roughness of surface. A special protection was applied to provide the erosive testing only within chosen limited area. The preliminary test allowed selecting erosive test duration. It also gave information that depths of eroded material were less than thickness of tested layers. The duration of each test was 90 s. All erosive tests were performed in the same

The erosion was determined using scanning profilometer. The eroded areas, and noneroded area that was treated as a reference surface, were 3D measured. The depth and volume of eroded region was calculated based on TalyMap Platinium 5.0 program. For each examined surface layers the testing was performed on three areas. Final erosive losses were

surface roughness measurement was repeated (Radziejewska, 2011).

determined as an average value for medium depths and volumes.

**2.3.3 Contact stiffness** 

given loading *F* [N].

**2.3.4 Wear test** 

conditions.

from three areas of measurement.

and significance evaluation has been determined according to criteria based on I. P. Guilford theory. The confidence level α = 0.1 was adopted. Credibility of the equations was assessed based on the following criteria:



Table 1. The input values of the hybrid treatment experiment for steel 304 alloyed with Stellite 6.

#### **2.3.1 Microstructure and macro-stress analysis**

Microstructure analysis and measurements of a size of the melted zone was performed on an optical microscope at magnifications from 50 to 1000 X, and for selected samples on a scanning microscope. For this purpose, after the laser alloying and the laser-mechanical treatment all samples were cut perpendicular to the treated surface and metallographic micro-sections were made in a direction perpendicular to the direction of feed. For selected samples the micro-sections in a direction parallel to the feed samples were made additionally. Surface analysis of chemical composition was also carried out for selected samples after the alloying and the hybrid treatment.

A study of internal macro-stresses and phase composition was conducted on Bruker's D-8 Advance diffractometer, with the Mo anode lamp. The measurements of internal macrostresses were carried out on surface at a distance of 0.3 and 0.5 mm from the surface. Calculations were performed for elasticity indexes *E*=210 GPa and ν=0.28. An evaluation of the degree of plastic deformation, caused by surface burnishing process, was carried out on the basis of changes in microhardness of the material. Measurements were made at load of 0.2 N, in the central zone of the melting for both samples - alloyed with the oscillations as well as laser-mechanically treated with oscillations. The microhardness result is an average of 5 measurements.

#### **2.3.2 Surface roughness measurements**

The surface topography was examined. The measurements were conducted on a scanning profilometer Form Talysurf after laser alloying and hybrid process performed at different parameters. Surface roughness measurements were performed for each track of the laser alloying, and the laser alloying with oscillations combined with micro-hammering. The 3D roughness measurements were conducted in central area of the melting path. The values of surface topography parameters were determined for scanned area 1.4×4 mm. The measurements were conducted at steps dx = 0.5 µm, dy = 5 µm, with the stylus radius of 2 µm. Profile measurements were carried out in the middle of the zone of the melting on measuring section equal to 4 mm, parallel to direction of the feed rate. Roughness parameters are the average values of 16 measured profiles.

#### **2.3.3 Contact stiffness**

256 CO2 Laser – Optimisation and Application

and significance evaluation has been determined according to criteria based on I. P. Guilford theory. The confidence level α = 0.1 was adopted. Credibility of the equations was assessed

> Parameter Input values Unit *Vrev* 3500 4200 5000 5950 7100 rev/min *Vf* 150 230 360 570 900 mm/min *X* 5 6 7 8,5 10 mm

Table 1. The input values of the hybrid treatment experiment for steel 304 alloyed with

Microstructure analysis and measurements of a size of the melted zone was performed on an optical microscope at magnifications from 50 to 1000 X, and for selected samples on a scanning microscope. For this purpose, after the laser alloying and the laser-mechanical treatment all samples were cut perpendicular to the treated surface and metallographic micro-sections were made in a direction perpendicular to the direction of feed. For selected samples the micro-sections in a direction parallel to the feed samples were made additionally. Surface analysis of chemical composition was also carried out for selected

A study of internal macro-stresses and phase composition was conducted on Bruker's D-8 Advance diffractometer, with the Mo anode lamp. The measurements of internal macrostresses were carried out on surface at a distance of 0.3 and 0.5 mm from the surface.

An evaluation of the degree of plastic deformation, caused by surface burnishing process, was carried out on the basis of changes in microhardness of the material. Measurements were made at load of 0.2 N, in the central zone of the melting for both samples - alloyed with the oscillations as well as laser-mechanically treated with oscillations. The

The surface topography was examined. The measurements were conducted on a scanning profilometer Form Talysurf after laser alloying and hybrid process performed at different parameters. Surface roughness measurements were performed for each track of the laser alloying, and the laser alloying with oscillations combined with micro-hammering. The 3D roughness measurements were conducted in central area of the melting path. The values of surface topography parameters were determined for scanned area 1.4×4 mm. The measurements were conducted at steps dx = 0.5 µm, dy = 5 µm, with the stylus radius of 2 µm. Profile measurements were carried out in the middle of the zone of the melting on measuring section equal to 4 mm, parallel to direction of the feed rate. Roughness

ν=0.28.

Calculations were performed for elasticity indexes *E*=210 GPa and

based on the following criteria:

Stellite 6.


**2.3.1 Microstructure and macro-stress analysis** 

samples after the alloying and the hybrid treatment.

microhardness result is an average of 5 measurements.

parameters are the average values of 16 measured profiles.

**2.3.2 Surface roughness measurements** 


The process of contact deformation was carried on the stand using a measuring method of the surface approach proposed by Demkin 1956. The device enables measurement the approach *a* with accuracy 1 µm as a function of applied nominal pressure *q*. Contact is realized between flat and rough surface of the sample, and the smooth and rigid surface of the counter-sample, made of sintered carbide. The counter-samples have three punches, 5mm diameter and nominal area of 58.875 mm2 each. The sample undergoes deformation under the punches. Both samples are placed in a specially constructed device, which is mounted inside the laboratory precision hydraulic press that allows applying the normal pressure up to 1000 MPa. The applied pressure are measured using the compression proving ring, while the approach of samples is measured using the inductive sensor. The results of measurements were recorded in the form of the approach value *a* [µm] for the given loading *F* [N].

The study was carried out to nominal pressure 270 MPa, then the unloading to 0 N. The testing enabled to determine the values of total deformation, denoted as *a1* and *a2,* and plastic deformation, *apl*, as well as the elastic one, *ae*. It also allowed determining the curves of approach - nominal pressure relation. The values of deformation are the averaged values from three areas of measurement.

Due to the large differences in the surface geometric structure, which has a significant influence on the contact stiffness, the surfaces of the samples were subjected to grinding in order to form similar surface topography. Application of the grinding allowed to eliminate the influence of differences in surface roughness on contact stiffness due to the various processes applied previously. It allowed for the analysis of properties of the surface layer, in this case the burnished layer, on the process of contact deformation. After grinding the surface roughness measurement was repeated (Radziejewska, 2011).

#### **2.3.4 Wear test**

The slurry erosive wear test was conducted on test stand equipped with the chamber, the sprinkler head with nozzle of 10 mm diameter, the mixer and a sample holder. The pressure in the power system filled with compressed air was 5 Barr. The erosive tests were performed using 15% aqueous suspension of SiC particles. The size of SiC particles was 42.5 – 46.5 µm. The samples were placed 100 mm from the nozzle. The suspension angle between the sample and surface was 900. Before the test the samples were polished to reach the same roughness of surface. A special protection was applied to provide the erosive testing only within chosen limited area. The preliminary test allowed selecting erosive test duration. It also gave information that depths of eroded material were less than thickness of tested layers. The duration of each test was 90 s. All erosive tests were performed in the same conditions.

The erosion was determined using scanning profilometer. The eroded areas, and noneroded area that was treated as a reference surface, were 3D measured. The depth and volume of eroded region was calculated based on TalyMap Platinium 5.0 program. For each examined surface layers the testing was performed on three areas. Final erosive losses were determined as an average value for medium depths and volumes.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 259

Surface plastic deformation caused a complete reconstruction of material microstructure near the surface. The microstructure, after laser alloying, is shown in Figure 4. There are equiaxed grains of a size of several micrometers. The intensive flattening of grains in the direction perpendicular to the surface is presented, Fig.5. There are no cracks or chipping. The coating particles, which were not melted by laser, can be observed on the surface in some places. They were probably stuck in the surface of already melted and solidified material by the micro-hammers. The largest deformation of grains is in the zone about 100- 150 µm from the surface. As it was expected the varied plastic deformation thickness of the zone was determined depending on the impact of the micro-hammers on the surface and the

Fig. 4. The surface layer microstructure of steel 0H18N9 after laser alloying with Stellite 6 using oscillations; a- close to surface, b- central part of alloyed zone. The treatment

Fig. 5. The surface layer microstructure after hybrid treatment at the parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min, *V rev*= 5950 rev/min, *X* = 6 mm, using oscillations; a- close to

The X –ray diffraction XRD showed the presence of cobalt austenite, tungsten carbides and chromium as well as chromium oxides, cobalt and cobalt ferrite. In almost all samples the dominant phase was cobalt austenite. The analysis of chemical composition in the melting

a b

a b

temperature at which the process was carried out.

parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min; (SEM).

surface, b- central part of alloyed zone; (SEM).

#### **3. Plastic deformations due to laser-burnishing**

#### **3.1 Microstructure and size of plastic deformation zone**

Studies of the microstructure showed that both after the alloying and the hybrid treatment the surface layer is homogeneous, free of pores and micro-cracks. A very fine dendrites structure, oriented in the direction of heat dissipation, is formed. The oscillations caused an increase in the width of the melted zone in relation to the alloyed samples without oscillations by the value of oscillation amplitude. Additionally the application of oscillations resulted in more even thickness of surface layer. Figure 2a shows the shape of the melted zone, while Figure 2b presents the melted zone at same processing parameters using oscillations.

Fig. 2. Shape of melted zone after: a - laser alloying of steel 0H18N9 with Stellite 6 at treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 900 mm/min; b - alloying using the oscillations in perpendicular direction.

The study proved constant thickness of the melting zone. When the treatment is conducted at the lowest speed, 150 mm/min, minor changes in the thickness of the melting, related to waves of the bottom of the zone melting, can be observed (Fig. 3).

Fig. 3. The surface layer after laser alloying of steel 0H18 N9 with Stellite 6 at treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 150 mm/min using the oscillations. The cross-section parallel to the direction of the sample motion.

Studies of the microstructure showed that both after the alloying and the hybrid treatment the surface layer is homogeneous, free of pores and micro-cracks. A very fine dendrites structure, oriented in the direction of heat dissipation, is formed. The oscillations caused an increase in the width of the melted zone in relation to the alloyed samples without oscillations by the value of oscillation amplitude. Additionally the application of oscillations resulted in more even thickness of surface layer. Figure 2a shows the shape of the melted zone, while Figure 2b presents the melted zone at same processing parameters using

Fig. 2. Shape of melted zone after: a - laser alloying of steel 0H18N9 with Stellite 6 at treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 900 mm/min; b - alloying using the

waves of the bottom of the zone melting, can be observed (Fig. 3).

The study proved constant thickness of the melting zone. When the treatment is conducted at the lowest speed, 150 mm/min, minor changes in the thickness of the melting, related to

Fig. 3. The surface layer after laser alloying of steel 0H18 N9 with Stellite 6 at treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 150 mm/min using the oscillations. The cross-section

a

b

**3. Plastic deformations due to laser-burnishing 3.1 Microstructure and size of plastic deformation zone** 

oscillations.

oscillations in perpendicular direction.

parallel to the direction of the sample motion.

Surface plastic deformation caused a complete reconstruction of material microstructure near the surface. The microstructure, after laser alloying, is shown in Figure 4. There are equiaxed grains of a size of several micrometers. The intensive flattening of grains in the direction perpendicular to the surface is presented, Fig.5. There are no cracks or chipping. The coating particles, which were not melted by laser, can be observed on the surface in some places. They were probably stuck in the surface of already melted and solidified material by the micro-hammers. The largest deformation of grains is in the zone about 100- 150 µm from the surface. As it was expected the varied plastic deformation thickness of the zone was determined depending on the impact of the micro-hammers on the surface and the temperature at which the process was carried out.

Fig. 4. The surface layer microstructure of steel 0H18N9 after laser alloying with Stellite 6 using oscillations; a- close to surface, b- central part of alloyed zone. The treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min; (SEM).

Fig. 5. The surface layer microstructure after hybrid treatment at the parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min, *V rev*= 5950 rev/min, *X* = 6 mm, using oscillations; a- close to surface, b- central part of alloyed zone; (SEM).

The X –ray diffraction XRD showed the presence of cobalt austenite, tungsten carbides and chromium as well as chromium oxides, cobalt and cobalt ferrite. In almost all samples the dominant phase was cobalt austenite. The analysis of chemical composition in the melting

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 261

0.02

Table 2. The microhardness as function of depth for samples after hybrid treatment for

Figure 7 shows the exemplary microhardness distributions after alloying and the hybrid treatment for the samples 1 and 2. The treatment process was carried out at the same feed rate, *Vf* = 230 mm/s, and at the same temperature. Both samples exhibit a substantial increase in microhardness of the hybrid treated material in relation to only alloyed. Additionally, the diagram Fig.7 shows the differences in microhardness of alloyed layers

On the basis of the results the relative percentage increase in microhardness, caused by surface plastic deformation, was determined for each sample, and the thickness of the zones of plastic deformation was estimated under the assumption that minimal increase in the

1 4200 230 6 527 480 430 410 350 2 5950 230 6 530 520 460 430 400 3 4200 570 6 540 430 380 360 290 4 5950 570 6 610 550 480 450 370 5 4200 230 8,5 640 520 440 420 290 6 5950 230 8,5 700 570 490 440 360 7 4200 570 8,5 580 550 510 440 350 8 5950 570 8,5 630 600 540 440 400 9 3500 360 7 630 500 560 520 410 10 7100 360 7 680 570 460 410 310 11 5000 150 7 660 480 450 410 400 12 5000 900 7 570 430 410 370 270 13 5000 360 5 540 460 420 380 290 14 5000 360 10 500 450 420 410 280 15 5000 360 7 610 480 430 410 280 16 5000 360 7 670 570 380 410 280 17 5000 360 7 630 490 490 420 380 18 5000 360 7 640 480 430 410 290 19 5000 360 7 620 530 450 400 350 20 5000 360 7 610 510 440 420 290

X HV

Treatment parameter Microhardness after hybrid treatment at selected

HV 0.2

0.6 [mm]

levels from surface

HV 0.3

HV 0.4

HV

Sample

Vrev [rev/min]

variable process parameters.

generated in the same conditions.

Vf [mm/min]

zone showed the increase in concentration of Co, W, Ni, and C in relation to the core. A homogeneous distribution of elements was found in the melted zone for both groups of samples - laser alloyed and hybrid treated. In Fig.6 the exemplary surface distributions of concentration of iron and cobalt for the hybrid treated sample, at the edge of the hybrid laser path, are shown.

Fig. 6. Surface distributions of elements in the melted zone after laser-mechanical treatment: a - Fe, b – Co; (EDS-SEM).

Measurements of dimensions of the alloyed zone showed that thickness of the melted zone ranges from 0.43 mm to 0.87 mm, while its width is from 3.68 mm to 4.6 mm. The size of the zone depends on the sample feed rates. The largest size of melted zone was found for the lowest feed rate of 150 mm/min. The oscillation motion caused an increase in the width of the melting zone by the value of the oscillation amplitude, i.e. 2 mm, compared to the laser alloying without oscillations. Shape changes of the melting also resulted from the oscillations, shown in Fig. 3.

#### **3.2 Microhardness of strain hardening zone**

Table 2 presents the results of microhardness study. Microhardness values refer to five distances from the surface: 0.02, 0.2, 0.3, 0.4 and 0.6 mm. After laser alloying the surface layer material had microhardness 300 - 420 HV. In most cases the increase in microhardness was recognized at the surface compared to the bottom of the melting. This is due to the presence of fine grains in the subsurface zone. Differences in microhardness observed for the same depth of the melted zone are due to different thickness and porosity of Stellite layer deposited before the process of laser-mechanical stated. These differences determine the chemical and phase composition of the alloyed layer. In order to eliminate these distortions all calculations, related to the assessment of surface plastic deformation and thickness of the hardening zone, were carried out for one laser "path" where the zone only laser-alloyed and alloyed with micro-hammering existed.

Microhardness of layer generated by laser alloying combined with micro-hammering is 530- 670 HV at the material surface and about 400 HV at the melting of the bottom. For all tested samples the increase in microhardness at the surface can be observed. It is related to the process of burnishing. The thickness of the strain hardening zone varies depending on the burnishing force as well as the intensity and temperature of the process. In all cases it is thicker than a textured zone observed on the metallographic cross-sections.

zone showed the increase in concentration of Co, W, Ni, and C in relation to the core. A homogeneous distribution of elements was found in the melted zone for both groups of samples - laser alloyed and hybrid treated. In Fig.6 the exemplary surface distributions of concentration of iron and cobalt for the hybrid treated sample, at the edge of the hybrid

Fig. 6. Surface distributions of elements in the melted zone after laser-mechanical treatment:

Measurements of dimensions of the alloyed zone showed that thickness of the melted zone ranges from 0.43 mm to 0.87 mm, while its width is from 3.68 mm to 4.6 mm. The size of the zone depends on the sample feed rates. The largest size of melted zone was found for the lowest feed rate of 150 mm/min. The oscillation motion caused an increase in the width of the melting zone by the value of the oscillation amplitude, i.e. 2 mm, compared to the laser alloying without oscillations. Shape changes of the melting also resulted from the

Table 2 presents the results of microhardness study. Microhardness values refer to five distances from the surface: 0.02, 0.2, 0.3, 0.4 and 0.6 mm. After laser alloying the surface layer material had microhardness 300 - 420 HV. In most cases the increase in microhardness was recognized at the surface compared to the bottom of the melting. This is due to the presence of fine grains in the subsurface zone. Differences in microhardness observed for the same depth of the melted zone are due to different thickness and porosity of Stellite layer deposited before the process of laser-mechanical stated. These differences determine the chemical and phase composition of the alloyed layer. In order to eliminate these distortions all calculations, related to the assessment of surface plastic deformation and thickness of the hardening zone, were carried out for one laser "path" where the zone only

Microhardness of layer generated by laser alloying combined with micro-hammering is 530- 670 HV at the material surface and about 400 HV at the melting of the bottom. For all tested samples the increase in microhardness at the surface can be observed. It is related to the process of burnishing. The thickness of the strain hardening zone varies depending on the burnishing force as well as the intensity and temperature of the process. In all cases it is

thicker than a textured zone observed on the metallographic cross-sections.

a b

laser path, are shown.

a - Fe, b – Co; (EDS-SEM).

oscillations, shown in Fig. 3.

**3.2 Microhardness of strain hardening zone** 

laser-alloyed and alloyed with micro-hammering existed.


Table 2. The microhardness as function of depth for samples after hybrid treatment for variable process parameters.

Figure 7 shows the exemplary microhardness distributions after alloying and the hybrid treatment for the samples 1 and 2. The treatment process was carried out at the same feed rate, *Vf* = 230 mm/s, and at the same temperature. Both samples exhibit a substantial increase in microhardness of the hybrid treated material in relation to only alloyed. Additionally, the diagram Fig.7 shows the differences in microhardness of alloyed layers generated in the same conditions.

On the basis of the results the relative percentage increase in microhardness, caused by surface plastic deformation, was determined for each sample, and the thickness of the zones of plastic deformation was estimated under the assumption that minimal increase in the

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 263

HV 0.2

1 4200 230 6 46 45 34 28 9 0.55 0.75 2 5950 230 6 71 63 44 43 38 1.2 1.62 3 4200 570 6 59 13 0 0 0 0.25 0.54 4 5950 570 6 74 67 60 50 42 1.2 2.67 5 4200 230 8.5 78 30 13 14 - 0.5 0.77 6 5950 230 8.5 89 36 32 16 38 0.7 1.08 7 4200 570 8.5 32 45 34 10 - 0.4 0.75 8 5950 570 8.5 50 43 32 13 14 0.65 1.30 9 3500 360 7 62 39 56 33 8 0.5 0.94 10 7100 360 7 100 68 53 37 3 0.55 1.00 11 5000 150 7 89 55 50 37 33 1.1 1.26 12 5000 900 7 68 39 24 12 4 0.45 1.05 13 5000 360 5 54 44 35 27 - 0.5 0.77 14 5000 360 10 32 18 2 8 4 0.25 0.43 15 5000 360 7 74 41 23 28 - 0.5 0.88 16 5000 360 7 91 78 9 17 - 0.4 0.67 17 5000 360 7 70 29 32 17 12 0.55 0.80 18 5000 360 7 83 41 23 28 - 0.5 0.82 19 5000 360 7 77 66 29 14 3 0.5 0.85 20 5000 360 7 65 34 19 17 - 0.5 0.74 Table 3. The percentage change in the microhardness value at different depths, the thickness of the plastic deformation zone, *Gpl*, and the ratio of the plastic deformation thickness to the

The influence of hybrid treatment parameters on features characterizing plastic deformation zone, e.g. thickness, change in microhardness compared to the microhardness after laser alloying and ratio of thickness of the plastic deformation zone to thickness of alloyed zone,

ΔHV = 0.53 Vrev0.66 Vf

0.6 [mm]

microhardness **[%]** 

HV 0.3

HV 0.4

HV



Thickness of plastic deformati on zone [mm]

Gpl/Gla

Treatment parameter Relative change in

X HV 0.02

Sample

Vrev [rev/min]

thickness of the alloyed zone, *Gpl/Gla*.

**3.3 Statistical analysis of results** 

was presented in form of the regression function.

Gpl = 0.0026 Vrev1.01 Vf

Vf [mm/min]

layer is 10% (Tab.3). The micro-hammering caused the relative increase in microhardness of SL of about 32-100% at the surface compared to microhardness of SL due to laser alloying. This effect is due to surface strain hardening. The smallest increase in microhardness, 32%, at the surface was found for the sample that was burnished at the lowest temperature when distance between the hammers and the beam axis was 10 mm. In this case, the smallest depth of the plastic deformation zone was also recognized. The increase in microhardness, over 60%, was found for most of the samples (2, 4, 5, 6, 10, 11, 15-20) burnished with large impact forces of micro-hammers on surface, rotational speeds of the head above 5000 rev/min and temperature of treatment from the middle range. For the samples (1, 13) burnished at high temperature the degree of strain hardening is the order of 40-50%, which is probably related to the partial recovery of the material at high temperature.

Fig. 7. Microhardness after: ♦ ■ laser alloying of steel 0H18 N9 with Stellite 6 treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 230 mm/min and alloying combined with microhammering ▲ *X* = 6 mm, *Vrev*= 4200 rev/min, **×** *Vrev*= 5950 rev/min,

The thickness of the plastic deformation zone is from 0.25 to 1.2 mm, depending on the parameters of laser-mechanical treatment. It grows with increase of the impact forces of burnishing elements on the surface and the rise of temperature in which the process of burnishing was undergoing. This effect is due to the increase in material plasticity with temperature growth.

Plastic deformation of the surface layer caused changes in residual stresses. In order to assess the state of stresses in the surface layer the preliminary studies of internal macrostresses were performed. The selected for study samples were laser alloyed at feed rate of 230 mm/min and dynamically burnished at temperature from the upper range, *Vrev* = 5950 rev/min, *X* = 6 mm. The measurements were carried out on the surface and at the depth of 0.3 and 0.5 mm beneath the surface. The results confirmed the stresses change, from tensile +398 MPa after the alloying process to compressive stresses -800 MPa caused by the burnishing process. After the dynamic burnishing the compressive stresses across the whole depth of the melted zone were found.

layer is 10% (Tab.3). The micro-hammering caused the relative increase in microhardness of SL of about 32-100% at the surface compared to microhardness of SL due to laser alloying. This effect is due to surface strain hardening. The smallest increase in microhardness, 32%, at the surface was found for the sample that was burnished at the lowest temperature when distance between the hammers and the beam axis was 10 mm. In this case, the smallest depth of the plastic deformation zone was also recognized. The increase in microhardness, over 60%, was found for most of the samples (2, 4, 5, 6, 10, 11, 15-20) burnished with large impact forces of micro-hammers on surface, rotational speeds of the head above 5000 rev/min and temperature of treatment from the middle range. For the samples (1, 13) burnished at high temperature the degree of strain hardening is the order of 40-50%, which

0 0,2 0,4 0,6 0,8 1 1,2 1,4

distance from surface [mm]

Fig. 7. Microhardness after: ♦ ■ laser alloying of steel 0H18 N9 with Stellite 6 treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 230 mm/min and alloying combined with micro-

The thickness of the plastic deformation zone is from 0.25 to 1.2 mm, depending on the parameters of laser-mechanical treatment. It grows with increase of the impact forces of burnishing elements on the surface and the rise of temperature in which the process of burnishing was undergoing. This effect is due to the increase in material plasticity with

Plastic deformation of the surface layer caused changes in residual stresses. In order to assess the state of stresses in the surface layer the preliminary studies of internal macrostresses were performed. The selected for study samples were laser alloyed at feed rate of 230 mm/min and dynamically burnished at temperature from the upper range, *Vrev* = 5950 rev/min, *X* = 6 mm. The measurements were carried out on the surface and at the depth of 0.3 and 0.5 mm beneath the surface. The results confirmed the stresses change, from tensile +398 MPa after the alloying process to compressive stresses -800 MPa caused by the burnishing process. After the dynamic burnishing the compressive stresses across the whole

hammering ▲ *X* = 6 mm, *Vrev*= 4200 rev/min, **×** *Vrev*= 5950 rev/min,

is probably related to the partial recovery of the material at high temperature.

0

100

200

300

HV0.02

temperature growth.

depth of the melted zone were found.

400

500

600


Table 3. The percentage change in the microhardness value at different depths, the thickness of the plastic deformation zone, *Gpl*, and the ratio of the plastic deformation thickness to the thickness of the alloyed zone, *Gpl/Gla*.

#### **3.3 Statistical analysis of results**

The influence of hybrid treatment parameters on features characterizing plastic deformation zone, e.g. thickness, change in microhardness compared to the microhardness after laser alloying and ratio of thickness of the plastic deformation zone to thickness of alloyed zone, was presented in form of the regression function.

$$\mathbf{G}\_{\rm pl} = 0.0026 \text{ V}\_{\rm rev} \, ^{1.01} \mathbf{V}\_{\rm f} ^{0.19} \mathbf{X} \cdot ^{1.16} \tag{1}$$

$$
\Delta \text{HV} = 0.53 \text{ V}\_{\text{rev}} \alpha \text{.66 V}\_{\text{f}} \text{-0.082 } \text{X} \text{ -0.61} \tag{2}
$$

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 265

Figure 9 shows the relation between the increase in material microhardness at the depth of 0.2 mm and the rotational speed of the head, and the distance from the tool axis to the laser beam at constant feed rate 360 mm/min. The microhardness growth, characterizing the degree of strain hardening at the surface, depends mainly on the rotational speed of the head and the temperature of the process zone, which is a function of the distance between

Fig. 9. Dependence of microhardness increase at a depth of 0.2 mm on rotational speed of the head and distance between the tool axis *X* and the laser beam, for fixed feed rate 360

Figure 10 shows the influence of the rotational speed of the head and the distance of the tool from surface, at a fixed feed rate *Vf* = 360 mm/min, on the *Gpl/Gla* parameter. It is evident that at the highest speed and the distances of less than 9 mm the ratio is bigger than 1. At speeds below 4000 rev/min and the distance of less than 8 mm the thickness of the strainhardening zone in relation to the thickness of the alloyed layer is less than 0.5, although it

Fig. 10. Influence of rotational speed and the distance between the beam axis and the tools on the ratio of the thickness of strain hardening zone to the thickness of alloyed zone *Gpl/Gla*

does not ensure the presence of compressive stresses in the entire alloyed zone.

the burnishing tool and the laser beam.

at a fixed feed rate *Vf* = 360 mm/min.

mm/min.

$$\mathbf{G}\_{\rm pl}/\mathbf{G}\_{\rm la} = 0.00035 \text{ V}\_{\rm rev} 1^{02} \text{ V}\_{\rm l}^{0.2} \text{ X}^{-1.09} \tag{3}$$

Table 4 contains the value of multiple correlation coefficients *R*, the value of the Fisher's number *F* and *T*-Student coefficient describing the significance of subsequent independent variables *T1, T2, T3*, and *T4*.


Table 4. The regression function for the thickness of the plastic deformation zone, Gpl, change in microhardness ,ΔHV, and ratio of thickness of the plastic deformation zones to thickness of alloyed zone Gpl/Gla of hybrid treated steel 304.

Multiple correlation coefficients of the equations are high and the relation between the studied properties is significant. For all equations, the condition *F > Fkr* is fulfilled. For the first and third equations all the factors are significant *t > tkr* at confidence level α = 0.1. Only in the case of the function 2 which shows the relation of the microhardness increase the factor *T2* describing influence of feed rate is insignificant for the assumed level of confidence.

Figure 8 shows the graphical interpretation of the thickness of strain hardening depending on the rotational speed of the burnishing head and the distance between the tool from the axis of the laser beam for fixed feed rate, *Vf* = 360 mm/s, according to the relation 1, Table 4. The strain hardening thickness almost linearly increases with speed growing and decreases with increasing distance from the axis. With increasing of the head rotational speed the intensity of the burnishing process and the impact forces of micro-hammers on machined surface increases. It induced an enlargement of the plastic deformation depth of the material in the entire range of temperatures applied in the burnishing. The effect of temperature on the depth of the plastic deformation zone is stronger for raised values of the impact forces and higher intensity of the burnishing process.

Fig. 8. Effect of rotational speed and the distance between laser beam and the head on the thickness plastic deformation zone at fixed feed rate, *Vf* = 360 mm/min.

Table 4 contains the value of multiple correlation coefficients *R*, the value of the Fisher's number *F* and *T*-Student coefficient describing the significance of subsequent independent

> Relation R F T1 T2 T3 1 0.78 8.4 3.15 1.5 3.59 2 0.68 4.6 2.65 0.85 2.45 3 0.74 6.3 2.81 1.46 2.97

Table 4. The regression function for the thickness of the plastic deformation zone, Gpl, change in microhardness ,ΔHV, and ratio of thickness of the plastic deformation zones to

describing influence of feed rate is insignificant for the assumed level of confidence.

Multiple correlation coefficients of the equations are high and the relation between the studied properties is significant. For all equations, the condition *F > Fkr* is fulfilled. For the first and third equations all the factors are significant *t > tkr* at confidence level α = 0.1. Only in the case of the function 2 which shows the relation of the microhardness increase the factor *T2*

Figure 8 shows the graphical interpretation of the thickness of strain hardening depending on the rotational speed of the burnishing head and the distance between the tool from the axis of the laser beam for fixed feed rate, *Vf* = 360 mm/s, according to the relation 1, Table 4. The strain hardening thickness almost linearly increases with speed growing and decreases with increasing distance from the axis. With increasing of the head rotational speed the intensity of the burnishing process and the impact forces of micro-hammers on machined surface increases. It induced an enlargement of the plastic deformation depth of the material in the entire range of temperatures applied in the burnishing. The effect of temperature on the depth of the plastic deformation zone is stronger for raised values of the impact forces

Fig. 8. Effect of rotational speed and the distance between laser beam and the head on the

thickness plastic deformation zone at fixed feed rate, *Vf* = 360 mm/min.

0.2 X -1.09 (3)

Gpl/Gla = 0.00035 Vrev1.02 Vf

thickness of alloyed zone Gpl/Gla of hybrid treated steel 304.

and higher intensity of the burnishing process.

variables *T1, T2, T3*, and *T4*.

Figure 9 shows the relation between the increase in material microhardness at the depth of 0.2 mm and the rotational speed of the head, and the distance from the tool axis to the laser beam at constant feed rate 360 mm/min. The microhardness growth, characterizing the degree of strain hardening at the surface, depends mainly on the rotational speed of the head and the temperature of the process zone, which is a function of the distance between the burnishing tool and the laser beam.

Fig. 9. Dependence of microhardness increase at a depth of 0.2 mm on rotational speed of the head and distance between the tool axis *X* and the laser beam, for fixed feed rate 360 mm/min.

Figure 10 shows the influence of the rotational speed of the head and the distance of the tool from surface, at a fixed feed rate *Vf* = 360 mm/min, on the *Gpl/Gla* parameter. It is evident that at the highest speed and the distances of less than 9 mm the ratio is bigger than 1. At speeds below 4000 rev/min and the distance of less than 8 mm the thickness of the strainhardening zone in relation to the thickness of the alloyed layer is less than 0.5, although it does not ensure the presence of compressive stresses in the entire alloyed zone.

Fig. 10. Influence of rotational speed and the distance between the beam axis and the tools on the ratio of the thickness of strain hardening zone to the thickness of alloyed zone *Gpl/Gla* at a fixed feed rate *Vf* = 360 mm/min.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 267


0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 mm

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 mm

Profile # 1 / 11 Pt = 33.6 µm Scale = 100 µm

Profile # 1 / 16 Pt = 38 µm Scale = 100 µm

Fig. 11. The surface topography views of the laser path and section of the central part of path, and perpendicular and parallel profiles to the laser path and: a – laser alloyed surface,

b – after hybrid treatment.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 mm

Length = 4.66 mm Pt = 255 µm Scale = 400 µm µm

µm


µm


0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 mm

Length = 3.23 mm Pt = 168 µm Scale = 300 µm µm

a

a

b

#### **4. Surface roughness after hybrid treatment**

#### **4.1 Surface topography**

The study showed that of the laser alloying combined with the burnishing can provide reduction of surface roughness in relation to the heights obtained in the laser melting process. The shape of the melted zone has improved as well. In the case of profile parameters cut-off 0.8 was used for all surfaces. In Figure 11a the examples of the surface topography views of the laser path and section of the central part of the path are shown. Below the profiles perpendicular and parallel to the direction of movement are visible. Figure 11b presents geometrical features of hybrid treated surface. Table 5 shows the 3D parameters *Sa, Sz* , their changes compared to the parameters after laser alloying *Sa(la)/Sa(h), Sz(la)/Sz(h)* and the 2D roughness parameters *Ra, RSm* of the surface which underwent hybrid treatment.

A change in shape of the melted zone due to the burnishing is noticeable. The heights of asperities at the alloying zone boundary decreased significantly, while the shape of asperities in the central area of the melting underwent "flattening" in comparison with only alloyed material.


Table 5. Surface topography and roughness parameters after the hybrid treatment.

The study showed that of the laser alloying combined with the burnishing can provide reduction of surface roughness in relation to the heights obtained in the laser melting process. The shape of the melted zone has improved as well. In the case of profile parameters cut-off 0.8 was used for all surfaces. In Figure 11a the examples of the surface topography views of the laser path and section of the central part of the path are shown. Below the profiles perpendicular and parallel to the direction of movement are visible. Figure 11b presents geometrical features of hybrid treated surface. Table 5 shows the 3D parameters *Sa, Sz* , their changes compared to the parameters after laser alloying *Sa(la)/Sa(h), Sz(la)/Sz(h)* and the 2D

A change in shape of the melted zone due to the burnishing is noticeable. The heights of asperities at the alloying zone boundary decreased significantly, while the shape of asperities in the central area of the melting underwent "flattening" in comparison with only

Process parameters 3D topography parameters 2D roughness

[µm] Sa(la)/Sa(h) S z(la) /S z(h)

Sz

1 4200 230 6 8.16 60.6 2,70 1,85 2.68 0.195 2 5950 230 6 11.1 87.6 2,00 1,28 4.83 0.201 3 4200 570 6 8.64 81.2 3,00 1,60 4.36 0.203 4 5950 570 6 10.1 77.6 2,56 1,68 3.95 0.19 5 4200 230 8.5 8.67 61.6 2,54 1,82 1.62 0.251 6 5950 230 8.5 11.4 73 1,95 1,53 1.79 0.242 7 4200 570 8.5 7.61 57.6 3,40 2,26 1.67 0.226 8 5950 570 8.5 8.4 52.8 3,08 2,46 1.87 0.282 9 3500 360 7 6.16 44.6 3,90 2,80 1.71 0.231 10 7100 360 7 9.01 66.4 2,66 1,88 2.9 0.25 11 5000 150 7 13.3 82.4 2,49 1,61 3.58 0.238 12 5000 900 7 9.93 80.2 3,55 1,90 4.44 0.207 13 5000 360 5 10.6 86.4 2,26 1,45 4.3 0.213 14 5000 360 10 7.96 51.2 3,02 2,44 0.87 0.28 15 5000 360 7 9.44 67.8 2,54 1,84 2.22 0.221 16 5000 360 7 12.1 68.6 1,98 1,82 2.44 0.219 17 5000 360 7 14.8 95 1,62 1,32 2.93 0.235 18 5000 360 7 5.97 45.6 4,02 2,74 2.03 0.234 19 5000 360 7 5.81 49.6 4,13 2,52 1.69 0.212 20 5000 360 7 7.84 57.6 3,06 2,17 2.24 0.245

parameters

RSm [mm]

Ra [µm]

roughness parameters *Ra, RSm* of the surface which underwent hybrid treatment.

Sa [µm]

Table 5. Surface topography and roughness parameters after the hybrid treatment.

**4. Surface roughness after hybrid treatment** 

**4.1 Surface topography** 

alloyed material.

Vrev [rev/min]

Vf [mm/min]

X [mm]

No

Fig. 11. The surface topography views of the laser path and section of the central part of path, and perpendicular and parallel profiles to the laser path and: a – laser alloyed surface, b – after hybrid treatment.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 269

When analyzing the influence of process parameters on roughness sampling, *RSm,* a good correlation was also stated; *R=0.82, F=11.3.*Polynomial function, for which all the coefficients

 RSm = 0.085-0.000004 Vrev+ 0.00002 Vf + 0.017 X (5) With an increase in the rotational speed of the head the roughness spacing decreases, while the increasing distance between the head and the laser beam and in the feed rate, causes lowering of *RSm*. Analysis of the equation shows that the rise of temperature of the burnishing process affects the spatial property of surface geometrical structure increasing

The analysis of the parameter *Sz(la)/Sz(h)* allows the assessment of the degree of plastic

 Sz(la)/Sz(h) = 3.26+0.00039Vrev-0.00026 Vf-0.29 X (6) The biggest change in the asperity magnitude is obtained using the highest rotational speeds of the burnishing head, small feed rates and small *X* distances. At these parameters the large

The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the parameters *Sz(la)/Sz(h),* for constant value of feed speed

Fig. 13. The influence of rotational speed of the head, *Vrev*, and the *X* distance on changes of

The erosive tests were performed for sample 15 after laser–mechanical treatment carried at parameters presented in Table 4. It was stated that the increase of hardness compared to only laser alloyed Stellite layer was 70% while depth of plastic deformation was about 0.5

the surface topography parameter *Sz(la)/Sz(h)* for fixed feed rate *Vf* =360 mm/min.

forces of micro-hammer impact on surface and high temperature of treatment occur.

of the equation are significant at the confidence level of 0.1, shows good fit.

deformation of surface asperities due to the micro-hammering.

the distance between micro-asperities.

*Vf* =360 mm/min, is shown in Figure 13.

**5. Surface layer properties** 

**5.1 Wear resistance** 

#### **4.2 Correlations between roughness and treatment parameters**

The analysis of the influence of process parameters (*X, Vf, Vrev*) on roughness parameters *Ra, RSm* and changes on topography parameters *Sz/Szh* was done, a good correlation was also stated.

The following relation between *Ra* and the treatment parameters was found:

$$\text{Ra} = 6 + 0.00037 \text{V}\_{\text{rev}} + 0.0017 \text{V}\_{\text{I}} \text{-} 0.76 \text{X} \tag{4}$$

Multiple correlation coefficients, describing the relation between *Ra* and the parameters of hybrid treatment are high *R=0.83*. The dependence between studied properties is significant: *F=12.1, F > Fkr*. All coefficients are significant at the accepted level of confidence. Figure 3 shows a graphical interpretation of polynomial function that describes the relation between *Ra* and the rotational speed of the burnishing head, *Vrev*, and the distance between the tool axle and the axis of the laser beam, *X*, for fixed feed rate *Vf* = 360 mm/s. The parameter *Ra* grows with increasing, *Vrev*, *Vf,* and it lowers with the *X* distance increasing. With rise of the rotational speed of head, *Vrev,* the intensity of the burnishing process, and the forces of impact of micro-hammers on machined surface, grow. The increase in feed rate reduces temperature and intensity of the process of burnishing that means the number of microhammer strokes per unit area, leading to roughness asperity enlargement. The increasing of the *X* distance causes temperature reduces in the zone of mechanical treatment. This is also associated with the lowering of plastic properties of material, smaller plastic deformations of surface asperities and increase of the height parameter, *Ra*. high temperature of treatment occur.

The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the selected surface topography parameters, *Ra* for constant value of feed speed *Vf* =360 mm/min, are shown in Figure 12.

Fig. 12. The influence of the rotational speed of the head, *Vrev*, and the distance between tool axle and laser beam axis, *X*, on roughness parameter *Ra* for fixed feed rate *Vf* = 360 mm/min.

When analyzing the influence of process parameters on roughness sampling, *RSm,* a good correlation was also stated; *R=0.82, F=11.3.*Polynomial function, for which all the coefficients of the equation are significant at the confidence level of 0.1, shows good fit.

$$\text{RSm} = 0.085 \text{-0.000004 V}\_{\text{rev}} + 0.00002 \text{ V}\_{\text{f}} + 0.017 \text{ X} \tag{5}$$

With an increase in the rotational speed of the head the roughness spacing decreases, while the increasing distance between the head and the laser beam and in the feed rate, causes lowering of *RSm*. Analysis of the equation shows that the rise of temperature of the burnishing process affects the spatial property of surface geometrical structure increasing the distance between micro-asperities.

The analysis of the parameter *Sz(la)/Sz(h)* allows the assessment of the degree of plastic deformation of surface asperities due to the micro-hammering.

$$\text{Sz}\_{\text{(lb)}/\text{Sz}\_{\text{(lb)}}} = 3.26 \pm 0.00039 \text{V}\_{\text{rev}} \text{-0.00026 V} \text{-0.29 X} \tag{6}$$

The biggest change in the asperity magnitude is obtained using the highest rotational speeds of the burnishing head, small feed rates and small *X* distances. At these parameters the large forces of micro-hammer impact on surface and high temperature of treatment occur.

The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the parameters *Sz(la)/Sz(h),* for constant value of feed speed *Vf* =360 mm/min, is shown in Figure 13.

Fig. 13. The influence of rotational speed of the head, *Vrev*, and the *X* distance on changes of the surface topography parameter *Sz(la)/Sz(h)* for fixed feed rate *Vf* =360 mm/min.

#### **5. Surface layer properties**

#### **5.1 Wear resistance**

268 CO2 Laser – Optimisation and Application

The analysis of the influence of process parameters (*X, Vf, Vrev*) on roughness parameters *Ra, RSm* and changes on topography parameters *Sz/Szh* was done, a good correlation was also

 Ra = 6+0.00037Vrev + 0.0017Vf - 0.76X (4) Multiple correlation coefficients, describing the relation between *Ra* and the parameters of hybrid treatment are high *R=0.83*. The dependence between studied properties is significant: *F=12.1, F > Fkr*. All coefficients are significant at the accepted level of confidence. Figure 3 shows a graphical interpretation of polynomial function that describes the relation between *Ra* and the rotational speed of the burnishing head, *Vrev*, and the distance between the tool axle and the axis of the laser beam, *X*, for fixed feed rate *Vf* = 360 mm/s. The parameter *Ra* grows with increasing, *Vrev*, *Vf,* and it lowers with the *X* distance increasing. With rise of the rotational speed of head, *Vrev,* the intensity of the burnishing process, and the forces of impact of micro-hammers on machined surface, grow. The increase in feed rate reduces temperature and intensity of the process of burnishing that means the number of microhammer strokes per unit area, leading to roughness asperity enlargement. The increasing of the *X* distance causes temperature reduces in the zone of mechanical treatment. This is also associated with the lowering of plastic properties of material, smaller plastic deformations of surface asperities and increase of the height parameter, *Ra*. high temperature of treatment

The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the selected surface topography parameters, *Ra* for constant

Fig. 12. The influence of the rotational speed of the head, *Vrev*, and the distance between tool

axle and laser beam axis, *X*, on roughness parameter *Ra* for fixed feed rate

value of feed speed *Vf* =360 mm/min, are shown in Figure 12.

**4.2 Correlations between roughness and treatment parameters** 

The following relation between *Ra* and the treatment parameters was found:

stated.

occur.

*Vf* = 360 mm/min.

The erosive tests were performed for sample 15 after laser–mechanical treatment carried at parameters presented in Table 4. It was stated that the increase of hardness compared to only laser alloyed Stellite layer was 70% while depth of plastic deformation was about 0.5

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 271

The results of contact strain and contact stiffness are shown in Table. The analysis of the total value of elastic and plastic strain shows that the surfaces after the burnishing undergo significantly lower elastic and plastic deformation in contact with counter sample. For samples without grinding this effect is associated primarily with surface topography. In the process of burnishing the rebuilding of surface topography occurs: surface irregularities have smaller heights, while their shapes become preferable. Therefore, the real contact area is increased and the unit pressure on individual asperities is reduced. At nominal pressure of 270 MPa the plastic deformation is 88 μm for samples that were only alloyed, whereas the burnished surface is deformed by 52 μm in height. It is the effect of more profitable

Grinding process has provided similar geometric structure: the surface heights and the shape of asperities are similar for all samples. In this case, examining the process of deformation of the surface under the influence of contact pressure, the differences in elastic

> a2 [µm]

Laser alloying & grinding 40 28 12 6.75 Hybrid & grinding 24 14 10 11.25 Plasma sprayed 57 36 27 4.74 Plasma sprayed & grinding 40 28 12 7.11 Table 7. The contact strain and the contact stiffness for nominal pressures 270 MPa. The hybrid treatment parameters: *P*=2 kW, *d*=3mm *Vf* =360 mm/min, *Vrev*=5000 rev/min,

The proposed new type of hybrid treatment, using the head for dynamic burnishing and applying the oscillatory motion, has lead to large plastic deformation of surface layer of steel 304 alloyed with Stellite 6. Application of the new head with two rows of hammers highly intensified the process and enabled the burnishing at various temperatures in one operation. The forces of the micro-hammer impact were changing in a wide range. The introduction of the oscillations increased the width of the melted zone by the value of the oscillation amplitude, provided more uniform surface plastic deformation, and also allowed for obtained favourable surface topography. The design of the head allowed the treatment in high as well as low temperature in the single pass. The temperature of the burnishing was around 850-200 K, depending on treatment parameters. The studies of residual stresses have shown the temperature in the zone of plastic deformation is sufficient for transforming the tensile stresses into compressive once at the material surface even at the highest temperature

Laser alloying 138 90 88 3.38 Hybrid 80 28 52 1.96

Contact strain at nominal pressure 270 MPa

> a2e [µm]

a2pl

Contact stiffness

[µm] q=270 [MPa]

geometric structure and the strengthening of surface layer due to the burnishing.

and plastic properties of the surface layer material can be established.

Treatment

*X*=7 mm.

**6. Discussions** 

of the burnishing.

**5.2 Contact stiffness** 

mm. The tests were done also for the plasma sprayed Stellite 6 layer and the layer formed by laser alloying at feed rate 360 mm/min.

Fig. 14. The profiles after erosive wear test: PS-plasma sprayed Stellite 6 layer, LA- laser alloyed layer (*P*=2 kW, *d*=3 mm *Vf* =360 mm/min), Hybrid - sample 13 after laser-mechanical treatment (*P*=2 kW, *d*=2.5 mm *Vf* =360 mm/min, *Vrev* = 5000 rev/min, *X* = 5 mm).

Figure 5 shows exemplary surface profiles after erosive wear after laser-mechanical treatment, laser alloying and plasma sprayed Stellite 6 layer. The eroded surface is visible in the middle. The non-eroded side material parts were used as the reference surface. The profiles of examined surfaces show significant differences in erosive depth. The maximal depth was smaller than the thickness of tested layers. The measurement of surface topography was performed on three selected areas and the medium depth and volume per 1 mm2 was chosen for quantitative analysis. This allowed compare erosive wear resistance of different surface layers of Stellite 6. Table 6 contains the average value of depth and volume of eroded surfaces whereas Figure 5 presents average eroded materials volumes loss.

Among Stellite 6 layers produced by plasma spraying, laser alloying and laser alloying combined with burnishing; the surfaces after hybrid treatment had the highest resistance to slurry erosive wear. The examined surfaces after laser-mechanical treatment showed the average depth of erosive loss 16.2µm. Surfaces after laser alloying showed little less of erosive resistance, 13-30 %, than hybrid treated surfaces. Their erosive depth was 22 µm.

The largest erosive loss was stated for plasma sprayed Stellite 6 layer, for which the average depth was 82 µm. Its erosive resistance is five times worse than the one of hybrid treated layer.


Table 6. The average depth and volume of erosive losses of Stellite 6 layer after slurry erosive test

#### **5.2 Contact stiffness**

270 CO2 Laser – Optimisation and Application

mm. The tests were done also for the plasma sprayed Stellite 6 layer and the layer formed by

Fig. 14. The profiles after erosive wear test: PS-plasma sprayed Stellite 6 layer, LA- laser alloyed layer (*P*=2 kW, *d*=3 mm *Vf* =360 mm/min), Hybrid - sample 13 after laser-mechanical

of eroded surfaces whereas Figure 5 presents average eroded materials volumes loss.

Among Stellite 6 layers produced by plasma spraying, laser alloying and laser alloying combined with burnishing; the surfaces after hybrid treatment had the highest resistance to slurry erosive wear. The examined surfaces after laser-mechanical treatment showed the average depth of erosive loss 16.2µm. Surfaces after laser alloying showed little less of erosive resistance, 13-30 %, than hybrid treated surfaces. Their erosive depth was 22 µm.

The largest erosive loss was stated for plasma sprayed Stellite 6 layer, for which the average depth was 82 µm. Its erosive resistance is five times worse than the one of hybrid treated

[µm]

Laser alloying 22.2 0.0992 Hybrid treatment 16.2 0.0699 Plasma spraying 81.9 0.3621

Table 6. The average depth and volume of erosive losses of Stellite 6 layer after slurry

The volume of loss [mm3]

Treatment The depth of erosive loss

Figure 5 shows exemplary surface profiles after erosive wear after laser-mechanical treatment, laser alloying and plasma sprayed Stellite 6 layer. The eroded surface is visible in the middle. The non-eroded side material parts were used as the reference surface. The profiles of examined surfaces show significant differences in erosive depth. The maximal depth was smaller than the thickness of tested layers. The measurement of surface topography was performed on three selected areas and the medium depth and volume per 1 mm2 was chosen for quantitative analysis. This allowed compare erosive wear resistance of different surface layers of Stellite 6. Table 6 contains the average value of depth and volume

treatment (*P*=2 kW, *d*=2.5 mm *Vf* =360 mm/min, *Vrev* = 5000 rev/min, *X* = 5 mm).

laser alloying at feed rate 360 mm/min.

layer.

erosive test

The results of contact strain and contact stiffness are shown in Table. The analysis of the total value of elastic and plastic strain shows that the surfaces after the burnishing undergo significantly lower elastic and plastic deformation in contact with counter sample. For samples without grinding this effect is associated primarily with surface topography. In the process of burnishing the rebuilding of surface topography occurs: surface irregularities have smaller heights, while their shapes become preferable. Therefore, the real contact area is increased and the unit pressure on individual asperities is reduced. At nominal pressure of 270 MPa the plastic deformation is 88 μm for samples that were only alloyed, whereas the burnished surface is deformed by 52 μm in height. It is the effect of more profitable geometric structure and the strengthening of surface layer due to the burnishing.

Grinding process has provided similar geometric structure: the surface heights and the shape of asperities are similar for all samples. In this case, examining the process of deformation of the surface under the influence of contact pressure, the differences in elastic and plastic properties of the surface layer material can be established.


Table 7. The contact strain and the contact stiffness for nominal pressures 270 MPa. The hybrid treatment parameters: *P*=2 kW, *d*=3mm *Vf* =360 mm/min, *Vrev*=5000 rev/min, *X*=7 mm.

#### **6. Discussions**

The proposed new type of hybrid treatment, using the head for dynamic burnishing and applying the oscillatory motion, has lead to large plastic deformation of surface layer of steel 304 alloyed with Stellite 6. Application of the new head with two rows of hammers highly intensified the process and enabled the burnishing at various temperatures in one operation. The forces of the micro-hammer impact were changing in a wide range. The introduction of the oscillations increased the width of the melted zone by the value of the oscillation amplitude, provided more uniform surface plastic deformation, and also allowed for obtained favourable surface topography. The design of the head allowed the treatment in high as well as low temperature in the single pass. The temperature of the burnishing was around 850-200 K, depending on treatment parameters. The studies of residual stresses have shown the temperature in the zone of plastic deformation is sufficient for transforming the tensile stresses into compressive once at the material surface even at the highest temperature of the burnishing.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 273

3. The decisive influence on the thickness of the plastic deformation zone has temperature of the material in the region of burnishing. Thickness from 0.25 to 1.2 mm of the strain hardened zone was determined depending on the applied parameters of hybrid treatment. It enables the use of the hybrid treatment for the majority of layers produced

4. The residual stress measurements showed the change in stresses within the melting zone from tensile stresses after the laser alloying to compressive ones after the hybrid

5. The hybrid treatment causes an increase in surface smoothness compared to the laser alloying. More than a threefold decrease in the average height of roughness, *Sa*, due to

6. The increase of the contact stiffness in relation to the laser alloying and the Stellite 6

7. The better slurry erosive wear resistance than for the plasma sprayed layer and the

8. The study of the correlation of treatment parameters with the state of surface layer showed that the dependences between the investigated properties are significant. Therefore, the controlling the hybrid treatment is possible and its industrial

Abbas, G., West, D.R. (1991). Laser Surface Cladding of Stellite and Stellite-SiC Composite

Anthony, T.R., Cline, H.E. (1977). Surface Rippling Induced by Surface-Tension Gradients

Arutunjan R,W., Baranow W.Yu., Bolszow L.A. Majuta D.D., Sebrant A.Yu.,(1989). *Laser beam effects on materials,* Nauka, ISBN -5-02-000747-X*,* Moscow*,* Russia De Hosson, J. Th. M., Noordhuis J. (1989). Surface Modification by Means of Laser Melting

Demkin, M.B. (1959). A device for measuring the deformation at the point contact of two

Filipowski, R. (1996). Application of matrix calculus for determining the coefficients of the

Ignatiev, M., Kovalev, E., Melekhin, I., Sumurov, I., Surlese, S. (1993). Investigation of the

Meijer, J. (2004). Laser Beam Machining (LBM), state of the art and new opportunities. *J of* 

*Materials Processing Technology*, Vol. 149, pp. 2-17, ISSN 0924-0136

surfaces under compression. *Bulletyn Izobretanii,* Vol. 19, pp. 15-19

Deposits for Enhanced Hardness and Wear. *Wear*, Vol. 143, pp. 87-95, ISSN 0043-

During Laser Surface Melting and Alloying. *J Appl. Phys.,* Vol. 48, pp. 1265-1272,

Combined with Shot Peening. *Material Science and Engineering,* Vol. A121, pp. 1211-

linear regression for varying degrees of a matrix describing the set of normal equations. *The Archive of Mechanical Engineering,* Vol. 43, pp. 5-17, ISSN 0137-4478 Grum, J. Sturm, R. (2004). A new experimental technique for measuring strin and residual

stresses during a laser remelting process. *J of Materials Processing Technology*, Vol.

hardening of titanium alloy by laser nitriding. *Wear,* Vol. 166, pp. 233-236, ISSN

layer, formed by plasma spraying after hybrid treatment was stated.

lower erosive rate compared to the laser alloyed layer were recognised.

by LBM.

treatment.

**8. References** 

1648

ISSN 1089-7550

1220, ISSN 0267-0836

0043-1648

147, pp. 351-358, ISSN 0924-0136

the burnishing process, was observed.

applications are recommended.

The results indicate that due to the burnishing at high temperature large plastic deformation of surface layer is possible to be obtain, without cracks and other defects of loosen structure that are characteristic for the classical burnishing of hard and brittle materials.

Good correlation between the process parameters and the features of plastic deformation zones and surface roughness was found. It enables controlling of the hybrid treatment. The variable degree of plastic deformation, strain hardening and thickness of plastic deformation zone can be governed by the controlling the impact force of micro-hammers on the surface and the temperature of metal in the treated zone. The application of high temperatures lowers the hardness while the plasticity of the material undergoes increasing, which in turn provides greater degree of plastic deformation and strain hardening of material. The hardness, depth of strain hardening zone increased, and residual stresses changed. Further increase of plastic deformation is possible to be obtained by the use of higher forces of micro-hammer impact on the surface. For the tested material the strain hardening, 32-100%, at the surface was obtained. Despite high degree of deformation there were no cracks no spallings. Fine grain material, homogeneous chemical and phase composition, was found.

The thickness of the strain hardened zone varying from 0.25 to 1.2 mm was obtained. This range is similar to the thickness of the typical mostly produced by LBM alloyed and cladding layers. It was found that with proper selection of process parameters it is possible to obtain the depth of strain hardening zone greater than the depth of the melting. This ensures the presence of compressive stress across the alloyed layer and provides greater durability, especially of parts subjected to fatigue during operation life.

The results of measurement of surface topography, contact stiffness and slurry erosive wear showed that laser-mechanical treatment allows the attainment better properties than the plasma sprayed and laser alloyed Stellite 6 layers have, and it can be used in specific industrial applications.

On the basis of regression equations and well-known effect of temperature on plastic properties of the material, it is possible to select parameters of the hybrid treatment in order to obtain the expected degree and thickness of the plastic deformation zone for other materials and layers. The hybrid treatment enables to combine in a single operation the advantages of laser treatment in the form of preferred microstructure, good adhesion, beneficial chemical composition of the surface layer and burnishing treatment. It ensures the increase of material hardness, improved surface topography and favourable compressive stresses of formed layer.

#### **7. Conclusion**


#### **8. References**

272 CO2 Laser – Optimisation and Application

The results indicate that due to the burnishing at high temperature large plastic deformation of surface layer is possible to be obtain, without cracks and other defects of loosen structure

Good correlation between the process parameters and the features of plastic deformation zones and surface roughness was found. It enables controlling of the hybrid treatment. The variable degree of plastic deformation, strain hardening and thickness of plastic deformation zone can be governed by the controlling the impact force of micro-hammers on the surface and the temperature of metal in the treated zone. The application of high temperatures lowers the hardness while the plasticity of the material undergoes increasing, which in turn provides greater degree of plastic deformation and strain hardening of material. The hardness, depth of strain hardening zone increased, and residual stresses changed. Further increase of plastic deformation is possible to be obtained by the use of higher forces of micro-hammer impact on the surface. For the tested material the strain hardening, 32-100%, at the surface was obtained. Despite high degree of deformation there were no cracks no spallings. Fine grain material, homogeneous chemical and phase

The thickness of the strain hardened zone varying from 0.25 to 1.2 mm was obtained. This range is similar to the thickness of the typical mostly produced by LBM alloyed and cladding layers. It was found that with proper selection of process parameters it is possible to obtain the depth of strain hardening zone greater than the depth of the melting. This ensures the presence of compressive stress across the alloyed layer and provides greater

The results of measurement of surface topography, contact stiffness and slurry erosive wear showed that laser-mechanical treatment allows the attainment better properties than the plasma sprayed and laser alloyed Stellite 6 layers have, and it can be used in specific

On the basis of regression equations and well-known effect of temperature on plastic properties of the material, it is possible to select parameters of the hybrid treatment in order to obtain the expected degree and thickness of the plastic deformation zone for other materials and layers. The hybrid treatment enables to combine in a single operation the advantages of laser treatment in the form of preferred microstructure, good adhesion, beneficial chemical composition of the surface layer and burnishing treatment. It ensures the increase of material hardness, improved surface topography and favourable compressive

1. The hybrid treatment with the new dynamic burnishing head provides the extended range of plastic deformation of surface layer of steel 304, alloyed with Stellite 6. The increase in microhardness, caused by surface strain hardening, was 32-100% depending on the impact forces of micro-hammers on the surface and the temperature of the metal

2. Due to the treatment at high temperature, despite the high degree of plastic deformation, no cracking and spallings or other phenomena proving loosen

durability, especially of parts subjected to fatigue during operation life.

that are characteristic for the classical burnishing of hard and brittle materials.

composition, was found.

industrial applications.

stresses of formed layer.

in the treated zone.

microstructure of the material were recognized.

**7. Conclusion** 


**10** 

*Brazil* 

**Covering with Carbon Black** 

G. Vasconcelos1, D. C. Chagas1 and A. N. Dias2

*1Institute of Advanced Studies - IEAv, EFO-L, S. J. dos Campos, SP, 2University of Vale do Paraíba - UNIVAP, S. J. dos Campos, SP* 

**of AISI 4340 Steel** 

**and Thermal Treatment by CO2 Laser Surfaces** 

The application of photo-absorbing coatings is a common practice, especially when lasers of low density power are used. These materials normally MoS2, graphite and carbon black, futher the coupling of incident radiation, reducing the losses by reflection, common to the

In a previous work, using graphite coatings, it was observed that part of the coating, after irradiation, remained on the metal surface. In pin on disc tests, it was observed a reduction in the coefficient of friction surface with this coating. REIS, J. L., (2009) improvement on the surface hardness, even using laser low of energy density power. This hardenning process was attributed to better coupling in the region of beam interaction with the metal surface. The laser hardening consists in heating and rapid cooling the steel surface. If the power density is enough, a layer on the steel surface will reach the austenitizing temperature (during heating) and then with rapid cooling, place the formation of martensites (Ganeev, R. A., 2002). The depth of the surface treated is determined by the law of thermal conductivity, where the propagation of heat occurs in a region of higher temperature to a region of lower temperature (Benedeck, J.; Shachrai, A.; Levin, L., 1980). The laser hardening allows the hardening of specific areas with controlled depth and with minimal surface deformation when compared to other methods. It also promotes, improves the mechanical properties and fatigue resistance, attraction, wear (reducing the friction factor) and increased resistance to corrosion (Dohotre, N. B., 1998; Machado, I. F., 2006). This work will evaluate the use of carbon black to replace the graphite used in the work of REIS, J. L. 2009, to eliminate the stage of solution preparation, grinding mills at high

The steel used in this work is AISI 4340. Its chemical composition was assessed by the optical spectrometer Thermo Scientific, Model ARL 3460 OES Metals Analyzer, presented in

**1. Introduction** 

energy.

Table 1.

**2. Methodology** 

process, when CO2 lasers are used as radiation source.


### **Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel**

G. Vasconcelos1, D. C. Chagas1 and A. N. Dias2 *1Institute of Advanced Studies - IEAv, EFO-L, S. J. dos Campos, SP, 2University of Vale do Paraíba - UNIVAP, S. J. dos Campos, SP Brazil* 

#### **1. Introduction**

274 CO2 Laser – Optimisation and Application

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Tsai, C.-H. , Ou, C.-H. (2004). Machining a smooth surface ceramic material by laser fracture

Tian, Y., Shin, Y.C. (2007). Laser-assisted burnishing of metals. *Int. J of Machine Tools and* 

*Material Processing Technology*, Vol. 205, pp. 249-58, ISSN 0924-0136

*Manufacture,* Vol. 47(1), pp. 14-22, ISSN 0890-6955

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Vol. A208, pp. 143-147, ISSN 0921-5093

Przybylski, W. (1986). *Burnishing technology*, ISBN 83-204-0742-7, WNT, Warsaw, Poland Radziejewska J., Kalita W., Bartoszewicz A. Modification of surface layer properties by laser

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alloying combined with burnishing. *Proceedings of Laser Technologies in Welding and Materials Processing*, pp. 162-164, ISBN 966-8872-01-0 Katsiveli Crimea, Ukraine,

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simultaneously laser alloyed and burnished steel. *J of Materials Processing* 

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of residual stresses in laser melted Ti-6Al-V alloy, *Material Science and Engineering,*

ball-burnishing process. *J of Materials Processing Technology*, Vol. 140, pp. 248–254,

using ball grinding, ball burnishing and polishing process on machine centre. *J of* 

machining technique*. Materials Processing Technology,* Vol. 155–156, pp. 1797–1804,

The application of photo-absorbing coatings is a common practice, especially when lasers of low density power are used. These materials normally MoS2, graphite and carbon black, futher the coupling of incident radiation, reducing the losses by reflection, common to the process, when CO2 lasers are used as radiation source.

In a previous work, using graphite coatings, it was observed that part of the coating, after irradiation, remained on the metal surface. In pin on disc tests, it was observed a reduction in the coefficient of friction surface with this coating. REIS, J. L., (2009) improvement on the surface hardness, even using laser low of energy density power. This hardenning process was attributed to better coupling in the region of beam interaction with the metal surface. The laser hardening consists in heating and rapid cooling the steel surface. If the power density is enough, a layer on the steel surface will reach the austenitizing temperature (during heating) and then with rapid cooling, place the formation of martensites (Ganeev, R. A., 2002). The depth of the surface treated is determined by the law of thermal conductivity, where the propagation of heat occurs in a region of higher temperature to a region of lower temperature (Benedeck, J.; Shachrai, A.; Levin, L., 1980). The laser hardening allows the hardening of specific areas with controlled depth and with minimal surface deformation when compared to other methods. It also promotes, improves the mechanical properties and fatigue resistance, attraction, wear (reducing the friction factor) and increased resistance to corrosion (Dohotre, N. B., 1998; Machado, I. F., 2006). This work will evaluate the use of carbon black to replace the graphite used in the work of REIS, J. L. 2009, to eliminate the stage of solution preparation, grinding mills at high energy.

#### **2. Methodology**

The steel used in this work is AISI 4340. Its chemical composition was assessed by the optical spectrometer Thermo Scientific, Model ARL 3460 OES Metals Analyzer, presented in Table 1.

Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 277

oxidation. Figure 3 shows the laser used and Figure 4, the diagram of the experimental set

Fig. 2. Histogram of particle size distribution of carbon black.

Fig. 3. Experimental set-up. CO2 laser. Highlighted in red rectangular box, located

up of the treatment process.

galvanometric mirrors.


Table 1. Chemical composition of steel AISI 4340 -% mass

Carbon black is formed by fine particles obtained by the process of pyrolysis or partial combustion of hydrocarbon gases or liquids. These nano-particulate structure, favors the coating with thin layers (Sector Report N 09, 1998). The shape of the particles was observed by scanning electron microscopy (SEM), Zeiss / EVO MA10, as shown in Figure 1.

Fig. 1. SEM of particles of carbon black (1320X)

The particle size of the lubricant can influence the thickness of the coating deposited and after irradiation with the laser beam. In order to determine the size distribution of particles, carbon black was subjected to particle size analysis through testing by laser diffraction (CILAS 1064L, range from 0.04 to 500μm). The results of this analysis are presented in Figure 2.

Samples of AISI 4340 steel with a thickness of 3mm and 20mm diameter, previously sanded (SiC paper 600), were coated with a solution prepared with 10g of carbon black and 0.1g of carboxilmetilcelulose in 100ml of ethanol.

This solution was mechanically mixed for 20 minutes in a plastic container with metal balls to the homogenization of the solution. Subsequently, the solution was sprayed with a pneumatic pistol on the surface of steel samples previously heated to 60°C. Then the samples are irradiated with a beam of CO2 laser (50W) and beam diameter of 300μm. In the region of action beam on the sample surfaces, we used a flow of nitrogen to prevent

Carbon black is formed by fine particles obtained by the process of pyrolysis or partial combustion of hydrocarbon gases or liquids. These nano-particulate structure, favors the coating with thin layers (Sector Report N 09, 1998). The shape of the particles was observed

The particle size of the lubricant can influence the thickness of the coating deposited and after irradiation with the laser beam. In order to determine the size distribution of particles, carbon black was subjected to particle size analysis through testing by laser diffraction (CILAS 1064L, range from 0.04 to 500μm). The results of this analysis are presented in

Samples of AISI 4340 steel with a thickness of 3mm and 20mm diameter, previously sanded (SiC paper 600), were coated with a solution prepared with 10g of carbon black and 0.1g of

This solution was mechanically mixed for 20 minutes in a plastic container with metal balls to the homogenization of the solution. Subsequently, the solution was sprayed with a pneumatic pistol on the surface of steel samples previously heated to 60°C. Then the samples are irradiated with a beam of CO2 laser (50W) and beam diameter of 300μm. In the region of action beam on the sample surfaces, we used a flow of nitrogen to prevent

by scanning electron microscopy (SEM), Zeiss / EVO MA10, as shown in Figure 1.

Table 1. Chemical composition of steel AISI 4340 -% mass

Fig. 1. SEM of particles of carbon black (1320X)

carboxilmetilcelulose in 100ml of ethanol.

*Steel 4340*

Figure 2.

*Fe C Mn Si Cr Ni Mo P S* 

95.79 0.361 0.638 0.261 0.794 1.702 0.221 0.024 0.008

oxidation. Figure 3 shows the laser used and Figure 4, the diagram of the experimental set up of the treatment process.

Fig. 2. Histogram of particle size distribution of carbon black.

Fig. 3. Experimental set-up. CO2 laser. Highlighted in red rectangular box, located galvanometric mirrors.

Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 279

The irradiated samples according to Table 2 were selected one that showed lower surface ablation and greater extension of the layer treated. These parameters were evaluated by measurements of roughness and optical microscopy (OM), respectively. Figure 6 shows a

Fig. 6. Cross section of the irradiated surface of the sample P2. Optical microscopy a) OM-

The average microhardness of AISI 4340 steel without heat treatment is 286 HV0,05. After the thermal treatment with CO2 laser, we can observe that the hardness of the material increased significantly, reaching an average of about 760 HV0,05. Figure 7 shows the microhardness profile of the cross section of the treated region. These results were obtained

**A B** 

Fig. 7. Microhardness profile of cross section of the treated region after treatment via laser-

According to Figure 7, there is increased hardness, occurred due to surface hardening

**3. Results and discussions** 

200X, b) OM-500X.

coated carbon black.

cross section of the treated region of the sample P2.

by means of microhardness Future-Tech / FM-700.

process, resulting in heating and cooling of the sample.

Fig. 4. Fitting of the experimental process. The laser beam is guided by a set of mirrors galvanometric controlled by software

The speed of scanning laser beam (mm/s), the resolution in pulses per inch (ppp) and number of heating cycles (NC) to be used in this experiment were selected from tests previously conducted (Chagas, D. C.; et al 2010). The Table 2 shows the parameters of the laser beam used in the treatment of the samples.



Fig. 5. Illustrates the layout of the treatment process, with laser beam, the samples previously coated with carbon black.

#### **3. Results and discussions**

278 CO2 Laser – Optimisation and Application

Fig. 4. Fitting of the experimental process. The laser beam is guided by a set of mirrors

**Samples Speed (mm/s) Resolution** 

Fig. 5. Illustrates the layout of the treatment process, with laser beam, the samples

**Hardened layer** 

Table 2. Parameters of laser used for surface hardening of AISI 4340.

**Coating of carbon black**

The speed of scanning laser beam (mm/s), the resolution in pulses per inch (ppp) and number of heating cycles (NC) to be used in this experiment were selected from tests previously conducted (Chagas, D. C.; et al 2010). The Table 2 shows the parameters of the

> P1 40 300 5 P2 60 300 5 P3 80 300 5

**(ppp)** 

**Steel AlSi 4340** 

**Laser beam** 

**Steel AlSi 4340** 

**Number of cicles** 

galvanometric controlled by software

previously coated with carbon black.

laser beam used in the treatment of the samples.

The irradiated samples according to Table 2 were selected one that showed lower surface ablation and greater extension of the layer treated. These parameters were evaluated by measurements of roughness and optical microscopy (OM), respectively. Figure 6 shows a cross section of the treated region of the sample P2.

Fig. 6. Cross section of the irradiated surface of the sample P2. Optical microscopy a) OM-200X, b) OM-500X.

The average microhardness of AISI 4340 steel without heat treatment is 286 HV0,05. After the thermal treatment with CO2 laser, we can observe that the hardness of the material increased significantly, reaching an average of about 760 HV0,05. Figure 7 shows the microhardness profile of the cross section of the treated region. These results were obtained by means of microhardness Future-Tech / FM-700.

Fig. 7. Microhardness profile of cross section of the treated region after treatment via lasercoated carbon black.

According to Figure 7, there is increased hardness, occurred due to surface hardening process, resulting in heating and cooling of the sample.

Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 281

The tests of atomic force microscopy (AFM) were performed to evaluate the morphology and surface topography. The results obtained by AFM indicate a possible crystallization of carbon black, coalescence of grains and the appearance of new phases from the process of

> 559.76 [nm]

0.00

2.00 um 5.00 x 5.00 um IEAv600\_5

Fig. 10. AFM image obtained by the carbon black surface after irradiation with lasers. The bright spots in the figure correspond to higher regions and the darker the lower regions,

In roughness tests obtained by AFM, there is heterogeneous across regions with different surface roughness, where in the region obtained a RA= 87.999nm and region B was obtained

The experiments conducted indicate that the use of carbon black is feasible, presenting results of microhardness, friction coefficient, tempera and extent of the treated layer, similar to results reported by REIS (2009), and also eliminates the grinding step, which is required

The use of nanoparticles of carbon black aids in the absorption of radiation incident on steel, influences the alteration of microstructure and promotes surface temperature of the surface

With the different laser parameters presented, it appears that high rates of speed in the beam of laser irradiation, the steel AISI 4340, showed no significant change in hardness, where the increase in hardness was 268 HV0.05 to 405 HV0.05. Unlike the lowest parameters that showed a homogeneous microstructure and with greater depths of layers treated, increasing by up to three times the surface hardness of steel, where the increase in hardness was 268 HV0.05 to 760 HV0.05. The extent of the treated layer, the homogeneity of the steel

In tribological test, it was observed that the uncoated sample has a higher coefficient of friction of 0.70 and the samples coated with carbon black and further treated with lasers,

and its microstructure can be controlled by varying the parameters of the laser.

according to the scale shown on the right side of the figure.

and still attached to the steel surface acting as a lubricant.

RA= 124.115nm.

**4. Conclusion** 

when using graphite.

heating by laser, as shown in Figure 10.

The sample was subjected to tribological tests to evaluate the friction coefficient. The parameters used in the test were: linear velocity of 10cm/s, the track radius of 5mm, 52100 steel balls with 6mm diameter, number of rounds equal to 2000 and load of 5N. In this test, the sample is supported on a support rotation and pressed with a steel ball with known load, as shows the scheme of Figure 8.

Fig. 8. Schematic drawing of the tribological tests

Then, the sample is rotated to evaluate the friction and the results obtained from the tribological tests, are presented in Figure 9.

Fig. 9. Results of the friction coefficients of steel ball (52100) in 4340 and uncoated.

The tests of atomic force microscopy (AFM) were performed to evaluate the morphology and surface topography. The results obtained by AFM indicate a possible crystallization of carbon black, coalescence of grains and the appearance of new phases from the process of heating by laser, as shown in Figure 10.

Fig. 10. AFM image obtained by the carbon black surface after irradiation with lasers. The bright spots in the figure correspond to higher regions and the darker the lower regions, according to the scale shown on the right side of the figure.

In roughness tests obtained by AFM, there is heterogeneous across regions with different surface roughness, where in the region obtained a RA= 87.999nm and region B was obtained RA= 124.115nm.

#### **4. Conclusion**

280 CO2 Laser – Optimisation and Application

The sample was subjected to tribological tests to evaluate the friction coefficient. The parameters used in the test were: linear velocity of 10cm/s, the track radius of 5mm, 52100 steel balls with 6mm diameter, number of rounds equal to 2000 and load of 5N. In this test, the sample is supported on a support rotation and pressed with a steel ball with known

Then, the sample is rotated to evaluate the friction and the results obtained from the

Fig. 9. Results of the friction coefficients of steel ball (52100) in 4340 and uncoated.

load, as shows the scheme of Figure 8.

Fig. 8. Schematic drawing of the tribological tests

tribological tests, are presented in Figure 9.

The experiments conducted indicate that the use of carbon black is feasible, presenting results of microhardness, friction coefficient, tempera and extent of the treated layer, similar to results reported by REIS (2009), and also eliminates the grinding step, which is required when using graphite.

The use of nanoparticles of carbon black aids in the absorption of radiation incident on steel, influences the alteration of microstructure and promotes surface temperature of the surface and still attached to the steel surface acting as a lubricant.

With the different laser parameters presented, it appears that high rates of speed in the beam of laser irradiation, the steel AISI 4340, showed no significant change in hardness, where the increase in hardness was 268 HV0.05 to 405 HV0.05. Unlike the lowest parameters that showed a homogeneous microstructure and with greater depths of layers treated, increasing by up to three times the surface hardness of steel, where the increase in hardness was 268 HV0.05 to 760 HV0.05. The extent of the treated layer, the homogeneity of the steel and its microstructure can be controlled by varying the parameters of the laser.

In tribological test, it was observed that the uncoated sample has a higher coefficient of friction of 0.70 and the samples coated with carbon black and further treated with lasers,

**11** 

*France* 

Afia Kouadri-David

**Welding of Thin Light Alloys Sheets** 

**by CO2 Laser Beam: Magnesium Alloys** 

*PSM Team, European University of Brittany, France, INSA of Rennes, LGCGM* 

Laser welding is an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics and other industries. In this document the research and progress in laser welding of magnesium alloys are critically reviewed from different perspectives. Some important laser processing parameters and their effects on weld quality are discussed. The microstructure, metallurgical defects and mechanical properties encountered in laser welding of magnesium alloys, such as porosity, grains size, crystallographic texture and loss of alloying elements are described. Mechanical properties of welds such as hardness, residual stresses and other important structural properties are discussed. The aim of the chapter is to review the recent progress in laser

Laser Beam Welding (LBW) consists in the laser beam focalisation on the workpiece surface. The high power density then created, induces metal ionisation and then plasma is formed. The vaporisation of the surface progressively forms a depression in the workpiece and then a keyhole, which allows the laser energy in-depth absorption. The melted metal will progressively fill the keyhole during the laser displacement, to form the weld. The two laser sources available are CO2 and Nd: YAG. Laser CO2 consists in a mixture of CO2, N2 and noble gases. The nitrogen discharges in CO2 molecules activate the laser emission. The Nd: YAG (neodymium-doped yttrium aluminium garnet) consists in Nd3+ ions inserted in YAG crystal, the excitation is supplied by laser diodes. Nd: YAG laser light (λ = 1.06 μm) has a much higher absorption degree than CO2 laser light (λ=10.6 μm). Both CO2 and Nd:YAG lasers operate in the infrared region of the electromagnetic radiation spectrum, invisible to the human eye. The Nd:YAG provides its primary light output in the near-infrared, at a wavelength of 1.06 microns. This wavelength is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 percent for most metals. The nearinfrared radiation permits the use of standard optics to achieve focused spot sizes as small as .001" in diameter. On the other hand, the far infrared (10.6 micron) output wavelength of the CO2 laser has an initial reflectance of about 80 percent to 90 percent for most metals and requires special optics to focus the beam to a minimum spot size of .003" to .004" diam. However, whereas Nd:YAG lasers might produce power outputs up to 500 watts, CO2

welding of magnesium alloys and to provide a basis for follow-on research.

**2. General principle of laser beam welding** 

**1***.* **Introduction** 

have coefficient of friction of the order of 0.20, favoring better properties mechanical equipment and increase the service life.

In the trial by AFM showed that the coating presents heterogeneity throughout the area, with variations in surface roughness in different regions, possible crystallization of carbon black, coalescence of grains and the appearance of new phases resulting from via laser heating process.

#### **5. Acknowledgement**

Thanks to CNPq by financial support, the Group DEDALO-IEAv, Dr. J. R. Martinelli of the IPEN-USP and to Mr. A. Zanatta of the CCM-ITA.

#### **6. References**


### **Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys**

Afia Kouadri-David *PSM Team, European University of Brittany, France, INSA of Rennes, LGCGM France* 

#### **1***.* **Introduction**

282 CO2 Laser – Optimisation and Application

have coefficient of friction of the order of 0.20, favoring better properties mechanical

In the trial by AFM showed that the coating presents heterogeneity throughout the area, with variations in surface roughness in different regions, possible crystallization of carbon black, coalescence of grains and the appearance of new phases resulting from via laser

Thanks to CNPq by financial support, the Group DEDALO-IEAv, Dr. J. R. Martinelli of the

Benedeck, J.; Shachrai, A.; Levin, L., 1980, Case hardening of steel by a CO2 laser beam,

Chagas, D. C.; Dias, A. N.; Vasconcelos, G.; Antunes, E. F., 2010, Surface treatment and

Dohotre, N. B., 1998, Lasers in Surface Engineering: Surface Engineering Series, Volume 1, ASM International – The Materials Information Society, Chapter 1 and 3. Ganeev, R. A., 2002, Low-power laser hardening of steels, Journal of Materials Processing

Machado, I. F., 2006, Technological advances in steels heat treatment, Journal of Materials

Reis, J. L., 2009, Thermal treatment of AISI M2 Steel by CO2 Laser, 104f. Master Dissertation

in Physics and Chemistry, Aeronautics Institute of Technology, São José dos

covering by carbon black AISI 4340 by CO2 laser action, ISSN 1983-1544. Ativ.P&D

equipment and increase the service life.

IPEN-USP and to Mr. A. Zanatta of the CCM-ITA.

Optics and Laser Technology. October.

Processing Technology, 172, 160-173.

Sector Report N 09, 1998, Chemical Complex, Carbon Black, BNDES.

heating process.

**6. References** 

**5. Acknowledgement** 

IEAv, v.3, p.84.

Tech., 121, 414-419.

Campos – SP.

Laser welding is an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics and other industries. In this document the research and progress in laser welding of magnesium alloys are critically reviewed from different perspectives. Some important laser processing parameters and their effects on weld quality are discussed. The microstructure, metallurgical defects and mechanical properties encountered in laser welding of magnesium alloys, such as porosity, grains size, crystallographic texture and loss of alloying elements are described. Mechanical properties of welds such as hardness, residual stresses and other important structural properties are discussed. The aim of the chapter is to review the recent progress in laser welding of magnesium alloys and to provide a basis for follow-on research.

#### **2. General principle of laser beam welding**

Laser Beam Welding (LBW) consists in the laser beam focalisation on the workpiece surface. The high power density then created, induces metal ionisation and then plasma is formed. The vaporisation of the surface progressively forms a depression in the workpiece and then a keyhole, which allows the laser energy in-depth absorption. The melted metal will progressively fill the keyhole during the laser displacement, to form the weld. The two laser sources available are CO2 and Nd: YAG. Laser CO2 consists in a mixture of CO2, N2 and noble gases. The nitrogen discharges in CO2 molecules activate the laser emission. The Nd: YAG (neodymium-doped yttrium aluminium garnet) consists in Nd3+ ions inserted in YAG crystal, the excitation is supplied by laser diodes. Nd: YAG laser light (λ = 1.06 μm) has a much higher absorption degree than CO2 laser light (λ=10.6 μm). Both CO2 and Nd:YAG lasers operate in the infrared region of the electromagnetic radiation spectrum, invisible to the human eye. The Nd:YAG provides its primary light output in the near-infrared, at a wavelength of 1.06 microns. This wavelength is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 percent for most metals. The nearinfrared radiation permits the use of standard optics to achieve focused spot sizes as small as .001" in diameter. On the other hand, the far infrared (10.6 micron) output wavelength of the CO2 laser has an initial reflectance of about 80 percent to 90 percent for most metals and requires special optics to focus the beam to a minimum spot size of .003" to .004" diam. However, whereas Nd:YAG lasers might produce power outputs up to 500 watts, CO2

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 285

Then, when laser power was too low, lack of penetration was observed whereas high laser power produced laser cutting. These observations are consistent with the LBW literature. From macrostructure point of view, many authors observed the same evolutions where at low beam powers, some chevronlike pattern which is also called "ripples" (Marya et al., 2001). The mechanism of ripples formation is related to the effects of surface tension on the weld pool during solidification (D'Annessa, 1970 as cited in Cao et al., 2006). They also observed that the thickness in the weld area was slightly higher, a phenomenon which is called "crowning" or "humping". High beam powers led to deep and wide beads, and reduce both ripples and crowning (Marya et al., 2001). However, others authors showed at high beam powers, spatters and the evaporative losses would be produced. Authors (Weisheit et al., 1997) investigated the laser parameters for several magnesium alloys. They reported that for thin AZ31 plates (1, 8 mm), a 1.5 kW beam power was sufficient for achieving full penetration. In our experiences, for achieving full penetration, 3 mm AZ91

The penetration depth and weld width both increase linearly with decreasing welding speed and decrease with increasing welding speed. The obtained results in the literature and reported on the figure 2A and 2B confirm the effects of welding speed on penetration depth and weld width at different levels of power for CO2 lasers: The penetration depth and

Fig. 2. Influence of welding speed on the penetration depth (A) and on the weld width (B)

**(A) Welding speed (mm/s) (B) Welding speed (mm/s)** 

Moreover, it was reported that the speed lead too either to improve or to decrease the weld quality, in particular by the formation of the defects such as cracking or the pores formation. Indeed, at low speeds, the interaction time between molten metal and surrounding air is large enough to allow pores to nucleate in large quantity, grow and escape from the molten pool as a result of buoyancy and convection flow. Moreover, when the welding speed is too slow, the bead produced by the superfluous heat exchange will extend to the side, and the

plates welds were produced at 4 kW (Kouadri & Barrallier, 2006).

weld width both decrease linearly with increasing welding speed.

for WE43 magnesium alloy (Dharhi et al., 2001a, 2001b)

**Penetration depth (mm) Bead width (mm)** 

**3.2 LBW welding speed, V (mm/min)** 

systems can easily supply 10,000 watts and greater. The two laser processes are differentiated by the fabrication and the shape of the beam. It is generally accepted that the heat input parameter, defined as the ratio of beam power to beam travel speed, is well suited for describing LBW process. However, our results and those of the literature show that this parameter was not convenient, and that, the effect of the laser power and the welding speed parameters have to be differentiated, in particular for the light alloys such as magnesium or aluminium alloys. As a result of these broad differences, the two laser types are usually employed for different applications. The powerful CO2 lasers overcome the high reflectance by keyholing, wherein the absorption approaches blackbody. The reflectivity of the metal is only important until the keyhole weld begins. Once the material's surface at the point of focus approaches its melting point, the reflectivity drops within microseconds.

#### **3. Optimisation and influence of CO2 laser beam parameters on thin sheets**

The present investigation is concerned with laser power, welding speed, defocusing distance and type of shielding gas and their effects on the fusion zone shape and final solidification structure of magnesium alloys.

#### **3.1 Laser power, P (kW)**

The laser power is a critical parameter to obtain a full penetration depth and to control the weld bead profile. High power density at the workpiece is crucial to achieve keyhole welding and to control the formation of welds. Studies realized in this domain showed this effect of laser power on the penetration depth and weld width. The increasing beam power led to deeper and wider beads. Figure 1 shows the effect of laser power on the penetration depth (Fig. 1A) and weld width (Fig. 1B) for WE43 alloy welded at a speed of 33 mm/s and a focused diameter of 0.25mm (Dharhi et al., 2001a, 2001b as cited in Cao et al., 2006).

Fig. 1. Influence of laser power on the penetration depth (A) and on the weld width (B) for WE43 alloy (Dharhi et al., 2001a, 2001b)

The penetration depth and weld width increased with increasing laser power due to higher power density. Our experiences showed too the same evolution: the weld width becomes larger with increasing laser power. For example the threshold power to achieve full penetration is 2,5 kW (i.e. a power density of 2 MW/cm2) for 3 mm AZ91 plates welded at a speed of 4 m/min and a focused diameter of 0.25 mm.

Then, when laser power was too low, lack of penetration was observed whereas high laser power produced laser cutting. These observations are consistent with the LBW literature. From macrostructure point of view, many authors observed the same evolutions where at low beam powers, some chevronlike pattern which is also called "ripples" (Marya et al., 2001). The mechanism of ripples formation is related to the effects of surface tension on the weld pool during solidification (D'Annessa, 1970 as cited in Cao et al., 2006). They also observed that the thickness in the weld area was slightly higher, a phenomenon which is called "crowning" or "humping". High beam powers led to deep and wide beads, and reduce both ripples and crowning (Marya et al., 2001). However, others authors showed at high beam powers, spatters and the evaporative losses would be produced. Authors (Weisheit et al., 1997) investigated the laser parameters for several magnesium alloys. They reported that for thin AZ31 plates (1, 8 mm), a 1.5 kW beam power was sufficient for achieving full penetration. In our experiences, for achieving full penetration, 3 mm AZ91 plates welds were produced at 4 kW (Kouadri & Barrallier, 2006).

#### **3.2 LBW welding speed, V (mm/min)**

284 CO2 Laser – Optimisation and Application

systems can easily supply 10,000 watts and greater. The two laser processes are differentiated by the fabrication and the shape of the beam. It is generally accepted that the heat input parameter, defined as the ratio of beam power to beam travel speed, is well suited for describing LBW process. However, our results and those of the literature show that this parameter was not convenient, and that, the effect of the laser power and the welding speed parameters have to be differentiated, in particular for the light alloys such as magnesium or aluminium alloys. As a result of these broad differences, the two laser types are usually employed for different applications. The powerful CO2 lasers overcome the high reflectance by keyholing, wherein the absorption approaches blackbody. The reflectivity of the metal is only important until the keyhole weld begins. Once the material's surface at the point of focus approaches its melting point, the reflectivity drops within microseconds.

**3. Optimisation and influence of CO2 laser beam parameters on thin sheets**  The present investigation is concerned with laser power, welding speed, defocusing distance and type of shielding gas and their effects on the fusion zone shape and final

The laser power is a critical parameter to obtain a full penetration depth and to control the weld bead profile. High power density at the workpiece is crucial to achieve keyhole welding and to control the formation of welds. Studies realized in this domain showed this effect of laser power on the penetration depth and weld width. The increasing beam power led to deeper and wider beads. Figure 1 shows the effect of laser power on the penetration depth (Fig. 1A) and weld width (Fig. 1B) for WE43 alloy welded at a speed of 33 mm/s and

a focused diameter of 0.25mm (Dharhi et al., 2001a, 2001b as cited in Cao et al., 2006).

Fig. 1. Influence of laser power on the penetration depth (A) and on the weld width (B) for

**Laser power (kW) Laser power (kW)** 

**Bead width (mm)** 

The penetration depth and weld width increased with increasing laser power due to higher power density. Our experiences showed too the same evolution: the weld width becomes larger with increasing laser power. For example the threshold power to achieve full penetration is 2,5 kW (i.e. a power density of 2 MW/cm2) for 3 mm AZ91 plates welded at a

solidification structure of magnesium alloys.

WE43 alloy (Dharhi et al., 2001a, 2001b)

speed of 4 m/min and a focused diameter of 0.25 mm.

**(A) (B)**

**3.1 Laser power, P (kW)** 

**Penetration depth (mm)** 

The penetration depth and weld width both increase linearly with decreasing welding speed and decrease with increasing welding speed. The obtained results in the literature and reported on the figure 2A and 2B confirm the effects of welding speed on penetration depth and weld width at different levels of power for CO2 lasers: The penetration depth and weld width both decrease linearly with increasing welding speed.

Fig. 2. Influence of welding speed on the penetration depth (A) and on the weld width (B) for WE43 magnesium alloy (Dharhi et al., 2001a, 2001b)

Moreover, it was reported that the speed lead too either to improve or to decrease the weld quality, in particular by the formation of the defects such as cracking or the pores formation.

Indeed, at low speeds, the interaction time between molten metal and surrounding air is large enough to allow pores to nucleate in large quantity, grow and escape from the molten pool as a result of buoyancy and convection flow. Moreover, when the welding speed is too slow, the bead produced by the superfluous heat exchange will extend to the side, and the

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 287

and lack of penetration when the power density was too low. Concerning LBW, increasing the laser power (P) and decreasing the welding speed (V) result in an increase of the power

This is also a very important technical parameter, because in a certain output power, it will decide the density of beam power which is the key factor for laser weld. But for the laser beam with high power, it is difficult to measure, what is produced by the nature of the beam diameter. For laser weld, the condition of high effective deep penetration weld is that the power density on the laser focus must exceed 106 W/cm2. We can adopt two methods to enhance the power density, one is to enhance the laser power, and the other one is to reduce the diameter of the beam. The power density has linear relation with the laser power, and has inverse-square ratio relation with beam diameter, so the effect of reducing beam diameter is better. In ours experiments, to realize full deep penetration weld of 2mm thick

For the sake of simplicity, the focal distance is defined as the distance between the focal point and the top surface of the sample. The position of focal points has an important influence on welding process and quality. The focal plane should be set where the maximum penetration depths or best process tolerances are produced. The laser welding usually needs some focus distance, because too high power density of the beam center at the laser focus is easy to vaporize and become bores. When the focus distance reduces to a certain value, the melting depth will suddenly change, which will establish necessary conditions for producing penetration pores. These most results in this domain showed that the focus distance influences not only the laser beam on the weld piece surface, but also the incidence direction of beam, so it has important influences to the melting depth and seam

In our experiences, the most acceptable weld profile was obtained at defocusing distance of – 0.2 mm for 3 mm thickness where weld bead depth / width ratio is maximum and fusion zone size is minimum. In order to obtain the optimum value, complete penetration butt welds were made using previously obtained optimum laser power (4 kW) and optimum welding speed (3 m/min) (Kouadri & Barrallier, 2006). Sound welds were achieved with a focal point on the surface, which is consistent with what we found for thin plates. The weld width increases with moving the focal point away from the surface (i.e. increasing focal distance) which was also observed by other authors. These results indicated that the most effective range of defocusing distance to get maximum penetration with acceptable weld

However, this distance has to be adjusted to obtain the best quality of welding. For example, the optimum defocusing distance to attain acceptable weld profile for 5 mm thickness was 0.4 mm under the surface of the workpiece (Cao et al., 2006). These results were consistent with the literature study (Dharhi et al., 2000, 2001, 2002 as cited in Cao et al., 2006). They studied 1–5kW CO2 laser welding of 2mm AZ91 and 4mm WE43-T6 alloys. Their results showed that an adequate weld could be obtained for a focal position on or 1mm under the

density. This tendency is consistent with all the previous studies on laser welding.

plates, we choose beam diameter of 1 mm with 4 kW CO2 at 3 m/min.

**3.4 Beam diameter** 

**3.5 LBW focal distance, f (mm)** 

profile lies between zero and – 1 mm.

shape.

heat influenced area will become too heat and extended, the seam metallographic structure crystal becomes thick, sometimes the cracking will appear, which will seriously influence the welding quality. When the welding speed achieves the lower limitation, the superfluous power absorption also will induce local evaporation loss and hollows. Moreover, using lower welding speeds induced no real change in the penetration depth but wider the weld width and especially the heat affected zone (HAZ) (Marya & Edwards, 2000).

From pores formation point of view, at high speeds, the pores do not have enough time to nucleate. The influence of the welding speed on pore formation was studied (Marya & Edwards, 2001). They found that the pore fraction goes to a maximum with increasing welding speed. The obtained results showed that using higher welding speeds reduced ripples but greatly increased crowning phenomena (Marya & Edwards, 2001), and the fusion zone appeared to be far more brittle (Watkins, 2003 as cited in Cao et al., 2006). Moreover, they showed a dependency between crowning and pores content, so that crowning is actually a relevant parameter to assess the weld quality. When welding speed was too high, lack of penetration was observed, whereas low welding speed produced laser cutting. This one is explained by the fact the power density increases with decreasing welding speed (Dharhi et al., 2000 as cited in Cao et al., 2006).

However these observations must be readjusted because the results depend on the nature of used magnesium alloys. Indeed, though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different metallurgical and thermophysical properties. For example, die cast AZ91D has a lower thermal conductivity of 51 W/m K as compared with 139 W/m K for wrought AZ21A alloy. Thus, for similar welding parameters, the AZ91D alloy has a higher weld depth and weld volume compared with AZ21A alloy. It was also reported that greater penetration depth could be reached in AM50 alloy compared with AZ91 alloy welded under similar conditions using a 6kW CO2 laser (Marya & Edwards, 2001). These observations explain why it is needed to systematically investigate the laser-welding characteristics of different magnesium alloys because of the difference in their thermal properties. In the same way, it is needed to take account of the geometry and thickness of the plates for readjusting the speed of welding. Many authors reported that a welding speed of 2.5-3 m/min was suitable for thin plates, when using 1.5 kW laser beam. Therefore, welding speed above 3 m/min should be achievable during CO2 laser welding of 2 mm thick plates (Weisheit et al., 1997).

#### **3.3 LBW density (W/cm2 )**

It's the one more important parameter: the power density is one of most pivotal parameters in laser weld. When the laser power density is lower than 106 W/cm2, the laser weld belongs to category of heat exchange weld. When the laser power density achieves only 106 W/cm2, the deep penetration weld can be formed and "keyhole effects" appears.

The "keyhole effects" is closely correlative to the laser power density which is more low, the "keyhole effects" is more unstable even can not be formed, and the melting pool is also small. The melting depth of laser weld is directly correlative to the laser output power density and which is the function of incidence beam power and beam diameter. Therefore, to enhance the power density, we can enhance laser power or decrease the laser speed. A good balance had to be found to avoid laser cutting when the power density was too high and lack of penetration when the power density was too low. Concerning LBW, increasing the laser power (P) and decreasing the welding speed (V) result in an increase of the power density. This tendency is consistent with all the previous studies on laser welding.

#### **3.4 Beam diameter**

286 CO2 Laser – Optimisation and Application

heat influenced area will become too heat and extended, the seam metallographic structure crystal becomes thick, sometimes the cracking will appear, which will seriously influence the welding quality. When the welding speed achieves the lower limitation, the superfluous power absorption also will induce local evaporation loss and hollows. Moreover, using lower welding speeds induced no real change in the penetration depth but wider the weld

From pores formation point of view, at high speeds, the pores do not have enough time to nucleate. The influence of the welding speed on pore formation was studied (Marya & Edwards, 2001). They found that the pore fraction goes to a maximum with increasing welding speed. The obtained results showed that using higher welding speeds reduced ripples but greatly increased crowning phenomena (Marya & Edwards, 2001), and the fusion zone appeared to be far more brittle (Watkins, 2003 as cited in Cao et al., 2006). Moreover, they showed a dependency between crowning and pores content, so that crowning is actually a relevant parameter to assess the weld quality. When welding speed was too high, lack of penetration was observed, whereas low welding speed produced laser cutting. This one is explained by the fact the power density increases with decreasing

However these observations must be readjusted because the results depend on the nature of used magnesium alloys. Indeed, though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different metallurgical and thermophysical properties. For example, die cast AZ91D has a lower thermal conductivity of 51 W/m K as compared with 139 W/m K for wrought AZ21A alloy. Thus, for similar welding parameters, the AZ91D alloy has a higher weld depth and weld volume compared with AZ21A alloy. It was also reported that greater penetration depth could be reached in AM50 alloy compared with AZ91 alloy welded under similar conditions using a 6kW CO2 laser (Marya & Edwards, 2001). These observations explain why it is needed to systematically investigate the laser-welding characteristics of different magnesium alloys because of the difference in their thermal properties. In the same way, it is needed to take account of the geometry and thickness of the plates for readjusting the speed of welding. Many authors reported that a welding speed of 2.5-3 m/min was suitable for thin plates, when using 1.5 kW laser beam. Therefore, welding speed above 3 m/min should be

achievable during CO2 laser welding of 2 mm thick plates (Weisheit et al., 1997).

the deep penetration weld can be formed and "keyhole effects" appears.

It's the one more important parameter: the power density is one of most pivotal parameters in laser weld. When the laser power density is lower than 106 W/cm2, the laser weld belongs to category of heat exchange weld. When the laser power density achieves only 106 W/cm2,

The "keyhole effects" is closely correlative to the laser power density which is more low, the "keyhole effects" is more unstable even can not be formed, and the melting pool is also small. The melting depth of laser weld is directly correlative to the laser output power density and which is the function of incidence beam power and beam diameter. Therefore, to enhance the power density, we can enhance laser power or decrease the laser speed. A good balance had to be found to avoid laser cutting when the power density was too high

width and especially the heat affected zone (HAZ) (Marya & Edwards, 2000).

welding speed (Dharhi et al., 2000 as cited in Cao et al., 2006).

**3.3 LBW density (W/cm2**

**)** 

This is also a very important technical parameter, because in a certain output power, it will decide the density of beam power which is the key factor for laser weld. But for the laser beam with high power, it is difficult to measure, what is produced by the nature of the beam diameter. For laser weld, the condition of high effective deep penetration weld is that the power density on the laser focus must exceed 106 W/cm2. We can adopt two methods to enhance the power density, one is to enhance the laser power, and the other one is to reduce the diameter of the beam. The power density has linear relation with the laser power, and has inverse-square ratio relation with beam diameter, so the effect of reducing beam diameter is better. In ours experiments, to realize full deep penetration weld of 2mm thick plates, we choose beam diameter of 1 mm with 4 kW CO2 at 3 m/min.

#### **3.5 LBW focal distance, f (mm)**

For the sake of simplicity, the focal distance is defined as the distance between the focal point and the top surface of the sample. The position of focal points has an important influence on welding process and quality. The focal plane should be set where the maximum penetration depths or best process tolerances are produced. The laser welding usually needs some focus distance, because too high power density of the beam center at the laser focus is easy to vaporize and become bores. When the focus distance reduces to a certain value, the melting depth will suddenly change, which will establish necessary conditions for producing penetration pores. These most results in this domain showed that the focus distance influences not only the laser beam on the weld piece surface, but also the incidence direction of beam, so it has important influences to the melting depth and seam shape.

In our experiences, the most acceptable weld profile was obtained at defocusing distance of – 0.2 mm for 3 mm thickness where weld bead depth / width ratio is maximum and fusion zone size is minimum. In order to obtain the optimum value, complete penetration butt welds were made using previously obtained optimum laser power (4 kW) and optimum welding speed (3 m/min) (Kouadri & Barrallier, 2006). Sound welds were achieved with a focal point on the surface, which is consistent with what we found for thin plates. The weld width increases with moving the focal point away from the surface (i.e. increasing focal distance) which was also observed by other authors. These results indicated that the most effective range of defocusing distance to get maximum penetration with acceptable weld profile lies between zero and – 1 mm.

However, this distance has to be adjusted to obtain the best quality of welding. For example, the optimum defocusing distance to attain acceptable weld profile for 5 mm thickness was 0.4 mm under the surface of the workpiece (Cao et al., 2006). These results were consistent with the literature study (Dharhi et al., 2000, 2001, 2002 as cited in Cao et al., 2006). They studied 1–5kW CO2 laser welding of 2mm AZ91 and 4mm WE43-T6 alloys. Their results showed that an adequate weld could be obtained for a focal position on or 1mm under the

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 289

from oxidation, combined with helium back shielding and nitrogen shielding to protect the optics. Wang et al., 2006 studied the influence of gas flow rate on weld width and reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation.

As magnesium is highly susceptible to oxidation, a protective atmosphere is required during welding. Surface cracking leading to laser welding was observed without gas protection. This is due to the oxide formation during welding in the magnesium alloys. To increase the magnesium alloy weldability, argon or helium are the most common choices. Argon is heavier than air so it provides a better shield than helium, but it ionizes easily and has much lower thermal conductivity than helium. This causes a problem with high power CO2 welding: The metal vapour emerging from the keyhole is partially ionized, with charged atoms and free electrons. The free electrons absorb some of the laser light, reducing the power available for welding. As the vapor absorbs energy, it heats up, increasing the number of free elections and further increasing absorption. Helium shield gas is more effective than argon in suppressing this effect because it cools the vapor plume and does not contribute many electrons itself. This welding gas often plays an active role in the welding process, such as increasing the welding speed and improving the mechanical properties of the joint. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases

In addition, often to increase the weld quality, helium/argon mixtures combining the benefits of both gases, i.e. the higher density of argon and the higher ionization potential of helium, may be used to obtain better protection of the weld zone in CO2 laser welding. Hiraga et al., 2001 studied 1.7 mm thick AZ31B-H24 butt joints and get some improvements using argon back shielding in addition to the helium centre shielding. With these two gases, weld profile is remarkably improved where fusion zone interfaces are almost parallel to each other. The melting depth increases with the increase of gas flux, but too much gas flux will induce the surface hollow even penetration of the melting pool. Indeed, higher porosity content was observed for He gas flow higher than 50 l/min. Using Ar back shielding gas allowed us to produce sound welds at lower welding speed, reducing sag of the weld pool. Our study led to the same conclusions and sound welds were produced (Kouadri & Barrallier, 2006). Then, the optimum shielding system consists in a top helium flow superior to 20 l/min and Ar back shielding. By adding single-sided access, laser welding is even

**4. Application of laser beam CO2 on thin sheets of magnesium alloy** 

The presented material is a cast magnesium alloy (AZ91D) welded by laser CO2 processing. The alloy used for the study of the laser welding is a ternary magnesium - aluminium - zinc of designation AZ91, according to standard ASTM. Laser welding of magnesium alloys appears to be a challenge itself. Indeed, the ability to produce laser welds depends on the properties of the material to be welded. Then, magnesium being characterised by quite unfavourable properties (i.e. low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coefficient of thermal expansion, low melting temperature, wide solidification temperature range, high solidification shrinkage, a tendency to form low melting-point constituents, low viscosity, low surface tensions, high solubility for hydrogen

and confirmed that a helium gas flow was the best choice.

in the liquid state), processing is expected to be an issue.

more strategically advantageous.

surface of the workpiece. Focal position on the workpiece surface had the smallest weld width while the weld width became larger when the focal position deviated above or below the surface.

Then, the optimal focal point position to weld thin plates lies on the top surface of the workpiece. Indeed, Weisheit et al., (1997, 1998) investigated 2.5kW CO2 laser welding of some magnesium alloys. For thin plates (2.5 and 3 mm), the best welds according to penetration depth, aspect ratio and sag were achieved when the focal point was adjusted on the surface of workpiece, whereas for thick plates (5 and 8 mm) a position of 2mm below the surface of workpiece proved to be the best. Thus, the focal position should be moved deeper into the material for thicker work pieces and the following used process. Lehner et al., 1999 further researched the tolerance of focal position. For 3mm AZ91 and AM50 die castings welded using a 3 kW Nd:YAG laser, the best focal position is approximately 0.8mm below the workpiece surface, with a tolerance of ±0.5 mm. For 5mm material, the focal position has to shift to about 1.2±0.2mm below the surface.

#### **3.6 LBW shielding gas flow, V (l/min)**

Shielding gas selection produces a best weld quality. With the welding laser, the welding gas is flushed onto the workpiece through a nozzle system in order to protect molten and heated metal from the atmosphere. Gases have different chemical reactions and physical properties, which affect their suitability as assist gases for different welding tasks. At least three important points must be considered: tendency to form plasma, influence on mechanical properties and shielding effect.

Three main types of shielding gases are used: helium, argon and nitrogen. Helium is a gas characterized by minimum molecular weight, maximum thermal conductivity, and maximum ionization energy, thereby making it the most suitable gas for suppressing plasma formation. Argon, on the other hand, becomes ionized relatively easily and is therefore more prone to forming excessive amounts of plasma, in particular at CO2 laser power over 3 kW. Carbon dioxide and nitrogen, on the other hand, are reactive gases, which may react with the weld metal to form oxides, carbides, or nitrides and get trapped in pores. This can result in welds with deficient mechanical properties. As a result, pure carbon dioxide or nitrogen are unsuitable as welding gases in certain applications in particular for the aluminium or magnesium alloys due to the oxidation.

To reduce the plasma effect, in these cases, it is advantageous to use inert gases such as helium or argon as welding gases, because there is no reaction on the weld metal and do not affect weld metallurgy. Indeed, in general, when the laser beam interacts with the workpiece, a hole is drilled through the thickness of the material. This hole or cavity is filled with plasma and surrounded by molten metal, thus, the high energy density of the focused beam could be lost easily. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and reported that a helium gas flow was the best choice. This plasma effect was reduced as a result of the higher ionization potential of helium, and then the weld profile was improved.

These welding gases have other functions, too. It protects the focusing optics against fumes and spatters and, in the case of CO2 lasers, also controls plasma cloud formation. Leong et al., 1998 when welding 1.8 mm thick AZ31B-H4 used helium top shielding gas to protect

surface of the workpiece. Focal position on the workpiece surface had the smallest weld width while the weld width became larger when the focal position deviated above or below

Then, the optimal focal point position to weld thin plates lies on the top surface of the workpiece. Indeed, Weisheit et al., (1997, 1998) investigated 2.5kW CO2 laser welding of some magnesium alloys. For thin plates (2.5 and 3 mm), the best welds according to penetration depth, aspect ratio and sag were achieved when the focal point was adjusted on the surface of workpiece, whereas for thick plates (5 and 8 mm) a position of 2mm below the surface of workpiece proved to be the best. Thus, the focal position should be moved deeper into the material for thicker work pieces and the following used process. Lehner et al., 1999 further researched the tolerance of focal position. For 3mm AZ91 and AM50 die castings welded using a 3 kW Nd:YAG laser, the best focal position is approximately 0.8mm below the workpiece surface, with a tolerance of ±0.5 mm. For 5mm material, the focal position has

Shielding gas selection produces a best weld quality. With the welding laser, the welding gas is flushed onto the workpiece through a nozzle system in order to protect molten and heated metal from the atmosphere. Gases have different chemical reactions and physical properties, which affect their suitability as assist gases for different welding tasks. At least three important points must be considered: tendency to form plasma, influence on

Three main types of shielding gases are used: helium, argon and nitrogen. Helium is a gas characterized by minimum molecular weight, maximum thermal conductivity, and maximum ionization energy, thereby making it the most suitable gas for suppressing plasma formation. Argon, on the other hand, becomes ionized relatively easily and is therefore more prone to forming excessive amounts of plasma, in particular at CO2 laser power over 3 kW. Carbon dioxide and nitrogen, on the other hand, are reactive gases, which may react with the weld metal to form oxides, carbides, or nitrides and get trapped in pores. This can result in welds with deficient mechanical properties. As a result, pure carbon dioxide or nitrogen are unsuitable as welding gases in certain applications in particular for

To reduce the plasma effect, in these cases, it is advantageous to use inert gases such as helium or argon as welding gases, because there is no reaction on the weld metal and do not affect weld metallurgy. Indeed, in general, when the laser beam interacts with the workpiece, a hole is drilled through the thickness of the material. This hole or cavity is filled with plasma and surrounded by molten metal, thus, the high energy density of the focused beam could be lost easily. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and reported that a helium gas flow was the best choice. This plasma effect was reduced as a result of the higher ionization potential of helium, and then the weld

These welding gases have other functions, too. It protects the focusing optics against fumes and spatters and, in the case of CO2 lasers, also controls plasma cloud formation. Leong et al., 1998 when welding 1.8 mm thick AZ31B-H4 used helium top shielding gas to protect

the surface.

to shift to about 1.2±0.2mm below the surface.

**3.6 LBW shielding gas flow, V (l/min)** 

mechanical properties and shielding effect.

profile was improved.

the aluminium or magnesium alloys due to the oxidation.

from oxidation, combined with helium back shielding and nitrogen shielding to protect the optics. Wang et al., 2006 studied the influence of gas flow rate on weld width and reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation.

As magnesium is highly susceptible to oxidation, a protective atmosphere is required during welding. Surface cracking leading to laser welding was observed without gas protection. This is due to the oxide formation during welding in the magnesium alloys. To increase the magnesium alloy weldability, argon or helium are the most common choices. Argon is heavier than air so it provides a better shield than helium, but it ionizes easily and has much lower thermal conductivity than helium. This causes a problem with high power CO2 welding: The metal vapour emerging from the keyhole is partially ionized, with charged atoms and free electrons. The free electrons absorb some of the laser light, reducing the power available for welding. As the vapor absorbs energy, it heats up, increasing the number of free elections and further increasing absorption. Helium shield gas is more effective than argon in suppressing this effect because it cools the vapor plume and does not contribute many electrons itself. This welding gas often plays an active role in the welding process, such as increasing the welding speed and improving the mechanical properties of the joint. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and confirmed that a helium gas flow was the best choice.

In addition, often to increase the weld quality, helium/argon mixtures combining the benefits of both gases, i.e. the higher density of argon and the higher ionization potential of helium, may be used to obtain better protection of the weld zone in CO2 laser welding. Hiraga et al., 2001 studied 1.7 mm thick AZ31B-H24 butt joints and get some improvements using argon back shielding in addition to the helium centre shielding. With these two gases, weld profile is remarkably improved where fusion zone interfaces are almost parallel to each other. The melting depth increases with the increase of gas flux, but too much gas flux will induce the surface hollow even penetration of the melting pool. Indeed, higher porosity content was observed for He gas flow higher than 50 l/min. Using Ar back shielding gas allowed us to produce sound welds at lower welding speed, reducing sag of the weld pool. Our study led to the same conclusions and sound welds were produced (Kouadri & Barrallier, 2006). Then, the optimum shielding system consists in a top helium flow superior to 20 l/min and Ar back shielding. By adding single-sided access, laser welding is even more strategically advantageous.

#### **4. Application of laser beam CO2 on thin sheets of magnesium alloy**

The presented material is a cast magnesium alloy (AZ91D) welded by laser CO2 processing. The alloy used for the study of the laser welding is a ternary magnesium - aluminium - zinc of designation AZ91, according to standard ASTM. Laser welding of magnesium alloys appears to be a challenge itself. Indeed, the ability to produce laser welds depends on the properties of the material to be welded. Then, magnesium being characterised by quite unfavourable properties (i.e. low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coefficient of thermal expansion, low melting temperature, wide solidification temperature range, high solidification shrinkage, a tendency to form low melting-point constituents, low viscosity, low surface tensions, high solubility for hydrogen in the liquid state), processing is expected to be an issue.

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 291

Al8Mn3. The typical overall microstructure of the HAZ is shown in figure 3b. The microstructure of HAZ has coarse grain polygonal Mg as the base metal. Nevertheless, eutectic grains disappeared whereas a continuous β-Al12Mg17 phase was created at grain boundary. At the fusion boundary, where a relatively large thermal gradient and small growth rate are established, the microstructure is predominantly cellular (Marya & Edwards, 2000). Grains usually grow epitaxially from the Fusion Zone (FZ) –Heat Affected

**Al8Mn3**

β**-Mg17Al12**

**(**α**-Mg +**  β**-Mg17Al12) eutectic phases**

Fig. 3. a) Microstructure of the base metal, solid solution α-Mg and (α-Mg + β-Mg17Al12) eutectic phase and precipitates (OM), b) Microstructure of the Heat Affected Zone (HAZ) by

a b

**100 μm**

Fig. 4. a) Microstructure in the welding zone and in the heat affected zone by (OM), b)

a b

**80 μm**

Microstructure of the fusion zone (SEM) (Kouadri & Barrallier, 2006, 2011)

**Heat Affected zone 50-200 μm** 

In the fusion zone (FZ), the microstructure is very different from the base metal (Figure 4a, 4b).

Zone (HAZ) interface.

α**-Mg phase**

SEM (Kouadri & Barrallier, 2006, 2011)

**Welded zone 10-25 μm** 

The used magnesium alloy in this study were obtained by high pressure die casting under neutral gas and did not undergo heat treatment to match the conditions generally encountered in automobile applications. The provided plates were sheared to recover 3 mm-thick samples. Their edges were machined by milling. The plates were welded together side by side using a laser beam CO2, which penetrated through the thickness of the plates. The welding was performed using a 4000 W CO2 laser in an inert helium atmosphere. The speed of welding was optimised in the range of 1.0 – 4.25 m/min and the power in the range of 1 – 4 kW.

The objective of this part is to show the evolutions of the metallurgical and mechanical properties generated by the laser CO2 in thin AZ91 magnesium alloy sheets. The presented results were obtained with optimized parameters of CO2 laser beam welding. This part shows the microstructure modifications (characterization of the grain size, chemical properties and the crystallographic texture) occurring during laser welding in every zone of the welded sheet. From mechanical properties point of view, we present the evolution of the hardness and the residual stresses. These one have been performed by taking into account the crystallographic texture. The strain measurements and the characterization of the crystallographic texture have been performed using X-ray diffraction techniques. The set of results demonstrated that laser welding induces the presence of several distinct zones which have distinct microstructural and mechanical properties.

#### **4.1 Study of metallurgical properties**

#### **4.1.1 Macrostructure analysis**

From macrostructure point of view, a narrow weld joint is an important characteristic of high power density welding. The 4 kW CO2 laser welding in an inert helium atmosphere (2 bars) with a speed of welding of 2 m/min of 3mm AZ91D plates showed that the fusion zones have widths of approximately 0.8 1.6 mm (Kouadri & Barrallier, 2006). The region with a width of about 200 – 500 μm between the base metal (BM) and the fusion zone (FZ) can be recognized as the heat affected zone (HAZ). However, the width of the HAZ is defined according to the variations of laser beam parameters. For example, in the literature, the 6 kW CO2 laser welding with a speed of welding of 3.5 m/min of wrought AZ31B alloy indicated that the width of the HAZ was 50–60 µm, but can be doubled at substantially slower speeds (Leong et al., 1998; Sanders et al., 1999). These results showed that the width of the HAZ is tightly connected to laser process parameters.

#### **4.1.2 Microstructure analysis**

From microstructure point of view, the microstructure of the laser welds is characteristic of a high-speed process in which heat is rapidly extracted from the molten fusion zone by surrounding base material. In our study, the mean grain sizes of the base metal (BM) range from 50 to 200 µm (figure 3a).

The BM is heterogeneous and characterised by a mixture of a large primary α-Mg phase and of a (α-Mg + β-Mg17Al12) eutectic phases. This later constituent is a so-called abnormal eutectic (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996) because of its lamellar shape. The base metal exhibits small precipitates dispersed in the matrix but mainly located at the grain boundaries. These precipitates are β-Mg17Al12 and to a lesser degree

The used magnesium alloy in this study were obtained by high pressure die casting under neutral gas and did not undergo heat treatment to match the conditions generally encountered in automobile applications. The provided plates were sheared to recover 3 mm-thick samples. Their edges were machined by milling. The plates were welded together side by side using a laser beam CO2, which penetrated through the thickness of the plates. The welding was performed using a 4000 W CO2 laser in an inert helium atmosphere. The speed of welding was optimised in the range of 1.0 – 4.25 m/min and the power in the range

The objective of this part is to show the evolutions of the metallurgical and mechanical properties generated by the laser CO2 in thin AZ91 magnesium alloy sheets. The presented results were obtained with optimized parameters of CO2 laser beam welding. This part shows the microstructure modifications (characterization of the grain size, chemical properties and the crystallographic texture) occurring during laser welding in every zone of the welded sheet. From mechanical properties point of view, we present the evolution of the hardness and the residual stresses. These one have been performed by taking into account the crystallographic texture. The strain measurements and the characterization of the crystallographic texture have been performed using X-ray diffraction techniques. The set of results demonstrated that laser welding induces the presence of several distinct zones which

From macrostructure point of view, a narrow weld joint is an important characteristic of high power density welding. The 4 kW CO2 laser welding in an inert helium atmosphere (2 bars) with a speed of welding of 2 m/min of 3mm AZ91D plates showed that the fusion zones have widths of approximately 0.8 1.6 mm (Kouadri & Barrallier, 2006). The region with a width of about 200 – 500 μm between the base metal (BM) and the fusion zone (FZ) can be recognized as the heat affected zone (HAZ). However, the width of the HAZ is defined according to the variations of laser beam parameters. For example, in the literature, the 6 kW CO2 laser welding with a speed of welding of 3.5 m/min of wrought AZ31B alloy indicated that the width of the HAZ was 50–60 µm, but can be doubled at substantially slower speeds (Leong et al., 1998; Sanders et al., 1999). These results showed that the width

From microstructure point of view, the microstructure of the laser welds is characteristic of a high-speed process in which heat is rapidly extracted from the molten fusion zone by surrounding base material. In our study, the mean grain sizes of the base metal (BM) range

The BM is heterogeneous and characterised by a mixture of a large primary α-Mg phase and of a (α-Mg + β-Mg17Al12) eutectic phases. This later constituent is a so-called abnormal eutectic (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996) because of its lamellar shape. The base metal exhibits small precipitates dispersed in the matrix but mainly located at the grain boundaries. These precipitates are β-Mg17Al12 and to a lesser degree

have distinct microstructural and mechanical properties.

of the HAZ is tightly connected to laser process parameters.

**4.1 Study of metallurgical properties** 

**4.1.1 Macrostructure analysis** 

**4.1.2 Microstructure analysis** 

from 50 to 200 µm (figure 3a).

of 1 – 4 kW.

Al8Mn3. The typical overall microstructure of the HAZ is shown in figure 3b. The microstructure of HAZ has coarse grain polygonal Mg as the base metal. Nevertheless, eutectic grains disappeared whereas a continuous β-Al12Mg17 phase was created at grain boundary. At the fusion boundary, where a relatively large thermal gradient and small growth rate are established, the microstructure is predominantly cellular (Marya & Edwards, 2000). Grains usually grow epitaxially from the Fusion Zone (FZ) –Heat Affected Zone (HAZ) interface.

Fig. 3. a) Microstructure of the base metal, solid solution α-Mg and (α-Mg + β-Mg17Al12) eutectic phase and precipitates (OM), b) Microstructure of the Heat Affected Zone (HAZ) by SEM (Kouadri & Barrallier, 2006, 2011)

In the fusion zone (FZ), the microstructure is very different from the base metal (Figure 4a, 4b).

Fig. 4. a) Microstructure in the welding zone and in the heat affected zone by (OM), b) Microstructure of the fusion zone (SEM) (Kouadri & Barrallier, 2006, 2011)

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 293

Fig. 6. a) Statistical distribution of the grain sizes of AZ91 alloys in welded zone, b) Volume fraction of the α-Mg grains and of the β-Mg17Al12 precipitates in the welded zone (Kouadri

**M1**

**Volume fraction (%)**

Generally, the laser welding leads to the redistribution of chemical composition or evaporative losses in the fusion zone. In the case of magnesium alloys, the temperatures reached within keyholes are far greater than the boiling temperatures of magnesium, aluminium or zinc (Liu et al., 2000). Thus, during welding, the preferential evaporative losses consist primarily of zinc and magnesium and then aluminum. This evaporation causes a variation of chemical composition in the fusion zone, especially at high laser power

In our studies, the welding of AZ91 D plates, the chemical composition (% weight.) of each individual phase in the different phases was studied using EDS analysis. The base metal constitutes a reference state for the comparison with the welded area. In the table 1 energy dispersive spectroscopy (EDS) analysis indicated that the α-Mg grain contains up to 8.1 and 1.15 wt. % of Al and Zn, respectively. The α-phase contains ≈ 32 wt. % Al (Kouadri & Barrallier, 2006). All these results were in complete agreement with the literature on AZ91

Chemical Composition (weight %) Al Mg Zn Mn Fe Si

α*, (*α *+* β

Table 1. Chemical composition of AZ91 alloy in base metal, BM, (EDS analysis) (Kouadri &

The tables 2 and 3 show the evolution of the chemical composition in the HAZ and in the

In the HAZ, higher Al and Zn concentrations in the α-phase were measured as compared to the base metal and to the welded zone (Kouadri & Barrallier, 2006). So, this confirms that β-Mg17Al12 and Al8Mn3 precipitates were diluted in a matrix made of over-saturated α-phase.

*)* 8,1 90,6 1,15 0,15 - -

**Volume fraction in the welded zone**

**M2+ M3**

**Precipitates Principal phase: α-Mg Eutectic phase (α-Mg + β-**

**Phases**

**Al12Mg17)**

**M4**

Precipitates β 32,5 67,5 - - - - Precipitates Al8Mn3 32,8 14,5 - 52,7 - -

& Barrallier, 2010).

M1

**Volume fraction (%)**

Barrallier, 2006).

welded zone.

**4.1.4 Chemical analysis** 

density which leads to a new chemical redistribution.

**Distribution of grain sizes in the welded zone**

M2

**0 10 20 30 40 50 Grain sizes (μm)**

M3

M4

alloy microstructure, which has been widely studied.

Base metal Matrix

The α-Mg microstructure is much finer ranging between 10µm and 25µm. There is β-Mg17Al12 precipitates too, clearly present and located in grain boundaries. So, the microstructure appears to be more homogenous at scale lengths of a few micrometers. This fine equiaxed grains in the fusion zones formed by cellular growth were also observed by the others authors (Cao et al., 2006) in Zr-containing ZE41A alloy. Weisheit et al., 1997, 1998 have also observed a cellular morphology in all joints except for the WE54 alloy which showed a more globular grain shape. It was further observed the equiaxed morphology in AM60B alloy occurring at low welding speeds. At higher welding speeds, however, the morphology changes from equiaxed to dendritic forms (Pastor et al., 2000). In the same way, our observations showed that the rapid cooling experienced during laser welding leads to a significant grain refinement with cellular growth in the fusion zone. In brief, the laser welding leads to a grain refinement some is the initial structure (Haferkamp et al., 1996, 1998). Only the grain morphology changes following the used laser parameters. Indeed, it was reported that the original microstructure has little influence on the fusion zone structure though magnesium alloys can be welded in different conditions (Weisheit et al., 1998).

#### **4.1.3 Distribution of grain size and volumetric fractions of phases by image analysis**

An example of the statistical distribution of the different grain sizes obtained by grain count is presented on the figure 5a.

Fig. 5. (a) Statistical distribution of the grain sizes of AZ91 alloys in the base metal. (b) Volume fraction of the α-Mg grains, of the eutectic (α-Mg grains β-Mg17Al12) phase and of the precipitates in the base metal (Kouadri & Barrallier, 2011).

This distribution shows the presence of four modes; M1, M2, M3 and M4: The precipitates (M1), whose main mode is about 10 µm and the principal α-Mg (M2 + M3) phase, whose main modes are 50 and 160 µm. Finally the principal mode of the eutectic phase (α-Mg + β-Al12Mg17) is about 220µm. The volumetric fractions calculation (figure 5b) demonstrates that the volumetric fraction of the base metal for the principal α-Mg phase is estimated at 85.4%, that of the eutectic phases at 13.8% and those of the precipitates at 0.8%. These results are in line with those in the literature and the diagram of the alloy phase AZ91 (StJohn et al., 2003). The large reduction of grains in the fusion zone is confirmed by statistical distribution of the grains (Figure 6a) where the principal mode is 16µm. The volumetric fraction of the principal phase represented by modes 2 and 3 constitutes 96% of the matrix (Figure 6b). The eutectic phase has almost disappeared in the welded zone.

Fig. 6. a) Statistical distribution of the grain sizes of AZ91 alloys in welded zone, b) Volume fraction of the α-Mg grains and of the β-Mg17Al12 precipitates in the welded zone (Kouadri & Barrallier, 2010).

#### **4.1.4 Chemical analysis**

292 CO2 Laser – Optimisation and Application

The α-Mg microstructure is much finer ranging between 10µm and 25µm. There is β-Mg17Al12 precipitates too, clearly present and located in grain boundaries. So, the microstructure appears to be more homogenous at scale lengths of a few micrometers. This fine equiaxed grains in the fusion zones formed by cellular growth were also observed by the others authors (Cao et al., 2006) in Zr-containing ZE41A alloy. Weisheit et al., 1997, 1998 have also observed a cellular morphology in all joints except for the WE54 alloy which showed a more globular grain shape. It was further observed the equiaxed morphology in AM60B alloy occurring at low welding speeds. At higher welding speeds, however, the morphology changes from equiaxed to dendritic forms (Pastor et al., 2000). In the same way, our observations showed that the rapid cooling experienced during laser welding leads to a significant grain refinement with cellular growth in the fusion zone. In brief, the laser welding leads to a grain refinement some is the initial structure (Haferkamp et al., 1996, 1998). Only the grain morphology changes following the used laser parameters. Indeed, it was reported that the original microstructure has little influence on the fusion zone structure

though magnesium alloys can be welded in different conditions (Weisheit et al., 1998).

Fig. 5. (a) Statistical distribution of the grain sizes of AZ91 alloys in the base metal. (b) Volume fraction of the α-Mg grains, of the eutectic (α-Mg grains β-Mg17Al12) phase and of

This distribution shows the presence of four modes; M1, M2, M3 and M4: The precipitates (M1), whose main mode is about 10 µm and the principal α-Mg (M2 + M3) phase, whose main modes are 50 and 160 µm. Finally the principal mode of the eutectic phase (α-Mg + β-Al12Mg17) is about 220µm. The volumetric fractions calculation (figure 5b) demonstrates that the volumetric fraction of the base metal for the principal α-Mg phase is estimated at 85.4%, that of the eutectic phases at 13.8% and those of the precipitates at 0.8%. These results are in line with those in the literature and the diagram of the alloy phase AZ91 (StJohn et al., 2003). The large reduction of grains in the fusion zone is confirmed by statistical distribution of the grains (Figure 6a) where the principal mode is 16µm. The volumetric fraction of the principal phase represented by modes 2 and 3 constitutes 96% of the matrix (Figure 6b). The

**M1**

**Volume fraction (%)**

the precipitates in the base metal (Kouadri & Barrallier, 2011).

**M**

eutectic phase has almost disappeared in the welded zone.

is presented on the figure 5a.

**M**

**M**

**Volume fraction (%)**

**Distribution of the grain sizes in the base metal**

**M**

**0 50 100 150 200 250 300 Grain sizes (μm)**

**4.1.3 Distribution of grain size and volumetric fractions of phases by image analysis**  An example of the statistical distribution of the different grain sizes obtained by grain count

 **Volume fraction in the base metal**

**Precipitates Principal phase α-Mg Eutectic phase (α-Mg + β-**

**M2+ M3**

**Al12Mg17) Phases**

**M4**

Generally, the laser welding leads to the redistribution of chemical composition or evaporative losses in the fusion zone. In the case of magnesium alloys, the temperatures reached within keyholes are far greater than the boiling temperatures of magnesium, aluminium or zinc (Liu et al., 2000). Thus, during welding, the preferential evaporative losses consist primarily of zinc and magnesium and then aluminum. This evaporation causes a variation of chemical composition in the fusion zone, especially at high laser power density which leads to a new chemical redistribution.

In our studies, the welding of AZ91 D plates, the chemical composition (% weight.) of each individual phase in the different phases was studied using EDS analysis. The base metal constitutes a reference state for the comparison with the welded area. In the table 1 energy dispersive spectroscopy (EDS) analysis indicated that the α-Mg grain contains up to 8.1 and 1.15 wt. % of Al and Zn, respectively. The α-phase contains ≈ 32 wt. % Al (Kouadri & Barrallier, 2006). All these results were in complete agreement with the literature on AZ91 alloy microstructure, which has been widely studied.


Table 1. Chemical composition of AZ91 alloy in base metal, BM, (EDS analysis) (Kouadri & Barrallier, 2006).

The tables 2 and 3 show the evolution of the chemical composition in the HAZ and in the welded zone.

In the HAZ, higher Al and Zn concentrations in the α-phase were measured as compared to the base metal and to the welded zone (Kouadri & Barrallier, 2006). So, this confirms that β-Mg17Al12 and Al8Mn3 precipitates were diluted in a matrix made of over-saturated α-phase.

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 295

However, close to the surface, the AZ91 alloy exhibits two preferential orientations

Fig. 7. Evolution of the crystallographic texture (pole figure 101) from the surface to a depth of 200 µm: a) Texture to the surface, b) Texture to 50 µm, c) Texture to 150 µm, d)

**c) to 150 μm d) to 200 μm** 

**a) On the surface b) to 50 μm** 

The {1011} pole figure showed that a large fraction of grains are oriented with pyramidal {1011} planes parallel to the surface of the sheet. The intensity of the center of this pole figure is a tenfold improvement in intensity compared with other peaks. A close look at the position reveals that the normal axis is tilted at an angle of ± 4° from the normal sheet direction to the transverse direction and around the welding direction. The ODF calculation allows us to say that 71% of the grains have an orientation {1011} 3413 corresponding to the 3 Euler angles (ϕ1 = 65,6°, φ = 60,8°, ϕ2 = 54,4°). Likewise, the {1010} pole figure shows the presence of a smaller proportion of grains with {1010} planes parallel to the surface. Such grains show two other poles {1010} Pi and {1010} Pj , on both sides of the centre, due to the multiplicity of the hexagonal symmetry, which is equal to three. These poles are tilted at 60° around the centre. The ODF calculation allows us to say that 6% about of the grains have an orientation {1010} 0334 corresponding to the 3 Euler angles (ϕ1 = 65°, φ = 90°, ϕ2 = 60°). The most likely explanation for the variation of texture between the surface and the depth of the fusion zone is that the differences of the thermophysical and thermomechanical properties of the investigated location affect the process of solidification and plastic deformation, leading to different final out-comes. In our study, the nature of this texture has been explained by the thermodynamic conditions of minimisation of surface energies

Texture to 200 µm (Kouadri & Barrallier, 2006, 2011).

concerning 77% of the grains (Figure 7a).

In the welded zone, the chemical composition of phases was much the same as in base metal (Kouadri & Barrallier, 2006) except in a thin superficial layer close to the surface. The Alcontent in the β-phase decreases from 30 % (weight) in the BM down to 17% (weight) in the fusion zone. Likewise, we could see a strong decrease of the Al-content in every crystalline phase. We did'nt observe evaporative losses due to a good optimisation of the laser parameters. Indeed, it is known that higher energy densities lead to greater evaporative losses, increased spatter, and uneven weld beads. Thus, minimizing the irradiance incident upon the workpiece would reduce the loss of high vapor pressure elements. For example, larger reductions of both Mg and Zn were also reported at slower travel speeds (Leong et al., 1998; Sanders et al., 1999). We can conclude in the case of the magnesium alloys that there aren't evaporative losses of Mg and Zn if the laser parameters are optimized. There is only a chemical redistribution of the overall Al quantity for example in our case, due to the solidification conditions (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996). This redistribution should then be carefully controlled and optimized by manipulation of welding parameters.


Table 2. Chemical composition of AZ91 alloy in the HAZ, (EDS analysis) (Kouadri & Barrallier, 2006).


Table 3. Chemical composition of AZ91 alloy in welded metal, (EDS analysis) (Kouadri & Barrallier, 2006).

#### **4.1.5 Texture characterisation**

In general, texture develops in a metal as a result of processes such as crystallisation, plastic deformation…. The practical importance of preferred crystallographic orientation results from the dependence of many mechanical and physical properties on crystal direction. Thus, a textured material will have, in general, anisotropic values for a number of parameters including the yield strength, Young's modulus and Poisson's ratio. In order to understand how preferred crystallographic orientations might occur in laser welding, it is necessary to consider the formation and structure of the fusion zone in detail.

Initially, in our study, the base metal is characterized by a random orientation. Likewise, there is no more texture in the HAZ with 90% of the grains being randomly oriented. This is consistent with Coelho et al., 2008, study. In the fusion zone, the microstructure consists of fine and randomly oriented equiaxed dendrites nucleated. The texture analysis showed clearly that there is no texture. Indeed, ODF calculation indicates that more than 99% of the grains are randomly oriented (Kouadri & Barrallier, 2006, 2011).

In the welded zone, the chemical composition of phases was much the same as in base metal (Kouadri & Barrallier, 2006) except in a thin superficial layer close to the surface. The Alcontent in the β-phase decreases from 30 % (weight) in the BM down to 17% (weight) in the fusion zone. Likewise, we could see a strong decrease of the Al-content in every crystalline phase. We did'nt observe evaporative losses due to a good optimisation of the laser parameters. Indeed, it is known that higher energy densities lead to greater evaporative losses, increased spatter, and uneven weld beads. Thus, minimizing the irradiance incident upon the workpiece would reduce the loss of high vapor pressure elements. For example, larger reductions of both Mg and Zn were also reported at slower travel speeds (Leong et al., 1998; Sanders et al., 1999). We can conclude in the case of the magnesium alloys that there aren't evaporative losses of Mg and Zn if the laser parameters are optimized. There is only a chemical redistribution of the overall Al quantity for example in our case, due to the solidification conditions (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996). This redistribution should then be carefully controlled and optimized by manipulation of

Chemical Composition (weight %) Al Mg Zn Mn Fe Si HAZ Matrix α 8,51 90,31 1,15 0,04 -

Table 2. Chemical composition of AZ91 alloy in the HAZ, (EDS analysis) (Kouadri &

Chemical Composition (weight %) Al Mg Zn Mn Fe Si welded zone Matrix α 8,3 90,3 1,1 0,2 - 0,1

Table 3. Chemical composition of AZ91 alloy in welded metal, (EDS analysis) (Kouadri &

In general, texture develops in a metal as a result of processes such as crystallisation, plastic deformation…. The practical importance of preferred crystallographic orientation results from the dependence of many mechanical and physical properties on crystal direction. Thus, a textured material will have, in general, anisotropic values for a number of parameters including the yield strength, Young's modulus and Poisson's ratio. In order to understand how preferred crystallographic orientations might occur in laser welding, it is

Initially, in our study, the base metal is characterized by a random orientation. Likewise, there is no more texture in the HAZ with 90% of the grains being randomly oriented. This is consistent with Coelho et al., 2008, study. In the fusion zone, the microstructure consists of fine and randomly oriented equiaxed dendrites nucleated. The texture analysis showed clearly that there is no texture. Indeed, ODF calculation indicates that more than 99% of the

necessary to consider the formation and structure of the fusion zone in detail.

grains are randomly oriented (Kouadri & Barrallier, 2006, 2011).

Precipitates β 26,83 69,63 3,50 0,04 -

Precipitates β 29,9 69,1 - - - -

welding parameters.

Barrallier, 2006).

Barrallier, 2006).

**4.1.5 Texture characterisation** 

However, close to the surface, the AZ91 alloy exhibits two preferential orientations concerning 77% of the grains (Figure 7a).

Fig. 7. Evolution of the crystallographic texture (pole figure 101) from the surface to a depth of 200 µm: a) Texture to the surface, b) Texture to 50 µm, c) Texture to 150 µm, d) Texture to 200 µm (Kouadri & Barrallier, 2006, 2011).

The {1011} pole figure showed that a large fraction of grains are oriented with pyramidal {1011} planes parallel to the surface of the sheet. The intensity of the center of this pole figure is a tenfold improvement in intensity compared with other peaks. A close look at the position reveals that the normal axis is tilted at an angle of ± 4° from the normal sheet direction to the transverse direction and around the welding direction. The ODF calculation allows us to say that 71% of the grains have an orientation {1011} 3413 corresponding to the 3 Euler angles (ϕ1 = 65,6°, φ = 60,8°, ϕ2 = 54,4°). Likewise, the {1010} pole figure shows the presence of a smaller proportion of grains with {1010} planes parallel to the surface. Such grains show two other poles {1010} Pi and {1010} Pj , on both sides of the centre, due to the multiplicity of the hexagonal symmetry, which is equal to three. These poles are tilted at 60° around the centre. The ODF calculation allows us to say that 6% about of the grains have an orientation {1010} 0334 corresponding to the 3 Euler angles (ϕ1 = 65°, φ = 90°, ϕ2 = 60°). The most likely explanation for the variation of texture between the surface and the depth of the fusion zone is that the differences of the thermophysical and thermomechanical properties of the investigated location affect the process of solidification and plastic deformation, leading to different final out-comes. In our study, the nature of this texture has been explained by the thermodynamic conditions of minimisation of surface energies

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 297

due to its finer microstructure and higher volume fraction of intermetallics such as Mg17Al12

Of the same form, through the thickness at a depth around 200 μm, the results show a different evolution than the surface concerning the BM and the HAZ, the hardness is lower. In the depth, the evolution of the microhardness shows no statistical variation of the microhardness between the base metal, the heat affected zone and the welded zone contrary to the surface. We can see that the hardness between the base metal, the heat affected zone and the core of the welded zone at a depth around 200 μm stays stable with a value of about 85 HV. Though significant grain coarsening occurred in the HAZ of AZ91 alloy, the hardness in the HAZ was still almost the same as that in base metal. These results join those of literature. Weisheit et al., 1997, studied 2.5kW CO2 laser welding of cast magnesium alloys such as AZ91 AM60, ZC63, ZE41, QE22 and WE54 and wrought alloys (AZ31, AZ61, ZW3 and ZC71). For as-cast alloys, there is an increase in hardness of the FZ but little variation in hardness occurs in the HAZ. These observations demonstrate that laser welding induces particular profiles in the zones studied. These differences in hardness distribution over laser weld joints indicate the inhomogeneity of the joints following used parameters. Hardness is influenced by the laser welding parameters but too the initial chemical composition and also depends on the manufacturing process of the magnesium alloy. The choice of the laser process CO2 or Nd:YAG influences too the hardness. This last point has been demonstrated by Hiraga et al., 2001, studied 2 kW CO2 and Nd:YAG laser welding of wrought AZ31B-H24 butt joints of 1.7mm thickness. Their results showed that the Nd:YAG

laser welded fusion zone is slightly harder than the FZ produced by CO2 laser.

**Microhardness from the base metal passing on the welded zone** 

**close to the surface**

Fig. 8. Hardness from the base metal passing on the heat affected zone and the welded zone close to the surface and in the thickness at a depth about 200 μm (Kouadri & Barrallier,

**-3000 -2000 -1000 0 1000 2000 3000**

**Base Metal (MB) Welded zone Base Metal (MB) (ZWS) HAZ HAZ**

(Watkins, 2003).

2011).

**60**

**70**

**80**

**90**

**100**

**110**

**120**

(Kalinyuk et al., 2003; Matysina, 1999) which results in the presence of columnar grain growth at the surface of the welded zone (Kurtz et al., 2001). Between the surface and the 200 µm depth (Figure 7b, 7c, 7d) the texture decreases to disappear completely from 200 µm. This change underlines the presence of a transition from columnar growth to equiaxial grain growth. These results showed that the laser welding led to a complex microstructure and induced high temperature and deformation gradients which may cause changes in crystalline orientations. The study of the texture evolution is then required to understand the anisotropic characteristic of the welds and its influence on mechanical properties. Compared with the literature, little study takes into account the texture due to laser welding. So our results showed that the laser welding can form a crystallographic texture and that it is necessary to study it thoroughly to apprehend the mechanical properties as well as possible.

#### **4.2 Experimental results of the mechanical properties**

#### **4.2.1 Hardness characterization**

From laser parameters point of view, hardness in the fusion zone was found to increase almost linearly with welding speed because higher welding speeds lead to a more significant refinement of the microstructure and more alloying elements into the matrix, even though hard intermetallics are reduced and more finely distributed at high cooling rates. The average hardness of CO2 laser welded joints decreases with slower welding speeds. Indeed, at low welding speeds the weld structure and hardness were nearly the same or sometimes lower as those in the base die-cast material. The decrease in the hardness of the HAZ was due to grain growth. However, these results depend too on the laser power and the nature of used alloy. In the literature, it was also reported that there was a gradual decrease in hardness of 6 kW CO2 laser welded joints from the BM to the HAZ to the FZ of AZ31BH24 alloy, with a minimum value in the FZ (Leong et al., 1998; Sanders et al., 1999). Dhahri et al., 2001 investigated WE43-T6 alloy using 5 kW CO2 laser. The hardness at the top and bottom of the welds was similar but the hardness in the middle of the bead was lower. In our studies, the 4 kW CO2 laser welding of die cast AZ91D alloy showed that there is an increase in hardness of the fusion zone but little variation in hardness occurs in the HAZ according to the localization of the measurements. Figure 8 shows an example of the hardness results, measured close to the surface on both sides of the linear weld in a profile including the base metal passing through the heat affected zone and the welded zone. The same measurement has been realized along the same profile and at a depth around 200 μm.

Close to the surface, the hardness varies from around 90 HV in the base metal and around 95 HV in the heat affected zone to 110 HV in the welded zone. The hardness in the HAZ is higher than in the BM, even though the size of the grains is identical. This augmentation of microhardness has in part been explained by the contribution of added elements and particularly the increase in the level of aluminium in this zone (10%). Other studies have also demonstrated the presence of precipitates which are formed in this zone considered to be a zone of diffusion which contributes towards augmenting the hardness (Shaw et al., 1997). In the fusion zone, compared to the base metal, the increase in hardness was probably

(Kalinyuk et al., 2003; Matysina, 1999) which results in the presence of columnar grain growth at the surface of the welded zone (Kurtz et al., 2001). Between the surface and the 200 µm depth (Figure 7b, 7c, 7d) the texture decreases to disappear completely from 200 µm. This change underlines the presence of a transition from columnar growth to equiaxial grain growth. These results showed that the laser welding led to a complex microstructure and induced high temperature and deformation gradients which may cause changes in crystalline orientations. The study of the texture evolution is then required to understand the anisotropic characteristic of the welds and its influence on mechanical properties. Compared with the literature, little study takes into account the texture due to laser welding. So our results showed that the laser welding can form a crystallographic texture and that it is necessary to study it thoroughly to apprehend the mechanical properties as

From laser parameters point of view, hardness in the fusion zone was found to increase almost linearly with welding speed because higher welding speeds lead to a more significant refinement of the microstructure and more alloying elements into the matrix, even though hard intermetallics are reduced and more finely distributed at high cooling rates. The average hardness of CO2 laser welded joints decreases with slower welding speeds. Indeed, at low welding speeds the weld structure and hardness were nearly the same or sometimes lower as those in the base die-cast material. The decrease in the hardness of the HAZ was due to grain growth. However, these results depend too on the laser power and the nature of used alloy. In the literature, it was also reported that there was a gradual decrease in hardness of 6 kW CO2 laser welded joints from the BM to the HAZ to the FZ of AZ31BH24 alloy, with a minimum value in the FZ (Leong et al., 1998; Sanders et al., 1999). Dhahri et al., 2001 investigated WE43-T6 alloy using 5 kW CO2 laser. The hardness at the top and bottom of the welds was similar but the hardness in the middle of the bead was lower. In our studies, the 4 kW CO2 laser welding of die cast AZ91D alloy showed that there is an increase in hardness of the fusion zone but little variation in hardness occurs in the HAZ according to the localization of the measurements. Figure 8 shows an example of the hardness results, measured close to the surface on both sides of the linear weld in a profile including the base metal passing through the heat affected zone and the welded zone. The same measurement has been realized along the same profile and at a depth around 200

Close to the surface, the hardness varies from around 90 HV in the base metal and around 95 HV in the heat affected zone to 110 HV in the welded zone. The hardness in the HAZ is higher than in the BM, even though the size of the grains is identical. This augmentation of microhardness has in part been explained by the contribution of added elements and particularly the increase in the level of aluminium in this zone (10%). Other studies have also demonstrated the presence of precipitates which are formed in this zone considered to be a zone of diffusion which contributes towards augmenting the hardness (Shaw et al., 1997). In the fusion zone, compared to the base metal, the increase in hardness was probably

well as possible.

μm.

**4.2.1 Hardness characterization** 

**4.2 Experimental results of the mechanical properties** 

due to its finer microstructure and higher volume fraction of intermetallics such as Mg17Al12 (Watkins, 2003).

Of the same form, through the thickness at a depth around 200 μm, the results show a different evolution than the surface concerning the BM and the HAZ, the hardness is lower. In the depth, the evolution of the microhardness shows no statistical variation of the microhardness between the base metal, the heat affected zone and the welded zone contrary to the surface. We can see that the hardness between the base metal, the heat affected zone and the core of the welded zone at a depth around 200 μm stays stable with a value of about 85 HV. Though significant grain coarsening occurred in the HAZ of AZ91 alloy, the hardness in the HAZ was still almost the same as that in base metal. These results join those of literature. Weisheit et al., 1997, studied 2.5kW CO2 laser welding of cast magnesium alloys such as AZ91 AM60, ZC63, ZE41, QE22 and WE54 and wrought alloys (AZ31, AZ61, ZW3 and ZC71). For as-cast alloys, there is an increase in hardness of the FZ but little variation in hardness occurs in the HAZ. These observations demonstrate that laser welding induces particular profiles in the zones studied. These differences in hardness distribution over laser weld joints indicate the inhomogeneity of the joints following used parameters. Hardness is influenced by the laser welding parameters but too the initial chemical composition and also depends on the manufacturing process of the magnesium alloy. The choice of the laser process CO2 or Nd:YAG influences too the hardness. This last point has been demonstrated by Hiraga et al., 2001, studied 2 kW CO2 and Nd:YAG laser welding of wrought AZ31B-H24 butt joints of 1.7mm thickness. Their results showed that the Nd:YAG laser welded fusion zone is slightly harder than the FZ produced by CO2 laser.

Fig. 8. Hardness from the base metal passing on the heat affected zone and the welded zone close to the surface and in the thickness at a depth about 200 μm (Kouadri & Barrallier, 2011).

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 299

stresses. This evolution is in line with mechanical equilibrium. The welded zone exhibits residual traction stresses which are counter balanced by compression stresses in the base

Furthermore, study of the state of surface stresses demonstrates some anisotropy: the residual stresses are not equibiaxial. We observe that the longitudinal component decreases from the center of the weld zone towards the base metal, whereas the transverse component remains high before a sudden reduction. These changes occur in the thermally affected zone, and are associated with numerous factors and by many authors with a zone of relaxation. The evolution of the longitudinal stress can be connected with the heat flow resulting from the mobile heat source that follows the welding direction. We can see two explanations of these evolutions. On the one hand, the effect of temperature and cooling speed gradients arise from the anisotropic heat flow (Teng et al., 2002), on the other, the anisotropy can be the result of a shrinking structure. The negligible thermal dilation of both plates prevents the free shrinkage of the weld line along the direction of the weld line. The same applies to the transverse component. The restricted shrinkage in the transverse plane arises from the clamping of the plates during welding. Even if the influence of the clamping is hard to evaluate experimentally, digital studies have shown that the field of residual stresses is strongly influenced by the geometry of the assembly (Jensen et al., 2002; Dai & Shaw, 2003).

**4.2.2.2 Distribution of the residual stresses though the thickness of the welded zone** 

Fig. 10. Longitudinal and transverse residual stresses from the welded zone close to the

**0 200 400 600 800 1000 1200 1400 1600 1800 2000**

**Decreasing in the core of the welded zone**

surface to the core of the welded zone (Kouadri & Barrallier, 2011)

The profiles of the average stresses through the thickness are plotted in the figure 10 for the

**Residual stresses in the welded zone from the surface to the core**

**longitudunal residual stresses transverse residual stresses**

metal (Pryds & Huang, 2000).

longitudinal and transversal residual stresses.

**Minimum close to the surface**

**Maximum in sub layer**

**0**

**20**

**40**

**60**

**80**

**100**

**120**

**1 2**

#### **4.2.2 Residual stresses results**

The origin of residual stresses and their evolution within a welded joint is difficult to evaluate because they are the result of a number of competing mechanisms: shrinkage changes in phase and microstructure (Dai & Shaw, 2003). In our case, the magnesium alloy does not undergo a phase transformation, as is the case for aluminium alloys. Numerous studies have shown that when this is the case residual stresses are primarily a consequence of an inhibited shrinkage in the weld line and of the modified microstructure which is linked to strong temperature gradients, and to their distribution within the material (Wagner, 1999; Cho et al., 2003; Mao et al., 2006).

#### **4.2.2.1 Distribution of the residual stresses at the surface of the assembled sheets**

The measurements were undertaken in the welded zone, perpendicular to the weld line towards the base metal. Figure 9 shows an example of the obtained results.

Fig. 9. Longitudinal and transverse residual stresses close to the surface from the base metal until the welded zone (Kouadri & Barrallier, 2011)

At the surface, the results demonstrated that the base metal presents a state of compression, whereas the weld line is submitted to residual traction stresses. This state of compression has been attributed to the nature of the cooling, linked to the moulding process. With laser welding, cooling occurs by the diffusion of heat through the outer surfaces of the plates which are in contact with the mould walls. Furthermore the machining by milling of the surface of the base metal before welding accentuates the state of compression and explains the raised values (- 120 MPa) observed at the surface of the base metal. However, in the weld line the heat is evacuated by the plates and not the free surfaces. This leads to traction

The origin of residual stresses and their evolution within a welded joint is difficult to evaluate because they are the result of a number of competing mechanisms: shrinkage changes in phase and microstructure (Dai & Shaw, 2003). In our case, the magnesium alloy does not undergo a phase transformation, as is the case for aluminium alloys. Numerous studies have shown that when this is the case residual stresses are primarily a consequence of an inhibited shrinkage in the weld line and of the modified microstructure which is linked to strong temperature gradients, and to their distribution within the material

**4.2.2.1 Distribution of the residual stresses at the surface of the assembled sheets** 

towards the base metal. Figure 9 shows an example of the obtained results.

The measurements were undertaken in the welded zone, perpendicular to the weld line

**Longitudinal and Transversal residual stresses on the surface from the welded zone to the base metal**

> **longitudinal stresses Transversal stresses**

Fig. 9. Longitudinal and transverse residual stresses close to the surface from the base metal

**-200**

**-150**

**BM SZWHAZ HAZ BM**

**-100**

**-50**

**0**

**-3000 -2000 -1000 0 1000 2000 3000**

**50**

**100**

At the surface, the results demonstrated that the base metal presents a state of compression, whereas the weld line is submitted to residual traction stresses. This state of compression has been attributed to the nature of the cooling, linked to the moulding process. With laser welding, cooling occurs by the diffusion of heat through the outer surfaces of the plates which are in contact with the mould walls. Furthermore the machining by milling of the surface of the base metal before welding accentuates the state of compression and explains the raised values (- 120 MPa) observed at the surface of the base metal. However, in the weld line the heat is evacuated by the plates and not the free surfaces. This leads to traction

**4.2.2 Residual stresses results** 

(Wagner, 1999; Cho et al., 2003; Mao et al., 2006).

until the welded zone (Kouadri & Barrallier, 2011)

stresses. This evolution is in line with mechanical equilibrium. The welded zone exhibits residual traction stresses which are counter balanced by compression stresses in the base metal (Pryds & Huang, 2000).

Furthermore, study of the state of surface stresses demonstrates some anisotropy: the residual stresses are not equibiaxial. We observe that the longitudinal component decreases from the center of the weld zone towards the base metal, whereas the transverse component remains high before a sudden reduction. These changes occur in the thermally affected zone, and are associated with numerous factors and by many authors with a zone of relaxation. The evolution of the longitudinal stress can be connected with the heat flow resulting from the mobile heat source that follows the welding direction. We can see two explanations of these evolutions. On the one hand, the effect of temperature and cooling speed gradients arise from the anisotropic heat flow (Teng et al., 2002), on the other, the anisotropy can be the result of a shrinking structure. The negligible thermal dilation of both plates prevents the free shrinkage of the weld line along the direction of the weld line. The same applies to the transverse component. The restricted shrinkage in the transverse plane arises from the clamping of the plates during welding. Even if the influence of the clamping is hard to evaluate experimentally, digital studies have shown that the field of residual stresses is strongly influenced by the geometry of the assembly (Jensen et al., 2002; Dai & Shaw, 2003).

#### **4.2.2.2 Distribution of the residual stresses though the thickness of the welded zone**

The profiles of the average stresses through the thickness are plotted in the figure 10 for the longitudinal and transversal residual stresses.

Fig. 10. Longitudinal and transverse residual stresses from the welded zone close to the surface to the core of the welded zone (Kouadri & Barrallier, 2011)

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 301

However, during laser welding of magnesium alloys, therefore, some processing problems and weld defects can be encountered such as an unstable weld pool, substantial spatter a strong tendency to drop-through for large weld pools (Leong et al., 1998; Haferkamp et al., 1998; Sanders et al., 1999), sag of the weld pool (especially for thick workpiece), undercut (Dubé et al., 2001), porous oxide inclusions, loss of alloying elements (Leong et al., 1998; Sanders et al., 1999), excessive pore formation (particularly for die castings) (Pastor et al., 2000; Zhao & DebRoy, 2001) and solidification cracking (Marya & Edwards, 2000). These defects are generally decreased by a good optimization of the laser parameters. In view of the results achieved in this study, the use of high-power intensity focused CO2 laser beam with optimized parameters and careful material preparation prior to welding can produce welds with high quality for the most magnesium alloys, in particular for AZ91D of our study (Kouadri & Barrallier, 2006, 2010). Welding speed of 2 m/min and laser power of 4kW let to a full penetration of 3mm thickness welded joint. Optimum weld profile was obtained when focal point was placed on the top surface. In comparison with the literature, all the investigated magnesium alloys showed tendencies for porosity and solidification cracking particularly, at high welding speed (*≥*4m/min). Porosity was prevented by accurate cleaning of the base metal before welding and optimizing the flow rate of argon shielding gas. In order to maintain the mechanical properties when welding magnesium alloys, the heat input and time of exposure to very high temperatures must be minimized. For LBW, the laser power (P) and weld speed (V) directly influence the heat input. This relationship is

Beyond of the optimization of laser parameters, it is believed that the efficiency of CO2 laser beam welding of magnesium alloys could be improved by cleaning the workpiece surface prior to welding. This is due to increasing surface roughness that means decreasing surface reflectivity and enhancing the laser energy coupling during welding. Recent efforts on C02 laser beam welding have resolved several of the initial problems associated with the welding of magnesium alloys. Consistent and repeatable welds can now be obtained without resorting to meticulous edge preparation. Moreover, elimination or reduction of the plasma is recommended for optimal welding of magnesium. This effect of plasma formation which affects the weld quality and the optics during welding has been clarified: trouble-free operation of the optics has been achieved with the use of inert gas shielding

However, several results showed that the weldability of thin magnesium plates was significantly better with the Nd:YAG laser. These observations were attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. Indeed, an advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material's emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Recently, tremendous efforts have been made to clarify the fundamental laser weldability of different types of magnesium alloys using both Nd:YAG and CO2 lasers. It is pointed out that improvements in the laser weldability of a range of magnesium alloys are possible by increasing the power density of the focused spot, and this can be achieved through higher average powers, improved beam focusing system, and decreasing beam reflectivity on

often used to determine the heat input.

such as helium.

The study through the thickness of the welded zone shows that in general the profiles of the stresses reproduce the asymmetry of the welding process. Their behaviour in tension and their variation have in part been explained by the influence of the thermal cycle on the origin of residual stresses and their evolution within the material. The residual stresses on the face exposed to the laser beam are elevated (up to 80 MPa) whereas the opposite face creates stresses of only 23 and 7 MPa respectively for the longitudinal and transverse stresses. This effect can also be explained by the fact that using inert gas ensures very rapid cooling of the superior face whereas the inferior face cools more slowly (Dong et al., 2004).

Finally we have noticed that over the first 200 micrometers the residual stresses present a particular evolution. The intensity of the stresses is not maximum at the surface of the welded zone (78 and 45 MPa respectively for the longitudinal and transverse constraints) as expected, but at a depth of 200µm the stress is 100 MPa. The presence of this maximum stress demonstrates that there is a stress gradient between the textured layer and the heart of the isotropic welded zone. This specific evolution has partly been explained by the plastic deformation of the superficial layer. By presenting a strong texture and an important loss of aluminum, the superficial layer is more sensitive to plastic deformation in the plane compared to the heart of the weld line which is isotropic (Hsiao et al., 2000). These results can be compared to a thin coat deposit because these thin coatings are textured and the maximum stresses are found at the interface (Pina et al., 1997; Cevat Sarioglu 2006).

In our study there is a transition zone with a continual evolution in properties, in particular an evolution of the texture between the outer surface and the depth at about 200 µm. It appears that the development of this texture affects the distribution of stresses with a relaxation of the stresses at the surface and a maximum in the under layer. We explain these modifications by the fact that the level of plastic flow, related to local stresses, is dependent on grain orientation (Su et al., 2002; Agnew & Duygulu, 2005; Wu et al., 2007).

In conclusion, these results showed that the laser welding processes influence the residual stress distribution. Whereas compressive stresses are obtained in the base metal, tensile stresses are obtained in the LBWelds due to thermal gradients and high residual stresses are observed in the LBW fusion zone. These results showed too that it is important to take into account the crystallographic texture to evaluate the residual stresses.

#### **5. Conclusions**

The influence of various welding parameters during continuous wave CO2 laser beam welding of thin plates of magnesium alloys was investigated in this chapter. It is known that the weldability of such materials is usually not excellent and lasers can be utilized to achieve good quality welds. The obtained results and the realized synthesis from the literature showed that the CO2 laser welding possesses comprehensive performances such as good technology and the technology of laser welding magnesium alloys plates is well appropriated. The keyhole welding mode is likely to be encountered in the laser welding of thin sheet magnesium. The results of a detailed investigation showed the influence of different parameters of the laser which have to be tightly combined to obtain a weld quality.

The study through the thickness of the welded zone shows that in general the profiles of the stresses reproduce the asymmetry of the welding process. Their behaviour in tension and their variation have in part been explained by the influence of the thermal cycle on the origin of residual stresses and their evolution within the material. The residual stresses on the face exposed to the laser beam are elevated (up to 80 MPa) whereas the opposite face creates stresses of only 23 and 7 MPa respectively for the longitudinal and transverse stresses. This effect can also be explained by the fact that using inert gas ensures very rapid cooling of the superior face whereas the inferior face cools more slowly (Dong et al.,

Finally we have noticed that over the first 200 micrometers the residual stresses present a particular evolution. The intensity of the stresses is not maximum at the surface of the welded zone (78 and 45 MPa respectively for the longitudinal and transverse constraints) as expected, but at a depth of 200µm the stress is 100 MPa. The presence of this maximum stress demonstrates that there is a stress gradient between the textured layer and the heart of the isotropic welded zone. This specific evolution has partly been explained by the plastic deformation of the superficial layer. By presenting a strong texture and an important loss of aluminum, the superficial layer is more sensitive to plastic deformation in the plane compared to the heart of the weld line which is isotropic (Hsiao et al., 2000). These results can be compared to a thin coat deposit because these thin coatings are textured and the

maximum stresses are found at the interface (Pina et al., 1997; Cevat Sarioglu 2006).

on grain orientation (Su et al., 2002; Agnew & Duygulu, 2005; Wu et al., 2007).

account the crystallographic texture to evaluate the residual stresses.

In our study there is a transition zone with a continual evolution in properties, in particular an evolution of the texture between the outer surface and the depth at about 200 µm. It appears that the development of this texture affects the distribution of stresses with a relaxation of the stresses at the surface and a maximum in the under layer. We explain these modifications by the fact that the level of plastic flow, related to local stresses, is dependent

In conclusion, these results showed that the laser welding processes influence the residual stress distribution. Whereas compressive stresses are obtained in the base metal, tensile stresses are obtained in the LBWelds due to thermal gradients and high residual stresses are observed in the LBW fusion zone. These results showed too that it is important to take into

The influence of various welding parameters during continuous wave CO2 laser beam welding of thin plates of magnesium alloys was investigated in this chapter. It is known that the weldability of such materials is usually not excellent and lasers can be utilized to achieve good quality welds. The obtained results and the realized synthesis from the literature showed that the CO2 laser welding possesses comprehensive performances such as good technology and the technology of laser welding magnesium alloys plates is well appropriated. The keyhole welding mode is likely to be encountered in the laser welding of thin sheet magnesium. The results of a detailed investigation showed the influence of different parameters of the laser which have to be tightly combined to obtain a weld

2004).

**5. Conclusions** 

quality.

However, during laser welding of magnesium alloys, therefore, some processing problems and weld defects can be encountered such as an unstable weld pool, substantial spatter a strong tendency to drop-through for large weld pools (Leong et al., 1998; Haferkamp et al., 1998; Sanders et al., 1999), sag of the weld pool (especially for thick workpiece), undercut (Dubé et al., 2001), porous oxide inclusions, loss of alloying elements (Leong et al., 1998; Sanders et al., 1999), excessive pore formation (particularly for die castings) (Pastor et al., 2000; Zhao & DebRoy, 2001) and solidification cracking (Marya & Edwards, 2000). These defects are generally decreased by a good optimization of the laser parameters. In view of the results achieved in this study, the use of high-power intensity focused CO2 laser beam with optimized parameters and careful material preparation prior to welding can produce welds with high quality for the most magnesium alloys, in particular for AZ91D of our study (Kouadri & Barrallier, 2006, 2010). Welding speed of 2 m/min and laser power of 4kW let to a full penetration of 3mm thickness welded joint. Optimum weld profile was obtained when focal point was placed on the top surface. In comparison with the literature, all the investigated magnesium alloys showed tendencies for porosity and solidification cracking particularly, at high welding speed (*≥*4m/min). Porosity was prevented by accurate cleaning of the base metal before welding and optimizing the flow rate of argon shielding gas. In order to maintain the mechanical properties when welding magnesium alloys, the heat input and time of exposure to very high temperatures must be minimized. For LBW, the laser power (P) and weld speed (V) directly influence the heat input. This relationship is often used to determine the heat input.

Beyond of the optimization of laser parameters, it is believed that the efficiency of CO2 laser beam welding of magnesium alloys could be improved by cleaning the workpiece surface prior to welding. This is due to increasing surface roughness that means decreasing surface reflectivity and enhancing the laser energy coupling during welding. Recent efforts on C02 laser beam welding have resolved several of the initial problems associated with the welding of magnesium alloys. Consistent and repeatable welds can now be obtained without resorting to meticulous edge preparation. Moreover, elimination or reduction of the plasma is recommended for optimal welding of magnesium. This effect of plasma formation which affects the weld quality and the optics during welding has been clarified: trouble-free operation of the optics has been achieved with the use of inert gas shielding such as helium.

However, several results showed that the weldability of thin magnesium plates was significantly better with the Nd:YAG laser. These observations were attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. Indeed, an advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material's emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Recently, tremendous efforts have been made to clarify the fundamental laser weldability of different types of magnesium alloys using both Nd:YAG and CO2 lasers. It is pointed out that improvements in the laser weldability of a range of magnesium alloys are possible by increasing the power density of the focused spot, and this can be achieved through higher average powers, improved beam focusing system, and decreasing beam reflectivity on

Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 303

this technique stays still in the developing stage and many mechanisms need to be studied because of many parameters which govern this process. However, with a good optimization, laser welding for the magnesium alloys seems to be the most appropriated joining technique and can promote their wider uses in aerospace, aircraft, automotive,

Agnew S.R., Duygulu Ö. (2005). Plastic anisotropy and the role of non-basal slip in

Cao, X., Jahazi, M., Immarigeon, J.P., Wallace, W. (2006). A review of laser welding

Cevat Sarioglu C. (2006). The effect of anisotropy on residual stress values and modification

Coelho, R.S., Kostka, A., Pinto, H., Riekehr, S., Koçak, M., Pyzalla, A.R. (2008).

Dai, K., Shaw L. (2003). Finite-Element Analysis of Effects of the Laser-Processed Bimaterial

Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2000). CO2 laser welding of

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Dong, W., Kokawa, H., Tsukamoto, S., Sato Y. S., Ogawa, M. (2004). Mechanism governing

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**6. References** 

1193.

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725–732.

725–732.

workpiece surface. In conclusion, the weldability problems of magnesium alloys are much more easily overcome when using Nd:YAG than CO2 laser. However, our studies showed that CO2 laser is more appropriate to weld cast magnesium alloy than wrought magnesium alloy.

From technology point of view, in comparison with the traditional welding method such as arc welding processes, the laser welding has high efficiency, small welding distortion, low labor costs and convenient construction, is easy to realize automatization, and can be the effective measure to enhance the weld quality. Laser welding processes offer great benefit over other welding processes, e.g., arc welding, resistance welding, etc., since less heat is coupled into the workpiece. The low-heat input will tend to keep a very narrow HAZ then, retaining some to the strength of the material. The benefits of low-heat input and extremely rapid cooling rate, all of which help to minimize the metallurgical problems in the fusion zone. For example, high cooling rate will tend to slow down the development of blisters because of the short time in which the diffusion of hydrogen can take place. In comparison with electron beam welding, even though this process offers the advantages of a high energy density welding process, a vacuum chamber is required, which is not always practical.

All advantages explain their integration in the advanced technology industries such as in aerospace, aircraft, automotive, electronics and other industries. Indeed, today, laser beam welding is being used in an increasingly wider range of industries, from the production of medical devices and microelectronics to shipbuilding. The automotive industry, in particular, takes advantage of this technology's benefits: low heat input, small heat-affected zone (HAZ), low distortion rate, good repeatability, reduced need for post processing and high welding speed. This last point is becoming critical for a successful application in the automotive industry because the increase in welding speed provided by laser welding has resulted in the need for an automated system. Another application of the laser process is the aircraft where weight and cost reduction in civil aircrafts by replacing rivets by advanced welding techniques has now been realized for skin-stringer joints. In this context the laser beam welding technology has proved to be very suitable for a number of reasons, for example, low distortion while processing at high speeds. These benefits have made laser welding the process of choice for many applications that previously used resistance welding. Compared for example with TIG welding, the welding speed with laser is generally three times higher.

However, although laser materials processing has gained widespread acceptability, the mechanisms and main factors controlling the process remain controversial and need further theoretical and experimental studies. Further work is needed to develop the weld process parameters necessary to achieve the materials characteristics required for the use of magnesium alloys in industrial applications. Improved gas shielding requirements are expected to be critical to obtaining welds with the required materials properties. Further studies are needed to determine the parameters controlling weld quality. Indeed, laser beam welding involves many variables: laser power, welding speed, defocusing distance and type of shielding gas, any of which may have an important effect on heat flow and fluid flow in the weld pool. This in turn will affect penetration depth, shape and final solidification structure of the fusion zone. These final states affect the mechanical behaviour. It is for that this technique stays still in the developing stage and many mechanisms need to be studied because of many parameters which govern this process. However, with a good optimization, laser welding for the magnesium alloys seems to be the most appropriated joining technique and can promote their wider uses in aerospace, aircraft, automotive, electronics and other industries.

#### **6. References**

302 CO2 Laser – Optimisation and Application

workpiece surface. In conclusion, the weldability problems of magnesium alloys are much more easily overcome when using Nd:YAG than CO2 laser. However, our studies showed that CO2 laser is more appropriate to weld cast magnesium alloy than wrought magnesium

From technology point of view, in comparison with the traditional welding method such as arc welding processes, the laser welding has high efficiency, small welding distortion, low labor costs and convenient construction, is easy to realize automatization, and can be the effective measure to enhance the weld quality. Laser welding processes offer great benefit over other welding processes, e.g., arc welding, resistance welding, etc., since less heat is coupled into the workpiece. The low-heat input will tend to keep a very narrow HAZ then, retaining some to the strength of the material. The benefits of low-heat input and extremely rapid cooling rate, all of which help to minimize the metallurgical problems in the fusion zone. For example, high cooling rate will tend to slow down the development of blisters because of the short time in which the diffusion of hydrogen can take place. In comparison with electron beam welding, even though this process offers the advantages of a high energy density welding process, a vacuum chamber is required, which is not always

All advantages explain their integration in the advanced technology industries such as in aerospace, aircraft, automotive, electronics and other industries. Indeed, today, laser beam welding is being used in an increasingly wider range of industries, from the production of medical devices and microelectronics to shipbuilding. The automotive industry, in particular, takes advantage of this technology's benefits: low heat input, small heat-affected zone (HAZ), low distortion rate, good repeatability, reduced need for post processing and high welding speed. This last point is becoming critical for a successful application in the automotive industry because the increase in welding speed provided by laser welding has resulted in the need for an automated system. Another application of the laser process is the aircraft where weight and cost reduction in civil aircrafts by replacing rivets by advanced welding techniques has now been realized for skin-stringer joints. In this context the laser beam welding technology has proved to be very suitable for a number of reasons, for example, low distortion while processing at high speeds. These benefits have made laser welding the process of choice for many applications that previously used resistance welding. Compared for example with TIG welding, the welding speed with laser is

However, although laser materials processing has gained widespread acceptability, the mechanisms and main factors controlling the process remain controversial and need further theoretical and experimental studies. Further work is needed to develop the weld process parameters necessary to achieve the materials characteristics required for the use of magnesium alloys in industrial applications. Improved gas shielding requirements are expected to be critical to obtaining welds with the required materials properties. Further studies are needed to determine the parameters controlling weld quality. Indeed, laser beam welding involves many variables: laser power, welding speed, defocusing distance and type of shielding gas, any of which may have an important effect on heat flow and fluid flow in the weld pool. This in turn will affect penetration depth, shape and final solidification structure of the fusion zone. These final states affect the mechanical behaviour. It is for that

alloy.

practical.

generally three times higher.


Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 305

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texture in hard-chronium electroplated coatings. *Surface and coatings technology*, Vol.

during solidification of ferritic steel. *Metallurgical and Materials Transactions A*, Vol.

rapidly solidified magnesium alloys. *Materials Sciences and Engineering A,* Vol. 226-

Based Alloys. *Metallurgical and Materials Transactions A*, Vol. 33, Issue 5, pp. 1461-

fatigue in butt-welded joints. *International Journal of Pressure Vessels and Piping*, Vol.

using a CO2 laser, *IIW seminar, Trends Weld, Lightweight automobile railroad vehicles*,

texture evolution and anisotropic stress–strain curves during large plastic strains in


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Hiraga, H., Inoue, T., Kamado, S. Kojima, Y. (2001). Effect of the shielding gas and laser

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**12** 

*Iran* 

Mohammadreza Riahi

**CO2 Laser and Micro-Fluidics** 

*Shahid Beheshti University/Laser and Plasma Research Institute* 

Microfluidic chips have attracted significant attention over the past decade due to their wide range of potential applications in the biomedical and chemical analysis field such as drug delivery, Point of care diagnostics (Jakeway et al, 2004), flow cytometry (Fu LM et al 2004; Chen & Wang, 2009; Lin et al, 2009), polymerize chain reaction (Suna et al, 2007; Sun & Kwok, 2006; Hsieh et al, 2009), electrophoresis (Fu et al, 2007, 2009) and many other

Traditionally, silicon and glass are the predominant materials employed in the design of microfluidic systems. This was primarily due to their excellent chemical, physical, electrical and optical properties. But fabrication of a microfluidic device on these materials needs standard photolithography equipments such as Reactive Ion Etching (RIE) system which are very expensive and increases the production costs specially in single-use applications

In recent years application of polymeric materials for microfluidic device fabrication is becoming more and more important. Different methods for microfluidic fabrication on polymers such as hot embossing (Gerlach et al, 2002), injection molding (Rotting et al, 2002),

Different kind of lasers such as UV (Ball et al, 2000) and Infrared lasers is used for laser micromachining of polymers. In infrared regime, CO2 laser has a predominant application

In this chapter, we will deal with application of a CO2 laser in microfluidic device fabrication. The application of CO2 laser for fabrication of a optofluidic device and

Application of the CO2 laser for microfluidic device fabrication was first proposed in 2002

CO2 laser emits radiation with the wavelength of 10.6 micrometer. A CO2 laser mostly interacts with a polymer, photo-thermally. When a CO2 laser is irradiated on a polymer surface, it is strongly absorbed and raises the temperature of the polymer. The polymer is

application of a optofluidic device for CO2 laser characterization is also presented.

soft lithography (Xia et al, 1998) and laser micromachining can be applied.

**1. Introduction** 

applications.

which are desired in order to avoid contamination.

due to it's excellent absorption in polymers.

by H. Klank et al (Klank et al, 2002).

**2. Interaction of a CO2 laser with polymers** 

then melted, decomposed and leaving a void in a workpiece.

high purity titanium using a Taylor-type crystal plasticity. *Acta Materialia*, Vol.55, pp. 423-432.

Zhao, H., DebRoy, T. (2001). Pore formation during laser beam welding of die cast magnesium alloy AM60B – mechanism and remedy*. Weld. J*., Vol. 80, Issue 8, pp. 204S–210S.

### **CO2 Laser and Micro-Fluidics**

### Mohammadreza Riahi

*Shahid Beheshti University/Laser and Plasma Research Institute Iran* 

#### **1. Introduction**

306 CO2 Laser – Optimisation and Application

Zhao, H., DebRoy, T. (2001). Pore formation during laser beam welding of die cast

pp. 423-432.

204S–210S.

high purity titanium using a Taylor-type crystal plasticity. *Acta Materialia*, Vol.55,

magnesium alloy AM60B – mechanism and remedy*. Weld. J*., Vol. 80, Issue 8, pp.

Microfluidic chips have attracted significant attention over the past decade due to their wide range of potential applications in the biomedical and chemical analysis field such as drug delivery, Point of care diagnostics (Jakeway et al, 2004), flow cytometry (Fu LM et al 2004; Chen & Wang, 2009; Lin et al, 2009), polymerize chain reaction (Suna et al, 2007; Sun & Kwok, 2006; Hsieh et al, 2009), electrophoresis (Fu et al, 2007, 2009) and many other applications.

Traditionally, silicon and glass are the predominant materials employed in the design of microfluidic systems. This was primarily due to their excellent chemical, physical, electrical and optical properties. But fabrication of a microfluidic device on these materials needs standard photolithography equipments such as Reactive Ion Etching (RIE) system which are very expensive and increases the production costs specially in single-use applications which are desired in order to avoid contamination.

In recent years application of polymeric materials for microfluidic device fabrication is becoming more and more important. Different methods for microfluidic fabrication on polymers such as hot embossing (Gerlach et al, 2002), injection molding (Rotting et al, 2002), soft lithography (Xia et al, 1998) and laser micromachining can be applied.

Different kind of lasers such as UV (Ball et al, 2000) and Infrared lasers is used for laser micromachining of polymers. In infrared regime, CO2 laser has a predominant application due to it's excellent absorption in polymers.

In this chapter, we will deal with application of a CO2 laser in microfluidic device fabrication. The application of CO2 laser for fabrication of a optofluidic device and application of a optofluidic device for CO2 laser characterization is also presented.

#### **2. Interaction of a CO2 laser with polymers**

Application of the CO2 laser for microfluidic device fabrication was first proposed in 2002 by H. Klank et al (Klank et al, 2002).

CO2 laser emits radiation with the wavelength of 10.6 micrometer. A CO2 laser mostly interacts with a polymer, photo-thermally. When a CO2 laser is irradiated on a polymer surface, it is strongly absorbed and raises the temperature of the polymer. The polymer is then melted, decomposed and leaving a void in a workpiece.

CO2 Laser and Micro-Fluidics 309

Cheng et. al. reported that the roughness of the machined channels can be treated by thermal annealing of the samples (Cheng et al 2004). Fig. 2. shows the surface of their work

Hong et. al. also reported that the roughness of the microfluidic structures can be drastically

Fig. 2. The SEM pictures showing the rugged interior surface of the trench after laser machining (a) and smooth surface after thermal annealing (b). The AFM topography of the annealed surface is shown in the inset with full scale of 38 nm in the *Z*-axes. The viewing angle is perpendicular to the plane of the side wall (Cheng et al 2004) - Reproduced by

PMMA micro fluidic structures can then be top covered by other polymers like PMMA or poly carbonate (PC) utilizing thermal-bonding process. Thermal bonding is a process of joining two materials by the mechanism of diffusion; and unity of the materials. This process is accomplished through the application of pressure at temperature higher that the

permission of Elsevier under the license no. 283342005275.

glass temperature of the polymers.

reduced by out of focus machining of PMMA (Hong, et al, 2010).

piece before and after annealing.

Different kind of polymers can be used for microfluidic applications by taking a choice care that the fluid in the device do not interact chemically with the device. However just some of the polymers can be machined with CO2 laser. Most of the polymers leave contamination and soot when exposed to CO2 laser irradiation. For example, polycarbonate (PC) leaves a brownish residue after exposing to the CO2 laser.

Among different kind of polymers, poly methyl methacrylate (PMMA) is the most suitable polymer for CO2 laser machining. When PMMA heats up by the CO2 laser, after passing the glass temperature, the material turns into a rubbery material and by increasing the temperature, the chains are broken by depropagation process (Arisawa & Brill, 1997; Ferriol et al, 2003), and decompose with a non-charring process to it's MMA monomer which is volatile

$$Solids \to Volattices\tag{1}$$

Fig. 1 shows the decomposition process of PMMA polymer.

Fig. 1. Decomposition process of PMMA.

Decomposition of PMMA into volatile MMA monomers makes a hole in the workpiece. The shape and size of the hole, highly depends on the thermal properties of PMMA, focusing parameters, laser beam profile, exposure time and even exposure strategy. At the beginning of the exposure, the shape of the channel is very similar to the laser beam profile but as time goes up, the shape of the hole becomes more conical.

#### **3. Fabrication of a channel on PMMA utilizing a CO2 laser**

Fabrication of a channel on the surface of PMMA can be performed by scanning a CO2 laser over the surface of the workpiece. Commercial CO2 engraving systems with laser powers about a few watts to a few tens of watts and scanning speeds from a few tens of mm/sec up to a few hundreds of mm/sec, are good choices for micro channel fabrication.

By scanning the PMMA surface with CO2 laser, different channels and cavities can be fabricated. However, the ablated structures may be very rugged such that those can not be used for microfluidic structures with optical detection. Martin et al. reported that the roughness of the machined surfaces depends on the grade of the PMMA sheets. He reported roughness of 1.54 microns and 0.42 microns for two different grades of PMMA (Martin et al, 2003). Presence of the different roughness should probably be sought in the chemical additives of the different types of PMMA.

Different kind of polymers can be used for microfluidic applications by taking a choice care that the fluid in the device do not interact chemically with the device. However just some of the polymers can be machined with CO2 laser. Most of the polymers leave contamination and soot when exposed to CO2 laser irradiation. For example, polycarbonate (PC) leaves a

Among different kind of polymers, poly methyl methacrylate (PMMA) is the most suitable polymer for CO2 laser machining. When PMMA heats up by the CO2 laser, after passing the glass temperature, the material turns into a rubbery material and by increasing the temperature, the chains are broken by depropagation process (Arisawa & Brill, 1997; Ferriol et al, 2003), and decompose with a non-charring process to it's MMA monomer which is volatile

Decomposition of PMMA into volatile MMA monomers makes a hole in the workpiece. The shape and size of the hole, highly depends on the thermal properties of PMMA, focusing parameters, laser beam profile, exposure time and even exposure strategy. At the beginning of the exposure, the shape of the channel is very similar to the laser beam profile but as time

Fabrication of a channel on the surface of PMMA can be performed by scanning a CO2 laser over the surface of the workpiece. Commercial CO2 engraving systems with laser powers about a few watts to a few tens of watts and scanning speeds from a few tens of mm/sec up

By scanning the PMMA surface with CO2 laser, different channels and cavities can be fabricated. However, the ablated structures may be very rugged such that those can not be used for microfluidic structures with optical detection. Martin et al. reported that the roughness of the machined surfaces depends on the grade of the PMMA sheets. He reported roughness of 1.54 microns and 0.42 microns for two different grades of PMMA (Martin et al, 2003). Presence of the different roughness should probably be sought in the chemical

*Solids Volatiles* → (1)

brownish residue after exposing to the CO2 laser.

Fig. 1. Decomposition process of PMMA.

additives of the different types of PMMA.

goes up, the shape of the hole becomes more conical.

**3. Fabrication of a channel on PMMA utilizing a CO2 laser** 

to a few hundreds of mm/sec, are good choices for micro channel fabrication.

Fig. 1 shows the decomposition process of PMMA polymer.

Cheng et. al. reported that the roughness of the machined channels can be treated by thermal annealing of the samples (Cheng et al 2004). Fig. 2. shows the surface of their work piece before and after annealing.

Hong et. al. also reported that the roughness of the microfluidic structures can be drastically reduced by out of focus machining of PMMA (Hong, et al, 2010).

Fig. 2. The SEM pictures showing the rugged interior surface of the trench after laser machining (a) and smooth surface after thermal annealing (b). The AFM topography of the annealed surface is shown in the inset with full scale of 38 nm in the *Z*-axes. The viewing angle is perpendicular to the plane of the side wall (Cheng et al 2004) - Reproduced by permission of Elsevier under the license no. 283342005275.

PMMA micro fluidic structures can then be top covered by other polymers like PMMA or poly carbonate (PC) utilizing thermal-bonding process. Thermal bonding is a process of joining two materials by the mechanism of diffusion; and unity of the materials. This process is accomplished through the application of pressure at temperature higher that the glass temperature of the polymers.

CO2 Laser and Micro-Fluidics 311

When a shape, is engraved in a raster scan mode by a CO2 laser engraving system on PMMA, the system scans a shape, row by row which each row has a certain overlap with a previous row. During the first row scan, a symmetrical V-shape channel is ablated on the PMMA surface. When the laser scans the subsequent rows, a small portion of the laser beam reflects from the wall of the channel produced by the previous scan to the bottom of the hole in the opposite side of the scanning direction as shown in Fig. 4a. After several scans, the reflected beam can ablate a considerable amount of PMMA material at the bottom of the hole at the opposite side of the scanning direction which can causes a bending shape in the

Fig. 4. Ablation of a PMMA hole with CO2 laser. a) Reflection of the laser from the walls of

It is found that the shape of the holes can be controlled by adjusting the scanning parameters such as resolution, power and scan speed. Some of the fabricated holes have very bent shapes and some are straight. Fig. 5 shows the ablated bending holes for different

an ablated hole. b) The shape of the hole after several scans (Riahi, 2012).

Fig. 5. The ablated bending holes for different scan parameters (Riahi, 2012).

**4.1 Fabrication of the bending cones** 

structure (Fig. 4b).

scan parameters.

In addition to fabrication of the holes, channels and cavities, CO2 laser machining can be used to make some complicated structures, like bending holes. These structures can also be molded with other materials such as PDMS to get the negative of the PMMA structures. In the next section fabrication technique of the other complicated structure with 3D structure is presented.

#### **4. Fabrication of a 3D Mixer with CO2 laser machining of PMMA and PDMS molding**

In this section we present application of a CO2 laser for fabrication of a 3D mixer with bending cones (Riahi, 2012). Mixers are the elements in microfluidic and micro total analysis systems which are used for mixing two or more liquids in biological and chemical analyses. Mixers can be divided into the two categories, active and passive. In active mixers, an external actuation mechanism is used to mix liquids in a microfluidic chamber. In passive mixers, there is no energy consumption and the structure of these devices is simpler than that of active devices. Different schemes such as a Tesla structure (Hong et al, 2004), a T mixer (Hoe et al, 2004), a 3D serpentine (Liu et al, 2000) and twisted shapes (Bertsch et al, 2001) are also used in passive micro mixers.

The technique which is presented here is based on the application of the CO2 laser for fabrication of some bending and straight cones on PMMA followed by PDMS molding. The designed mixer is shown in Fig. 3.

Fig. 3. Schematic of the designed 3D mixer (Riahi, 2012).

In addition to fabrication of the holes, channels and cavities, CO2 laser machining can be used to make some complicated structures, like bending holes. These structures can also be molded with other materials such as PDMS to get the negative of the PMMA structures. In the next section fabrication technique of the other complicated structure with 3D structure is

**4. Fabrication of a 3D Mixer with CO2 laser machining of PMMA and PDMS** 

In this section we present application of a CO2 laser for fabrication of a 3D mixer with bending cones (Riahi, 2012). Mixers are the elements in microfluidic and micro total analysis systems which are used for mixing two or more liquids in biological and chemical analyses. Mixers can be divided into the two categories, active and passive. In active mixers, an external actuation mechanism is used to mix liquids in a microfluidic chamber. In passive mixers, there is no energy consumption and the structure of these devices is simpler than that of active devices. Different schemes such as a Tesla structure (Hong et al, 2004), a T mixer (Hoe et al, 2004), a 3D serpentine (Liu et al, 2000) and twisted shapes (Bertsch et al,

The technique which is presented here is based on the application of the CO2 laser for fabrication of some bending and straight cones on PMMA followed by PDMS molding. The

presented.

**molding** 

2001) are also used in passive micro mixers.

Fig. 3. Schematic of the designed 3D mixer (Riahi, 2012).

designed mixer is shown in Fig. 3.

#### **4.1 Fabrication of the bending cones**

When a shape, is engraved in a raster scan mode by a CO2 laser engraving system on PMMA, the system scans a shape, row by row which each row has a certain overlap with a previous row. During the first row scan, a symmetrical V-shape channel is ablated on the PMMA surface. When the laser scans the subsequent rows, a small portion of the laser beam reflects from the wall of the channel produced by the previous scan to the bottom of the hole in the opposite side of the scanning direction as shown in Fig. 4a. After several scans, the reflected beam can ablate a considerable amount of PMMA material at the bottom of the hole at the opposite side of the scanning direction which can causes a bending shape in the structure (Fig. 4b).

Fig. 4. Ablation of a PMMA hole with CO2 laser. a) Reflection of the laser from the walls of an ablated hole. b) The shape of the hole after several scans (Riahi, 2012).

It is found that the shape of the holes can be controlled by adjusting the scanning parameters such as resolution, power and scan speed. Some of the fabricated holes have very bent shapes and some are straight. Fig. 5 shows the ablated bending holes for different scan parameters.

Fig. 5. The ablated bending holes for different scan parameters (Riahi, 2012).

CO2 Laser and Micro-Fluidics 313

The molded PDMS structures are then stacked to each other and three steel tubes are inserted into the input and output channels and the voids are filled with PDMS to form the

Optofluidics refers to a science that uses the optical property of fluids for adjusting, measuring the properties of a device. Some examples of such devices are, liquid mirrors (Wood, 1909), liquid-crystal displays (Haas, 1983) and liquid lenses (Kuiper & Hendriks,

Several techniques are used to fabricate a tunable lens array (Dong et al, 2006; Jeong et al,

In this section we show how a CO2 laser can be used for fabrication of an optofluidic device,

The liquid microlens array is an array of tunable liquid lenses which can be used for Medical

Fig. 8. shows the basic structure of the liquid lens array which is presented here. An array of the hexagonal holes with about 2mm width each, are first fabricated on a 1mm thick PMMA sheet. A thin layer of PDMS with the thickness of about 50 microns is fabricated and placed on the array of holes. A 1mm depth reservoir with an inlet and outlet for the fluid is also

By introducing a liquid into the reservoir and changing the pressure inside, the curvature of the PDMS sheet in place of the holes changes and produces convex lenses as shown in

stereoendoscopy, Telecommunication, Optical data storage, Photonic imaging, etc.

fabricated. The whole of the collection are placed on top of each other.

Fig. 8. The structure of a tunable liquid lens array.

final mixer structure. The fabricated mixer is shown in Fig. 7.

2004).

Fig. 9.

2004; Xu et al 2009)

liquid micro lens array (Riahi, 2011).

**5. Fabrication of the structures for optofluidics applications** 

#### **4.2 Ablation of the mixer structure**

To fabricate the mixer, a few straight cones and bending cones are ablated with CO2 laser on two different PMMA sheets. One of the PMMA sheets is CO2 laser cut to form a channel with two inputs and one output. The structures are then molded with PDMS and one is placed upside down on top of the other. Fig. 6. shows the fabricated channels and holes on the PMMA sheet and the molded PDMS structure.

Fig. 6. a) Fabricated structures on PMMA sheets. b) The PDMS molds of structures shown in part a. The straight and bending cones are clear (Riahi, 2012).

Fig. 7. The fabricated mixer (Riahi, 2012).

To fabricate the mixer, a few straight cones and bending cones are ablated with CO2 laser on two different PMMA sheets. One of the PMMA sheets is CO2 laser cut to form a channel with two inputs and one output. The structures are then molded with PDMS and one is placed upside down on top of the other. Fig. 6. shows the fabricated channels and holes on

Fig. 6. a) Fabricated structures on PMMA sheets. b) The PDMS molds of structures shown in

part a. The straight and bending cones are clear (Riahi, 2012).

Fig. 7. The fabricated mixer (Riahi, 2012).

**4.2 Ablation of the mixer structure** 

the PMMA sheet and the molded PDMS structure.

The molded PDMS structures are then stacked to each other and three steel tubes are inserted into the input and output channels and the voids are filled with PDMS to form the final mixer structure. The fabricated mixer is shown in Fig. 7.

### **5. Fabrication of the structures for optofluidics applications**

Optofluidics refers to a science that uses the optical property of fluids for adjusting, measuring the properties of a device. Some examples of such devices are, liquid mirrors (Wood, 1909), liquid-crystal displays (Haas, 1983) and liquid lenses (Kuiper & Hendriks, 2004).

Several techniques are used to fabricate a tunable lens array (Dong et al, 2006; Jeong et al, 2004; Xu et al 2009)

In this section we show how a CO2 laser can be used for fabrication of an optofluidic device, liquid micro lens array (Riahi, 2011).

The liquid microlens array is an array of tunable liquid lenses which can be used for Medical stereoendoscopy, Telecommunication, Optical data storage, Photonic imaging, etc.

Fig. 8. shows the basic structure of the liquid lens array which is presented here. An array of the hexagonal holes with about 2mm width each, are first fabricated on a 1mm thick PMMA sheet. A thin layer of PDMS with the thickness of about 50 microns is fabricated and placed on the array of holes. A 1mm depth reservoir with an inlet and outlet for the fluid is also fabricated. The whole of the collection are placed on top of each other.

Fig. 8. The structure of a tunable liquid lens array.

By introducing a liquid into the reservoir and changing the pressure inside, the curvature of the PDMS sheet in place of the holes changes and produces convex lenses as shown in Fig. 9.

CO2 Laser and Micro-Fluidics 315

Fig. 11. The fabricated tunable liquid lens array.

Fig. 12. Imaging from the letter "B" with the fabricated liquid lens array.

Fig. 9. Mechanism of convex micro lens creation by applying pressure in a water reservoir limited by PDMS and PMMA walls.

As shown in Fig. 10, a commercial CO2 laser engraving system is used for producing the patterns on PMMA sheets. This engraving machine is also used for fabrication of the reservoir on PMMA sheets.

Fig. 10. The commercial CO2 laser engraving system in production process of an array of hexagonal holes on a PMMA sheet.

Fig. 11 shows the fabricated tunable microlens array with this technique. Fig. 12 also shows this microlens array in imaging from a "B" letter.

Fig. 9. Mechanism of convex micro lens creation by applying pressure in a water reservoir

As shown in Fig. 10, a commercial CO2 laser engraving system is used for producing the patterns on PMMA sheets. This engraving machine is also used for fabrication of the

Fig. 10. The commercial CO2 laser engraving system in production process of an array of

Fig. 11 shows the fabricated tunable microlens array with this technique. Fig. 12 also shows

limited by PDMS and PMMA walls.

hexagonal holes on a PMMA sheet.

this microlens array in imaging from a "B" letter.

reservoir on PMMA sheets.

Fig. 11. The fabricated tunable liquid lens array.

Fig. 12. Imaging from the letter "B" with the fabricated liquid lens array.

CO2 Laser and Micro-Fluidics 317

normalized intensity

normalized intensity

normalized intensity

Fig. 13. (a) Square-well grating with n1 and n2 for the refractive indices of the land and the groove. (b)Wavefront of an incoming ray immediately after passing through the grating. (c), (d), (e) Simulation results of diffraction from the grating shown in (a) for γ = 0, γ = π=2, and γ = π, respectively. On the vertical axes, the maximum intensity has been normalized to unity. (f) Results of simulation of the intensity of the 1st order of diffraction versus phase

difference (Riahi et al, 2008).

#### **6. Fabrication of a beam profiler using the optical properties of liquids**

In the previous sections we focused on the application of the CO2 laser for fabrication of the devices used in microfluidics and optofluidics. In this section we look at the application of a fluid device which is used for CO2 laser characterization. We present a device called thermally tunable grating (TTG), which can be used as a CO2 laser beam profiler.

Thermally tunable grating is a family of the gratings which some of their specifications can be adjusted by the user. The tuning ability of a diffractive grating can be divided into two categories: first, gratings in which the diffractive angle can be tuned, and second, gratings in which the intensity of diffraction orders can be modulated which are called grating light valves (GLV). Electrostatic actuation is one of the methods used in MEMS based grating light valves system (Trisnadi et al, 2004). In this grating light valve system, tiny suspended ribbons are put together to form a specular surface. Electrostatic actuation lowers some of the ribbons, and a diffractive grating is formed. Electric field actuation has also been used to actuate an electro-optically controlled liquid crystal based GLV (Chen et al, 1995).

But in TTG device, thermal method is used for actuation of a grating which contains a liquid in it's grooves. Increasing temperature, changes the refractive index of the liquid and consequently the diffraction efficiency of the grating (Riahi et al, 2008).

#### **6.1 Principle of the method**

As shown in Fig. 13a, we suppose that the grooves of a transparent binary grating with refractive index n1 are filled with another transparent material with refractive index n2. Assume that a laser beam with wavelength λ is incident on this grating. If the period of the grating is large enough compared to the wavelength of light, the rays that pass through the n1 and n2 materials will have phases φ1 and φ2, respectively and the phase difference Δφ= φ1- φ2 as shown in Fig. 13b.

By changing Δφ, the intensity of the diffraction orders is changed as shown in Fig. 13c,d,e. It can be shown that the intensity of the first order of diffraction can be calculated as follow (Riahi et al, 2009):

$$I = I\_{\text{max}} S \dot{m}^2(\frac{\Delta \varphi}{2}) \tag{2}$$

It is clear now that if n1 or n2 are changed, the intensity of the first order of diffraction changes sinusoidally (Fig. 13f).

#### **6.2 Fabrication method**

Standard lithography technique is used for fabrication of the binary grating on a glass substrate (n=1.52). As shown in Fig 14, the grooves are then filled with nitrobenzene and a thin glass sheet with 250 microns thickness is placed on it. The high boiling point (T= 210:8 °C), low specific heat capacity (1:51 J/gK), and high dn/dT (−4:6 × 10−4 K−1 at 626.58 at T= 288 K) [36] make nitrobenzene suitable for this work. The refractive index of nitrobenzene is 1.546 at 656:28nm at 293:15 K.

In the previous sections we focused on the application of the CO2 laser for fabrication of the devices used in microfluidics and optofluidics. In this section we look at the application of a fluid device which is used for CO2 laser characterization. We present a device called

Thermally tunable grating is a family of the gratings which some of their specifications can be adjusted by the user. The tuning ability of a diffractive grating can be divided into two categories: first, gratings in which the diffractive angle can be tuned, and second, gratings in which the intensity of diffraction orders can be modulated which are called grating light valves (GLV). Electrostatic actuation is one of the methods used in MEMS based grating light valves system (Trisnadi et al, 2004). In this grating light valve system, tiny suspended ribbons are put together to form a specular surface. Electrostatic actuation lowers some of the ribbons, and a diffractive grating is formed. Electric field actuation has also been used to

But in TTG device, thermal method is used for actuation of a grating which contains a liquid in it's grooves. Increasing temperature, changes the refractive index of the liquid and

As shown in Fig. 13a, we suppose that the grooves of a transparent binary grating with refractive index n1 are filled with another transparent material with refractive index n2. Assume that a laser beam with wavelength λ is incident on this grating. If the period of the grating is large enough compared to the wavelength of light, the rays that pass through the n1 and n2 materials will have phases φ1 and φ2, respectively and the phase difference Δφ=

By changing Δφ, the intensity of the diffraction orders is changed as shown in Fig. 13c,d,e. It can be shown that the intensity of the first order of diffraction can be calculated as follow

> 2 max ( ) <sup>2</sup> *I I Sin* <sup>Δ</sup>

It is clear now that if n1 or n2 are changed, the intensity of the first order of diffraction

Standard lithography technique is used for fabrication of the binary grating on a glass substrate (n=1.52). As shown in Fig 14, the grooves are then filled with nitrobenzene and a thin glass sheet with 250 microns thickness is placed on it. The high boiling point (T= 210:8 °C), low specific heat capacity (1:51 J/gK), and high dn/dT (−4:6 × 10−4 K−1 at 626.58 at T= 288 K) [36] make nitrobenzene suitable for this work. The refractive index of nitrobenzene is

ϕ

= (2)

**6. Fabrication of a beam profiler using the optical properties of liquids** 

thermally tunable grating (TTG), which can be used as a CO2 laser beam profiler.

actuate an electro-optically controlled liquid crystal based GLV (Chen et al, 1995).

consequently the diffraction efficiency of the grating (Riahi et al, 2008).

**6.1 Principle of the method** 

φ1- φ2 as shown in Fig. 13b.

changes sinusoidally (Fig. 13f).

1.546 at 656:28nm at 293:15 K.

**6.2 Fabrication method** 

(Riahi et al, 2009):

Fig. 13. (a) Square-well grating with n1 and n2 for the refractive indices of the land and the groove. (b)Wavefront of an incoming ray immediately after passing through the grating. (c), (d), (e) Simulation results of diffraction from the grating shown in (a) for γ = 0, γ = π=2, and γ = π, respectively. On the vertical axes, the maximum intensity has been normalized to unity. (f) Results of simulation of the intensity of the 1st order of diffraction versus phase difference (Riahi et al, 2008).

CO2 Laser and Micro-Fluidics 319

Fig. 15. Diffraction order intensities at different temperatures: (a) T = 77 °C, (b) T = 108 °C, and (c) T = 140 °C. The maximum intensity is normalized to unity. (d) Experimental result of the intensity of the 1st order of diffraction versus temperature. The maximum intensity is

Fig. 16. Setup used for measurement of the temperature profile of the CO2 laser (Riahi et al,

The Image produced on the CCD camera and measured beam profile of the CO2 laser is

normalized to unity (Riahi et al, 2008).

2008).

shown in Fig. 17.

The diffraction pattern and intensity of the first order of diffraction versus temperature has been presented in Fig. 15.

#### **6.3 Measurement of the beam profile of a CO2 laser**

By changing the temperature, the intensity of the 1st order of diffraction is changed. The temperature of the TTG changes upon radiation by a CO2 laser beam. Radiation of a CO2 laser beam on a substrate warms it up and produces a temperature profile on the surface of the substrate. The temperature profile depends on the intensity profile of the laser beam. For example, if the laser profile is circular Gaussian, the temperature profile on the surface will be circular Gaussian in ideal case. Now if another visible laser is expanded and diffracted from the surface of the grating, the laser will be diffracted in different amounts from different parts of the grating, containing information on the temperature profile on the grating.

The setup shown in Fig. 16 is used to measure the beam profile of a CO2 laser. In this setup, a CO2 laser and a 658nm diode laser are made collinear with each other using a ZnSe window, and finally both lasers are irradiated on a 4mm × 4mm TTG device. The diode laser is expanded to about 3 cm diameter to cover the 4mm × 4mm TTG device with uniform intensity. The CO2 laser is passed through a shutter so that the irradiation time can be controlled. Immediately after the CO2 laser pulse, the CCD camera takes a picture from diffracted diode laser by a 4f imaging system using the 1st order of diffraction. It takes about 1 min for the device to get cool enough to repeat the experiment. The heat gun shown in Fig. 16. is used to keep the working area between point A and B as specified in Fig. 15d.

Fig. 14. Fabrication of the TTG device: (a) the grooves of the grating are filled with nitrobenzene and (b) a supporting glass is placed on the device (Riahi et al, 2008).

The diffraction pattern and intensity of the first order of diffraction versus temperature has

By changing the temperature, the intensity of the 1st order of diffraction is changed. The temperature of the TTG changes upon radiation by a CO2 laser beam. Radiation of a CO2 laser beam on a substrate warms it up and produces a temperature profile on the surface of the substrate. The temperature profile depends on the intensity profile of the laser beam. For example, if the laser profile is circular Gaussian, the temperature profile on the surface will be circular Gaussian in ideal case. Now if another visible laser is expanded and diffracted from the surface of the grating, the laser will be diffracted in different amounts from different parts of the grating, containing information on the temperature profile on the

The setup shown in Fig. 16 is used to measure the beam profile of a CO2 laser. In this setup, a CO2 laser and a 658nm diode laser are made collinear with each other using a ZnSe window, and finally both lasers are irradiated on a 4mm × 4mm TTG device. The diode laser is expanded to about 3 cm diameter to cover the 4mm × 4mm TTG device with uniform intensity. The CO2 laser is passed through a shutter so that the irradiation time can be controlled. Immediately after the CO2 laser pulse, the CCD camera takes a picture from diffracted diode laser by a 4f imaging system using the 1st order of diffraction. It takes about 1 min for the device to get cool enough to repeat the experiment. The heat gun shown in Fig. 16. is used to keep the working area between point A and B as specified

Fig. 14. Fabrication of the TTG device: (a) the grooves of the grating are filled with nitrobenzene and (b) a supporting glass is placed on the device (Riahi et al, 2008).

been presented in Fig. 15.

grating.

in Fig. 15d.

**6.3 Measurement of the beam profile of a CO2 laser** 

Fig. 15. Diffraction order intensities at different temperatures: (a) T = 77 °C, (b) T = 108 °C, and (c) T = 140 °C. The maximum intensity is normalized to unity. (d) Experimental result of the intensity of the 1st order of diffraction versus temperature. The maximum intensity is normalized to unity (Riahi et al, 2008).

Fig. 16. Setup used for measurement of the temperature profile of the CO2 laser (Riahi et al, 2008).

The Image produced on the CCD camera and measured beam profile of the CO2 laser is shown in Fig. 17.

CO2 Laser and Micro-Fluidics 321

problem for real time measurement. In this part, a thermally tunable grating with fast response time is presented, which makes the real time measurements feasible (Riahi &

The principle of this method is the same as what was mentioned in the previous section except that the device becomes a reflective instead of transitive and a thin supporting glass in the device is replaced by a double side polished silicon wafer. The silicon wafer plays the role of a reflector at 532 nm (40% of reflection) and also as an optical window for the CO2 laser. But the most important characteristic of the silicon is it's high thermal diffusivity. The

However, silicon can plays a role of a heat sink during the measurements and maked the

Fig. 18. Schematic setup for real time measurement of the CO2 laser beam profile (Riahi &

To measure the beam profile of a CO2 laser, a setup as shown in Fig. 18 was used. In this setup, a CO2 laser beam incident on the grating device from the silicon side is absorbed in the grating structure and warms it up. A 532 nm laser is expanded and irradiates the grating, from the glass side. After passing through the grating, the visible light reflects back from the silicon slab and is directed to a 4*f* imaging system. A high pass spatial filter is used

The response time of this system can be measured. For this reason the same setup as in Fig. 18 is used, except that a chopper is placed in front of the CO2 laser and a fast photo-detector is used instead of the CCD camera. By chopping the CO2 laser beam, the signal of the photodetector was monitored by an oscilloscope. As seen in Fig. 19, the response time of this

*Si* = (cm^/sec). It has to be mentioned that the thermal

α

*cu* = (cm^2/sec) which is

Latifi, 2011).

Latifi, 2011)

thermal diffusivity of silicon is 0.95

to keep the first order of diffraction for imaging.

device is about 10 milliseconds.

just a bit more than for silicon.

real time measurements feasible.

α

diffusivity of copper which is used as a very good heat sink is 1.1

Fig. 17. (a) Image produced on the CCD camera. (b) 3D intensity profile of "a" will be the same as the beam profile of the CO2 laser (Riahi et al, 2008).

The followings are some errors presented in this experiment.


Some of these errors are so small to affect the beam profile, but some of them might be important and have to be corrected.

#### **7. Real time measurement of the CO2 laser beam profile utilizing TTG**

In method we presented in the previous section, after each measurement, time had to be taken for the grating to cool down and get ready for another measurement. This can be a big

Fig. 17. (a) Image produced on the CCD camera. (b) 3D intensity profile of "a" will be the


Some of these errors are so small to affect the beam profile, but some of them might be

In method we presented in the previous section, after each measurement, time had to be taken for the grating to cool down and get ready for another measurement. This can be a big

**7. Real time measurement of the CO2 laser beam profile utilizing TTG** 

same as the beam profile of the CO2 laser (Riahi et al, 2008).

The followings are some errors presented in this experiment.

environmental errors



important and have to be corrected.

problem for real time measurement. In this part, a thermally tunable grating with fast response time is presented, which makes the real time measurements feasible (Riahi & Latifi, 2011).

The principle of this method is the same as what was mentioned in the previous section except that the device becomes a reflective instead of transitive and a thin supporting glass in the device is replaced by a double side polished silicon wafer. The silicon wafer plays the role of a reflector at 532 nm (40% of reflection) and also as an optical window for the CO2 laser. But the most important characteristic of the silicon is it's high thermal diffusivity. The thermal diffusivity of silicon is 0.95 α*Si* = (cm^/sec). It has to be mentioned that the thermal diffusivity of copper which is used as a very good heat sink is 1.1 α*cu* = (cm^2/sec) which is just a bit more than for silicon.

However, silicon can plays a role of a heat sink during the measurements and maked the real time measurements feasible.

Fig. 18. Schematic setup for real time measurement of the CO2 laser beam profile (Riahi & Latifi, 2011)

To measure the beam profile of a CO2 laser, a setup as shown in Fig. 18 was used. In this setup, a CO2 laser beam incident on the grating device from the silicon side is absorbed in the grating structure and warms it up. A 532 nm laser is expanded and irradiates the grating, from the glass side. After passing through the grating, the visible light reflects back from the silicon slab and is directed to a 4*f* imaging system. A high pass spatial filter is used to keep the first order of diffraction for imaging.

The response time of this system can be measured. For this reason the same setup as in Fig. 18 is used, except that a chopper is placed in front of the CO2 laser and a fast photo-detector is used instead of the CCD camera. By chopping the CO2 laser beam, the signal of the photodetector was monitored by an oscilloscope. As seen in Fig. 19, the response time of this device is about 10 milliseconds.

CO2 Laser and Micro-Fluidics 323

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Fig. 19. Detected signals from photo-diode when CO2 laser is chopped off and on

#### **8. Conclusion**

In this chapter, the relation between CO2 laser and fluid applications was presented. First, application of a CO2 laser for fabrication of microfluidic and optofluidic structures on PMMA polymer was presented. Then application of a fluidic device for measurement of a characteristic of a CO2 laser was discussed.

Application of the CO2 laser for microfluidic fabrication is a simple and low cost method which can be performed by a commercial CO2 laser engraving system. This method makes the final products very cheap which are suitable for single use applications.

Also it seems that the application of the CO2 laser in microfluidics shows a good potential for fabrication of some complicated structures even 3D structures for future works.

#### **9. References**


Fig. 19. Detected signals from photo-diode when CO2 laser is chopped off and on

the final products very cheap which are suitable for single use applications.

for fabrication of some complicated structures even 3D structures for future works.

Micromachining *Anal. Chem* , Vol. 72, pp. 497–501, ISSN 0003-2700.

In this chapter, the relation between CO2 laser and fluid applications was presented. First, application of a CO2 laser for fabrication of microfluidic and optofluidic structures on PMMA polymer was presented. Then application of a fluidic device for measurement of a

Application of the CO2 laser for microfluidic fabrication is a simple and low cost method which can be performed by a commercial CO2 laser engraving system. This method makes

Also it seems that the application of the CO2 laser in microfluidics shows a good potential

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**8. Conclusion** 

**9. References** 

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characteristic of a CO2 laser was discussed.


**13** 

*Portugal* 

Rui F. M. Lobo1,2

**Infrared Lasers in Nanoscale Science** 

*Faculdade de Ciencias e Tecnologia - New University of Lisbon, Caparica, 2Institute for Science and Technology of Materials and Surfaces (ICEMS)* 

*1Group for Nanoscale Science and Nanotechnology (GNCN), Physics Department,* 

In the nearly half a century scientists have already realized that, just as Feynman predicted, there is plenty of research room at the bottom of the matter world in a tiny universe so small that new methods for viewing it are still being discovered. Actually, nanoscience and nanotechnology have evolved into a revolutionary area of technology-based research, opening the door to precise engineering on the atomic scale and affecting everything from healthcare to the environment. Nanoscience research and education lead to nanotechnology, the manipulation of nanometer-length atoms, molecules, and supramolecular structures in order to generate larger structures with superior features. Because all natural materials and systems exist at a nanoscale level, nanotechnology impacts a variety of scientific fundamental and applied disciplines, from physics to medicine and engineering. Nanomaterials consisting of nano-sized building blocks exhibit unique and often superior properties relatively to their bulk counterpart. Due to the fact that most of the novel properties of nanomaterials are size-dependent, synthesis methods leading to better control of size, distribution and chemical content of the nanoparticles are imperative in modern

On its turn, the laser has been one of the top applied physics inventions that played a significant role in many fields of science and technology. It has been used in tackling and solving many scientific and technological problems, including interesting applications in the field of nanotechnology, biotechnology/medicine, environment, material characterization,

There are several gaseous molecules which serve as good laser media and the majority of them are simple molecules which provide emission in the ultraviolet. Infrared molecular gas lasers fall into two general categories, namely the middle- and far-infrared lasers, which

The N2 laser is known as a pulse ultraviolet laser and in addition it covers some lines in the infrared up to 8,2 μm. Normally, the pulse width is a few nanoseconds and a high-voltage power supply of 30-40 kV is necessary to excite it. The HF is a high power chemical laser media with an emission wavelength of about 2,7 μm, a laser pulse of the order of μs in duration and the output energy ranges from 1 J to more than 1 kJ per pulse. The DF and HBr chemical lasers emit larger wavelengths than the HF laser, and their output power is lower [1,2].

occur on rotational-vibrational transitions or on pure rotational transitions.

**1. Introduction** 

nanotechnologies.

and energy.


### **Infrared Lasers in Nanoscale Science**

Rui F. M. Lobo1,2

*1Group for Nanoscale Science and Nanotechnology (GNCN), Physics Department, Faculdade de Ciencias e Tecnologia - New University of Lisbon, Caparica, 2Institute for Science and Technology of Materials and Surfaces (ICEMS) Portugal* 

#### **1. Introduction**

324 CO2 Laser – Optimisation and Application

Liu Robin H., Mark Stremler A., Kendra V. Sharp, Michael G. Olsen, Juan G. Santiago,

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3, pp. 302–307, ISSN (printed) 1473-0197. ISSN (electronic) 1473-0189. Riahi M. (2012), Fabrication of a passive 3D mixer using CO2 laser ablation of PMMA and PDMS moldings *Microchemical Journal,* Vol. 100, pp. 14–20, ISSN 0026-265X. Riahi M. (2011), Fabrication and characterization of a tunable liquid lens array in water-

(EOSOF2011) 23-25 may, Munich, Germany

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Light Valve based optical write engines for high-speed digital imaging, presented at Photonics West2004—Micromachining and Microfabrication Symposium, 26 In the nearly half a century scientists have already realized that, just as Feynman predicted, there is plenty of research room at the bottom of the matter world in a tiny universe so small that new methods for viewing it are still being discovered. Actually, nanoscience and nanotechnology have evolved into a revolutionary area of technology-based research, opening the door to precise engineering on the atomic scale and affecting everything from healthcare to the environment. Nanoscience research and education lead to nanotechnology, the manipulation of nanometer-length atoms, molecules, and supramolecular structures in order to generate larger structures with superior features. Because all natural materials and systems exist at a nanoscale level, nanotechnology impacts a variety of scientific fundamental and applied disciplines, from physics to medicine and engineering. Nanomaterials consisting of nano-sized building blocks exhibit unique and often superior properties relatively to their bulk counterpart. Due to the fact that most of the novel properties of nanomaterials are size-dependent, synthesis methods leading to better control of size, distribution and chemical content of the nanoparticles are imperative in modern nanotechnologies.

On its turn, the laser has been one of the top applied physics inventions that played a significant role in many fields of science and technology. It has been used in tackling and solving many scientific and technological problems, including interesting applications in the field of nanotechnology, biotechnology/medicine, environment, material characterization, and energy.

There are several gaseous molecules which serve as good laser media and the majority of them are simple molecules which provide emission in the ultraviolet. Infrared molecular gas lasers fall into two general categories, namely the middle- and far-infrared lasers, which occur on rotational-vibrational transitions or on pure rotational transitions.

The N2 laser is known as a pulse ultraviolet laser and in addition it covers some lines in the infrared up to 8,2 μm. Normally, the pulse width is a few nanoseconds and a high-voltage power supply of 30-40 kV is necessary to excite it. The HF is a high power chemical laser media with an emission wavelength of about 2,7 μm, a laser pulse of the order of μs in duration and the output energy ranges from 1 J to more than 1 kJ per pulse. The DF and HBr chemical lasers emit larger wavelengths than the HF laser, and their output power is lower [1,2].

Infrared Lasers in Nanoscale Science 327

ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO2 and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by

The CO2 laser transitions are the 961 cm-1 transition of the 10,4 μm band and the 1064 cm-1 transition of the 9,4 μm band. Owing to the symmetry of the CO2 molecule, laser transitions occur to lower energy levels whose rotational quantum numbers are even, resulting in more

In general, there are two common different types of CO2 laser configurations. In one of them (longitudinally excited laser), the CO2 laser is excited by direct current and when the pressure raised from 103 Pa to 104 Pa, a peak power is obtainable by using a pulsed discharge. This is an arc maintained by an anode and a cathode at the ends of a long discharge tube. Another possibility is the transversely excited atmospheric pressure laser (TEA), excited by an arc discharge at roughly atmospheric pressure [4]. The current in the arc flows at right angles to the axis of the laser [5]. A TEA laser is always pulsed and many CO2

Some CO2 TEA lasers have been developed with additional techniques enabling us to achieve tuneable wavelengths, and in particular may reach oscillation threshold for several atomic or molecular transitions. The laser can then simultaneously oscillate on these transitions. In order to reach single mode operation, one has to first select a single transition. Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule CO2, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Because transmissive materials in the infrared are rather lossy, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected.

 A procedure towards optimization performance of a CO2 pulsed tuneable laser was developed which allows the power and the energy to be optimized [5]. The MTL3-GT is a very compact grating tuneable TEA laser version (Figure 1), and represents a significant improvement in performance and portability [6]. Combining a pulse mode with a grating tuning facility, it enables us to scan the working wavelength between 9.2 and 10.8 μm (operating on more than 60 lines), with repetition rates ranging from single-shot to 200 Hz. The maximum energy for this version is 50 mJ/pulse on the strongest lines. The MTL3-GT CO2 infrared laser works with a a gas mixture (40% He : 30% CO2 : 30% N2) and a chiller for high repetition rates. Actually, above 20 Hz, the number of HV discharges increases and the

The finest frequency selection may also be obtained through the use of an etalon.

laser needs to be cooled down in order to lower the temperature in the optical cavity.

Following an adequate procedure, the energy values could be optimized in intensity and stability, and therefore indirectly laser power. In addition, the same procedure allows to check the wavelengths of the laser emission lines in the absence of a spectrometer, using a previously established conversion table of the grating position versus line designation. With such method, many experiments can be performed in real time with simultaneous control of

than 30 laser lines in each of the two branches P and R [1,2].

pumps.

lasers are TEA lasers.

The CO2 laser is a gas laser electrically pumped, that emits in the mid-infrared. It gives a cw output at 10 μm in the infrared with a high efficiency and it is the most practical molecular laser. There are a large number of CO2 lasers, varying in structure, method of excitation and capacity, which can provide hundreds of laser lines, the main ones being between 9 and 11 μm. The output power of even a small CO2 laser is about 1 kW and large ones give over 10 kW. The usual way of obtaining single-line oscillation is to use a diffraction grating in conjunction with a laser resonator. If only mirrors are used, simultaneous oscillation on several lines in the neighborhood of 10,6 μm is commonly obtained [1,2]. Transverse excited atmosphere (TEA) CO2 lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures, and are suitable for average powers of tens of kilowatts [1,2].

Although N2O and CO laser have a lower output power than the CO2 laser, they have about one hundred laser lines each in the ranges 10-11 μm and 5-6,5 μm, respectivelly (considering the main isotopic species). The molecules NH3, OCS, CS also have quite a few laser lines in the infrared. With the SO2, HCN, H2O, many laser lines are obtained in the infrared from 30 μm up to submilimeter wavelengths [1,2].

Dye lasers are convenient tunable lasers in the visible but not so far in the infrared, mainly due to the lack of appropriate dyes, and in addition, since the dye laser medium is liquid, it is very inconvenient to handle.

In face of the real advantage of the laser as a very intense heating source that can be applied to a very small area, the most significant areas in which the CO2 laser has shown remarkable applications are in the general fields of materials processing and medical applications. This includes cutting, cauterizing, drilling, material removal, melting, welding, alloying, hardening, surgery, cancer treatment and so forth.

The carbon dioxide laser, invented by Patel [3], operates on rotational-vibrational transitions and is still one of the most useful among all the infrared molecular lasers. In general, it is one of the most powerful lasers currently available. It operates in the middle infrared wavelength region with the principal wavelength bands centering around 9.4 and 10.6 micrometers. It is also quite efficient, with a ratio of output power to pump power as large as 20%. It can operate at very high pressures because the energies of the upper laser levels are much closer to the ground state of the CO2 molecule than are energies of the upper laser levels of atomic lasers and so the electron temperature can be much lower, thereby allowing higher operating gas pressures. Higher operating gas pressures means a much greater population in the upper laser level per unit volume of the laser discharge and therefore much higher power output per unit volume of laser gain media. These lasers have produced cw powers of greater than 100 kW and pulsed energies of as much as 10 kJ. The gain occurs on a range of rotational-vibrational transitions that are dominated by either Doppler broadening or pressure broadening, depending upon the gas pressure.

Although laser radiation is obtainable with pure CO2 gas, the usual CO2 laser uses a mixture of He, N2 and CO2. The population inversion in the laser is achieved by a sequence of fundamental processes starting by vibrational excitation of nitrogen molecules in the electric discharge. The nitrogen molecules are left in a lower excited state and their transition to

The CO2 laser is a gas laser electrically pumped, that emits in the mid-infrared. It gives a cw output at 10 μm in the infrared with a high efficiency and it is the most practical molecular laser. There are a large number of CO2 lasers, varying in structure, method of excitation and capacity, which can provide hundreds of laser lines, the main ones being between 9 and 11 μm. The output power of even a small CO2 laser is about 1 kW and large ones give over 10 kW. The usual way of obtaining single-line oscillation is to use a diffraction grating in conjunction with a laser resonator. If only mirrors are used, simultaneous oscillation on several lines in the neighborhood of 10,6 μm is commonly obtained [1,2]. Transverse excited atmosphere (TEA) CO2 lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures, and are suitable for average

Although N2O and CO laser have a lower output power than the CO2 laser, they have about one hundred laser lines each in the ranges 10-11 μm and 5-6,5 μm, respectivelly (considering the main isotopic species). The molecules NH3, OCS, CS also have quite a few laser lines in the infrared. With the SO2, HCN, H2O, many laser lines are obtained in the infrared from 30

Dye lasers are convenient tunable lasers in the visible but not so far in the infrared, mainly due to the lack of appropriate dyes, and in addition, since the dye laser medium is liquid, it

In face of the real advantage of the laser as a very intense heating source that can be applied to a very small area, the most significant areas in which the CO2 laser has shown remarkable applications are in the general fields of materials processing and medical applications. This includes cutting, cauterizing, drilling, material removal, melting, welding, alloying,

The carbon dioxide laser, invented by Patel [3], operates on rotational-vibrational transitions and is still one of the most useful among all the infrared molecular lasers. In general, it is one of the most powerful lasers currently available. It operates in the middle infrared wavelength region with the principal wavelength bands centering around 9.4 and 10.6 micrometers. It is also quite efficient, with a ratio of output power to pump power as large as 20%. It can operate at very high pressures because the energies of the upper laser levels are much closer to the ground state of the CO2 molecule than are energies of the upper laser levels of atomic lasers and so the electron temperature can be much lower, thereby allowing higher operating gas pressures. Higher operating gas pressures means a much greater population in the upper laser level per unit volume of the laser discharge and therefore much higher power output per unit volume of laser gain media. These lasers have produced cw powers of greater than 100 kW and pulsed energies of as much as 10 kJ. The gain occurs on a range of rotational-vibrational transitions that are dominated by either Doppler

Although laser radiation is obtainable with pure CO2 gas, the usual CO2 laser uses a mixture of He, N2 and CO2. The population inversion in the laser is achieved by a sequence of fundamental processes starting by vibrational excitation of nitrogen molecules in the electric discharge. The nitrogen molecules are left in a lower excited state and their transition to

broadening or pressure broadening, depending upon the gas pressure.

powers of tens of kilowatts [1,2].

is very inconvenient to handle.

μm up to submilimeter wavelengths [1,2].

hardening, surgery, cancer treatment and so forth.

ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO2 and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.

The CO2 laser transitions are the 961 cm-1 transition of the 10,4 μm band and the 1064 cm-1 transition of the 9,4 μm band. Owing to the symmetry of the CO2 molecule, laser transitions occur to lower energy levels whose rotational quantum numbers are even, resulting in more than 30 laser lines in each of the two branches P and R [1,2].

In general, there are two common different types of CO2 laser configurations. In one of them (longitudinally excited laser), the CO2 laser is excited by direct current and when the pressure raised from 103 Pa to 104 Pa, a peak power is obtainable by using a pulsed discharge. This is an arc maintained by an anode and a cathode at the ends of a long discharge tube. Another possibility is the transversely excited atmospheric pressure laser (TEA), excited by an arc discharge at roughly atmospheric pressure [4]. The current in the arc flows at right angles to the axis of the laser [5]. A TEA laser is always pulsed and many CO2 lasers are TEA lasers.

Some CO2 TEA lasers have been developed with additional techniques enabling us to achieve tuneable wavelengths, and in particular may reach oscillation threshold for several atomic or molecular transitions. The laser can then simultaneously oscillate on these transitions. In order to reach single mode operation, one has to first select a single transition.

Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule CO2, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Because transmissive materials in the infrared are rather lossy, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected. The finest frequency selection may also be obtained through the use of an etalon.

 A procedure towards optimization performance of a CO2 pulsed tuneable laser was developed which allows the power and the energy to be optimized [5]. The MTL3-GT is a very compact grating tuneable TEA laser version (Figure 1), and represents a significant improvement in performance and portability [6]. Combining a pulse mode with a grating tuning facility, it enables us to scan the working wavelength between 9.2 and 10.8 μm (operating on more than 60 lines), with repetition rates ranging from single-shot to 200 Hz. The maximum energy for this version is 50 mJ/pulse on the strongest lines. The MTL3-GT CO2 infrared laser works with a a gas mixture (40% He : 30% CO2 : 30% N2) and a chiller for high repetition rates. Actually, above 20 Hz, the number of HV discharges increases and the laser needs to be cooled down in order to lower the temperature in the optical cavity.

Following an adequate procedure, the energy values could be optimized in intensity and stability, and therefore indirectly laser power. In addition, the same procedure allows to check the wavelengths of the laser emission lines in the absence of a spectrometer, using a previously established conversion table of the grating position versus line designation. With such method, many experiments can be performed in real time with simultaneous control of

Infrared Lasers in Nanoscale Science 329

unreliable, tedious and very sensitive to the rotation speed of the micrometer and stability of the energy signal. Thus, two acquisitions were made for each repetition rate, one by turning the micrometer clockwise and another counter clockwise. An average of them was calculated and definitive energy values were registered. This procedure was repeated in the opposite direction, in order to obtain an average, and also to confirm the reproducibility of the result. It was actually confirmed for every emission line and several values of repetition

Acquisitions recorded without concerns about the external factors and in different days revealed instability in energy values for each repetition rate measured (singleshot, 5 Hz, 10 Hz or 20 Hz), as displayed in Figure 2 (A). For the other three emission bands available (10R, 9P and 9R), the problem is also present. A variation in the power measured was observed in all possible cases, emission bands and repetition rates available. The values could vary from 1 mJ up to 5 mJ for energy and 10 mW to 50 mW for power (repetition rate was 10 Hz in this measurement). The final acquisitions were recorded taking into account the improved procedure regarding the verification of a correspondence between the micrometer drive readings and wavelengths. It can be observed in Figure 2 (B) that the energy values for each repetition rates available were smoother and without significant

Fig. 2. Relationship between emission line, energy and repetition rates (10P emission band-repetition rates single-shot, 5 Hz, 10 Hz and 20 Hz)

This has also been verified for the power values measured for the same repetition rates. The values still vary with the new procedure but in a much lower interval, between 0.5 mJ and 1 mJ for energy, and 5 mW and 10 mW for power values. The same improvement was verified for higher repetition rates up to 100 Hz. The confirmation could be observed not only for the 10P emission band (Figure 3), but also for the other three emission bands available (10R, 9P

rates [5].

deviations [5].

and 9R) [5].

power/energy and wavelength, and taking advantage of the full laser power for each selected wavelength.

Fig. 1. Schematics of the MTL3-GT TEA laser from Edinburgh Instruments [6]

One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behaviour was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used.

In order to overcome several error sources which are the causes of non-reproducibility in these type of lasers, the procedure used a continuous measurement of the energy line, making use of an infrared detector and power meter acquisition software. Such a display method reflects the inherent error associated with the grating tuning motion and therefore the micrometer hysteresis. The method is suitable to obtain the energy and power values for each emission laser TEA CO2 line optimized. The experimental set-up consists of the tuneable TEA CO2 laser, a pyroelectric energy detector connected to a handheld power/energy meter and a computer for acquisition purposes [5].

Since the laser is tuneable by wavelength, some specific emission lines of the CO2 molecule can be selected, making use of a micrometer. The correspondence between such emission lines and the micrometer driving position can be previously verified with an infrared spectrometer, in order to check those mentioned in the user's manual. Changing the position of the micrometer, one varies the angular position of the diffraction grid. This allows to scan among several emission lines, and so to choose the working wavelength.

Using a graphite target block, the pulse shape can be observed while the micrometer is moving. When the correct position is achieved, the focus should be round and symmetric (≈5 mm in diameter), displaying a strong luminosity and without sudden changes for consecutive shots. However, this method proved to be somewhat inaccurate and not very user friendly. To overcome these drawbacks, one must look at the real-time graphic line display of energy on the computer and follow its behaviour during the micrometer rotation, as well. The higher value of the energy line display corresponds, for each wavelength, to the desired position of the micrometer. This can be confirmed at any time by crossing the laser beam with the graphite target [5]. However, the micrometer hysteresis makes the procedure

power/energy and wavelength, and taking advantage of the full laser power for each

Fig. 1. Schematics of the MTL3-GT TEA laser from Edinburgh Instruments [6]

power/energy meter and a computer for acquisition purposes [5].

among several emission lines, and so to choose the working wavelength.

One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behaviour was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy

In order to overcome several error sources which are the causes of non-reproducibility in these type of lasers, the procedure used a continuous measurement of the energy line, making use of an infrared detector and power meter acquisition software. Such a display method reflects the inherent error associated with the grating tuning motion and therefore the micrometer hysteresis. The method is suitable to obtain the energy and power values for each emission laser TEA CO2 line optimized. The experimental set-up consists of the tuneable TEA CO2 laser, a pyroelectric energy detector connected to a handheld

Since the laser is tuneable by wavelength, some specific emission lines of the CO2 molecule can be selected, making use of a micrometer. The correspondence between such emission lines and the micrometer driving position can be previously verified with an infrared spectrometer, in order to check those mentioned in the user's manual. Changing the position of the micrometer, one varies the angular position of the diffraction grid. This allows to scan

Using a graphite target block, the pulse shape can be observed while the micrometer is moving. When the correct position is achieved, the focus should be round and symmetric (≈5 mm in diameter), displaying a strong luminosity and without sudden changes for consecutive shots. However, this method proved to be somewhat inaccurate and not very user friendly. To overcome these drawbacks, one must look at the real-time graphic line display of energy on the computer and follow its behaviour during the micrometer rotation, as well. The higher value of the energy line display corresponds, for each wavelength, to the desired position of the micrometer. This can be confirmed at any time by crossing the laser beam with the graphite target [5]. However, the micrometer hysteresis makes the procedure

selected wavelength.

detectors used.

unreliable, tedious and very sensitive to the rotation speed of the micrometer and stability of the energy signal. Thus, two acquisitions were made for each repetition rate, one by turning the micrometer clockwise and another counter clockwise. An average of them was calculated and definitive energy values were registered. This procedure was repeated in the opposite direction, in order to obtain an average, and also to confirm the reproducibility of the result. It was actually confirmed for every emission line and several values of repetition rates [5].

Acquisitions recorded without concerns about the external factors and in different days revealed instability in energy values for each repetition rate measured (singleshot, 5 Hz, 10 Hz or 20 Hz), as displayed in Figure 2 (A). For the other three emission bands available (10R, 9P and 9R), the problem is also present. A variation in the power measured was observed in all possible cases, emission bands and repetition rates available. The values could vary from 1 mJ up to 5 mJ for energy and 10 mW to 50 mW for power (repetition rate was 10 Hz in this measurement). The final acquisitions were recorded taking into account the improved procedure regarding the verification of a correspondence between the micrometer drive readings and wavelengths. It can be observed in Figure 2 (B) that the energy values for each repetition rates available were smoother and without significant deviations [5].

Fig. 2. Relationship between emission line, energy and repetition rates (10P emission band-repetition rates single-shot, 5 Hz, 10 Hz and 20 Hz)

This has also been verified for the power values measured for the same repetition rates. The values still vary with the new procedure but in a much lower interval, between 0.5 mJ and 1 mJ for energy, and 5 mW and 10 mW for power values. The same improvement was verified for higher repetition rates up to 100 Hz. The confirmation could be observed not only for the 10P emission band (Figure 3), but also for the other three emission bands available (10R, 9P and 9R) [5].

Infrared Lasers in Nanoscale Science 331

allows one to follow the formation rate of clusters and complexes during the adiabatic expansion. Selective photodissociation of van der Waals clusters by infrared lasers could be

Ca\*(3PJ) + HCl → CaCl(X;v'',J'') + H The reaction with the ground state Ca(1S0) is endothermic and this is why excited Ca atoms are required. When interrogating the centre of the reaction cell with a tuneable cw laser, Laser Induced Fluorescence (LIF) emission is observed on transitions in the CaCl(A-X) band system [2]. An example of a fraction of the related LIF excitation spectrum is shown in Figure

Fig. 4. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level

Important analytical applications are represented by measurements of the internal-state distribution of reaction products with LIF and spectroscopic investigations of collisioninduced energy-transfer processes. The high output power of pulsed CO2 lasers allows excitation of high vibrational levels by multiphoton absorption, which eventually may lead to the dissociation of the excited molecule. In some favorable cases the excited molecules or the dissociation fragments can even selectively react with other added components. Such selectively initiated chemical reactions can be induced by CO2 lasers which are particularly

As an example, let us consider the synthesis of SF5NF2 by multiphoton absorption of CO2 photons in a mixture of S2F10 and N2F4, which proceeds according to the following scheme:

> ν→ 2SF5

ν→ 2NF2

 SF5 + NF2 → SF5NF2 This laser-driven reaction proceeds much more quickly than the conventional high-

S2F10 + *nh*

N2F4 + *nh*

temperature synthesis without laser, even at the lower temperature of 350 K.

used for isotope separation [1].

population of the reaction product.

advantageous due to their large electrical efficiency.

4.

A typical example of a beam-gas collision is the process

Fig. 3. Power versus repetition rate for 10P emission band.

Along this book chapter, several examples of CO2 lasers applications to nanoscale science and nanotechnology, are explored and generally explained. These include examples in different topics, namely molecular photodynamics, tailored-size nanoparticles production, optical spectroscopy of nanopowders, infrared irradiation of nanostructures, desorption kinetics, photodynamic therapy, among others.

#### **2. Laser spectroscopy and photodynamics**

The combination of pulsed lasers, pulsed molecular beams and time-of-flight mass spectrometry represents a powerful technique for studying excitation, ionization and fragmentation of wanted molecules out of a large variety of different species present in a molecular beam [7]. The success of these two combined techniques is mainly due to the increase in the spectral resolution of absorption and fluorescence spectra by using collimated molecular beams with reduced transverse velocity components, and also to the fact that internal cooling of molecules during adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This particular aspect greatly reduces the number of absorbing levels and results in a huge simplification of the absorption spectrum [7].

In addition, the low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters. The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser intensities [1].

The structure of molecular complexes in their electronic ground state can be obtained from direct infrared laser absorption spectroscopy in pulsed supersonic-slit jet expansions. This

Along this book chapter, several examples of CO2 lasers applications to nanoscale science and nanotechnology, are explored and generally explained. These include examples in different topics, namely molecular photodynamics, tailored-size nanoparticles production, optical spectroscopy of nanopowders, infrared irradiation of nanostructures, desorption

The combination of pulsed lasers, pulsed molecular beams and time-of-flight mass spectrometry represents a powerful technique for studying excitation, ionization and fragmentation of wanted molecules out of a large variety of different species present in a molecular beam [7]. The success of these two combined techniques is mainly due to the increase in the spectral resolution of absorption and fluorescence spectra by using collimated molecular beams with reduced transverse velocity components, and also to the fact that internal cooling of molecules during adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This particular aspect greatly reduces the number of absorbing levels and results in a huge

In addition, the low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters. The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser

The structure of molecular complexes in their electronic ground state can be obtained from direct infrared laser absorption spectroscopy in pulsed supersonic-slit jet expansions. This

Fig. 3. Power versus repetition rate for 10P emission band.

kinetics, photodynamic therapy, among others.

simplification of the absorption spectrum [7].

intensities [1].

**2. Laser spectroscopy and photodynamics** 

allows one to follow the formation rate of clusters and complexes during the adiabatic expansion. Selective photodissociation of van der Waals clusters by infrared lasers could be used for isotope separation [1].

A typical example of a beam-gas collision is the process

$$\text{Ca}^\*(\text{°P}\_\text{I}) \text{ + HCl} \rightarrow \text{CaCl}(\text{X}; \text{v}^{\text{\textquotedblleft}}, \text{J}^{\text{\textquotedblright}}) + \text{H}^+$$

The reaction with the ground state Ca(1S0) is endothermic and this is why excited Ca atoms are required. When interrogating the centre of the reaction cell with a tuneable cw laser, Laser Induced Fluorescence (LIF) emission is observed on transitions in the CaCl(A-X) band system [2]. An example of a fraction of the related LIF excitation spectrum is shown in Figure 4.

Fig. 4. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level population of the reaction product.

Important analytical applications are represented by measurements of the internal-state distribution of reaction products with LIF and spectroscopic investigations of collisioninduced energy-transfer processes. The high output power of pulsed CO2 lasers allows excitation of high vibrational levels by multiphoton absorption, which eventually may lead to the dissociation of the excited molecule. In some favorable cases the excited molecules or the dissociation fragments can even selectively react with other added components. Such selectively initiated chemical reactions can be induced by CO2 lasers which are particularly advantageous due to their large electrical efficiency.

As an example, let us consider the synthesis of SF5NF2 by multiphoton absorption of CO2 photons in a mixture of S2F10 and N2F4, which proceeds according to the following scheme:

$$\begin{array}{c} \mathrm{S\_2F\_{10} + } \eta h \nu \to \mathrm{2SF\_5} \\\\ \mathrm{N\_2F\_4 + } \eta h \nu \to \mathrm{2NF\_2} \\\\ \mathrm{SF\_5 + NF\_2} \to \mathrm{SF\_5NF\_2} \end{array}$$

This laser-driven reaction proceeds much more quickly than the conventional hightemperature synthesis without laser, even at the lower temperature of 350 K.

Infrared Lasers in Nanoscale Science 333

τ v-T - relaxation time for vibration-translation transfer, i.e, the time needed to reach the

The first process can occur without collisions, but the other two are necesserelly collisional and therefore are pressure dependent. Considering vexc as the vibrational excitation velocity of a molecule by multiphoton ionization (i.e, 1/vexc will be the excitation time, which depends on radiation intensity and on the vibrational transition cross-section), a comparison of vexc with the several excitation velocities, gives rise to four different




The excitation spectrum obtained by LIF of the CN fragment (produced by multiphoton absorption in the infrared with a high power CO2 laser) of the gas H2C=CHCN, shows clearly the rotational fine structure of the (0,0) band of the CN violet emission, which allows to conclude that the rotational distribution is statistical and characterized by a certain Boltzmann temperature, confirming that the excitation energy is statistically redistributed in

The ability to energize a specific molecular bond and thereby promote a certain desired reaction pathway, has been a widely pursued goal, called mode-selective control in molecular physics. Actually, tunable infrared lasers are very convenient tools to divert a reaction from its dominant thermal pathway toward an envisaged possible product. However, the surplus of vibrational energy tends to be redistributed rapidly within a molecule. An initially excited, high-frequency localized mode can quickly de-excite by transferring its energy into combinations of lower frequency modes. In large molecules, in condensed phases, and at surfaces, huge numbers of low-frequency modes can accept energy, and energy randomization is very rapid (generally on the picosecond time scale or faster). This way, energy does not remain localized in a bond for a sufficiently long time to influence a chemical reaction. Therefore, the resulting chemistry is thermal rather than selective, which leads to the breaking of the weakest bond or to the reaction of the most reactive site. However, in small molecules with sparse vibrational modes, only a few or even

τintra v-v' - relaxation time for intramolecular vibrational energy transfer τv-v' - relaxation time for vibrational energy transfer among distinct molecules

complete thermal equilibrium

electromagnetic field.

the dissociation by multiphoton absorption [9].

situations:

modes.

system up.

Another example of CO2 laser-initiated reactions is the gas-phase telomerization of methyliodide CF3I with C2F4, which represents an exothermic radical chain reaction

$$\text{(CF}\_3\text{I)}^\* + \text{nC}\_2\text{F}\_4 \rightarrow \text{CF}\_3\text{(C}\_2\text{F}\_4\text{)}\_\text{nI} \tag{\text{n} = 1, 2, 3}$$

producing CF3(C2F4)nI with low values of n. The CO2 laser is in near resonance with the ν2+ν3 band of CF3I. The quantum yield for this reaction increases with increasing pressure in the irradiated cell [2].

The infrared lasers have the advantage that the contribution of scattering losses to the total beam attenuation is much smaller than in the visible range. For measurements of very low concentrations, on the other hand, visible dye lasers may be more advantageous because of the larger absorption cross sections for electronic transitions and the higher detector sensitivity.

The applications of lasers to chemical reactions in gas phase are usually classified in two categories: laser induced chemical reactions and laser catalyzed chemical reactions. In the first ones, the laser supplies all the energy thermodinamically needed for the occurence of the reaction and they correspond typically to unimolecular processes (dissociation by multiphotonic absorption); in the second ones (typically bimolecular reactions) only a partial energy amount is supplied and then reaction proceeds by itself. The dissociation by multiphotonic absorption has seen a huge growth in the last decades [8] due to the availability of high power infrared lasers and important technological applications, like isotopic separation. As an example, since in SF6 a mixture exists of 32SF6 and 34SF6, an infrared CO2 laser with λ = 10.61 μm only gives rise to the excitation of vibrational states of 32SF6 but not those of 34SF6; thus, when the continuum of 32SF6 vibrational states is reached after the absorption of 25 photons, only dissociation into 32SF5 and F is produced. This dissociation is fast and corresponds to a statistical mechanism. On its turn, when the wavelength is tuned to 10.82 μm, the dissociation takes place in the 34SF6 molecules.

The observation of dissociation phenomena in molecular beam apparatuses proves unequivocally that it is unimolecular and non-collisional, as it was shown through energetic and angular distributions of the SF5 fragment formed in the dissociation of a SF6 molecular beam, by a CO2 laser pulse of 5 J/cm3 [2]. Actually the results are consistent with RRKM unimolecular theory predictions. In this theory, it is assumed that energy is statistically distributed among the several available modes before dissociation takes place. This means that excitation energy is not localized in just one or a few modes, because in such cases, it will be not possible to reproduce with RRKM theory the above mentioned experimental results; in addition, the mean lifetime would be not about 10-8s (as predicted by RRKM) but much smaller [2].

The dissociation of a polyatomic molecule by multiphotonic absorption is in fact a statistical process (i.e, non-selective), and allows to consider distinct types of selectivity with lasers, based on the existing relation between the different relaxation times which are involved in a vibrationally excited molecule:

τintra v-v' << τv-v' << τ v-T

where:

Another example of CO2 laser-initiated reactions is the gas-phase telomerization of methyl-

(CF3I)\* + nC2F4 → CF3(C2F4)nI (n = 1, 2, 3) producing CF3(C2F4)nI with low values of n. The CO2 laser is in near resonance with the ν2+ν3 band of CF3I. The quantum yield for this reaction increases with increasing pressure in

The infrared lasers have the advantage that the contribution of scattering losses to the total beam attenuation is much smaller than in the visible range. For measurements of very low concentrations, on the other hand, visible dye lasers may be more advantageous because of the larger absorption cross sections for electronic transitions and the higher detector

The applications of lasers to chemical reactions in gas phase are usually classified in two categories: laser induced chemical reactions and laser catalyzed chemical reactions. In the first ones, the laser supplies all the energy thermodinamically needed for the occurence of the reaction and they correspond typically to unimolecular processes (dissociation by multiphotonic absorption); in the second ones (typically bimolecular reactions) only a partial energy amount is supplied and then reaction proceeds by itself. The dissociation by multiphotonic absorption has seen a huge growth in the last decades [8] due to the availability of high power infrared lasers and important technological applications, like isotopic separation. As an example, since in SF6 a mixture exists of 32SF6 and 34SF6, an infrared CO2 laser with λ = 10.61 μm only gives rise to the excitation of vibrational states of 32SF6 but not those of 34SF6; thus, when the continuum of 32SF6 vibrational states is reached after the absorption of 25 photons, only dissociation into 32SF5 and F is produced. This dissociation is fast and corresponds to a statistical mechanism. On its turn, when the

wavelength is tuned to 10.82 μm, the dissociation takes place in the 34SF6 molecules.

The observation of dissociation phenomena in molecular beam apparatuses proves unequivocally that it is unimolecular and non-collisional, as it was shown through energetic and angular distributions of the SF5 fragment formed in the dissociation of a SF6 molecular beam, by a CO2 laser pulse of 5 J/cm3 [2]. Actually the results are consistent with RRKM unimolecular theory predictions. In this theory, it is assumed that energy is statistically distributed among the several available modes before dissociation takes place. This means that excitation energy is not localized in just one or a few modes, because in such cases, it will be not possible to reproduce with RRKM theory the above mentioned experimental results; in addition, the mean lifetime would be not about 10-8s (as predicted by RRKM) but

The dissociation of a polyatomic molecule by multiphotonic absorption is in fact a statistical process (i.e, non-selective), and allows to consider distinct types of selectivity with lasers, based on the existing relation between the different relaxation times which are involved in a

τintra v-v' << τv-v' << τ v-T

iodide CF3I with C2F4, which represents an exothermic radical chain reaction

the irradiated cell [2].

sensitivity.

much smaller [2].

where:

vibrationally excited molecule:

τintra v-v' - relaxation time for intramolecular vibrational energy transfer

τv-v' - relaxation time for vibrational energy transfer among distinct molecules

τ v-T - relaxation time for vibration-translation transfer, i.e, the time needed to reach the complete thermal equilibrium

The first process can occur without collisions, but the other two are necesserelly collisional and therefore are pressure dependent. Considering vexc as the vibrational excitation velocity of a molecule by multiphoton ionization (i.e, 1/vexc will be the excitation time, which depends on radiation intensity and on the vibrational transition cross-section), a comparison of vexc with the several excitation velocities, gives rise to four different situations:


The excitation spectrum obtained by LIF of the CN fragment (produced by multiphoton absorption in the infrared with a high power CO2 laser) of the gas H2C=CHCN, shows clearly the rotational fine structure of the (0,0) band of the CN violet emission, which allows to conclude that the rotational distribution is statistical and characterized by a certain Boltzmann temperature, confirming that the excitation energy is statistically redistributed in the dissociation by multiphoton absorption [9].

The ability to energize a specific molecular bond and thereby promote a certain desired reaction pathway, has been a widely pursued goal, called mode-selective control in molecular physics. Actually, tunable infrared lasers are very convenient tools to divert a reaction from its dominant thermal pathway toward an envisaged possible product. However, the surplus of vibrational energy tends to be redistributed rapidly within a molecule. An initially excited, high-frequency localized mode can quickly de-excite by transferring its energy into combinations of lower frequency modes. In large molecules, in condensed phases, and at surfaces, huge numbers of low-frequency modes can accept energy, and energy randomization is very rapid (generally on the picosecond time scale or faster). This way, energy does not remain localized in a bond for a sufficiently long time to influence a chemical reaction. Therefore, the resulting chemistry is thermal rather than selective, which leads to the breaking of the weakest bond or to the reaction of the most reactive site. However, in small molecules with sparse vibrational modes, only a few or even

Infrared Lasers in Nanoscale Science 335

k(T) of the individual elementary reactions which occur on the molecular level have to be known. For example, the above mentioned H2/O2 combustion reaction consists of 38 elementary reactions involving a variety of reactive intermediates like H, O atoms and OH radicals [14]. Laser pump-and-probe techniques, which combine pulsed laser photolysis for reactive species generation with time-resolved laser-induced fluorescence (LIF) detection for reaction products, have paved the way for detailed studies of the molecular dynamics of the

The experimental possibilities for studying processes in technical combustion devices have expanded a lot in recent years as a result of the development of various pulsed high-power laser sources which provide high temporal, spectral and spatial resolution. Laser spectroscopic methods are important for non-intensive measurements in systems where complex chemical kinetics are coupled with transport processes. One of the key factors for improving the performance of many technical combustion devices is an optimum control of the ignition process. Optimized reproducible ignition ensures an efficient and safe operation. Actually, experimental studies on CO2 laser-induced thermal ignition of CH3OH/O2 mixtures have been performed. In a quartz cell equipped with SrF2 windows, CH3OH/O2 mixtures are ignited using a cw CO2 laser in the pulsed mode. The coincidence of the 9P(12) CO2 laser line in the (001)-(020) band with the R(12) CO stretch fundamental band of the methanol molecule at 9.6 μm allows controlled heating and ignition of the mixture. OH radicals formed during flame propagation were excited in the (v' = 3, v\* = 0) vibrational band of the OH (A2Σ+ - X2Π) transition around 248 nm using two tunable KrF excimer lasers (laser wavelength tunable in the range 247.9 - 248.9 nm with a bandwidth of typically 0.5 cm-1). The time delay between the two excimer laser pulses is 100 ns, in order to separate the signals induced by them. The fluorescence is collected using achromatic UV lens. Reflection filters are used to spectrally isolate the (v' = 3, v'' = 2) fluorescence band of the OH radical for detection. Fluorescence is detected by gated image-intensified CCD cameras. Excitation of two different optical transitions starting from the N'' = 8 and N'' = 11 rotational levels of the OH (X2Π - v'' = 0) vibrational state, allowed the measurement of spatially corresponding LIF image pairs. Assuming a Boltzmann distribution for the population of the OH (X2Π - v'' = 0) rotational states, the ratio of the two OH fluorescence

Interest in infrared laser-induced chemical reactions centered on the quest for mode selectivity was sparked by work on laser isotope separation in the 1970s. In much of this work it is common to find a strongly increasing yield with fluence. However, it is much less common to find an increase with pressure. The most dramatic example of an increasing yield with pressure is the IR laser-induced reaction of isomerization of methyl isocyanid to acetonitrile. [15]. It was then demonstrated that absorption and dissociation can be significantly enhanced through collisions and such reaction exhibits a sharp threshold pressure above which nearly complete isomerization occurs in a single pulse. Thus, the laser-initiated isomerization of methyl isocyanide is an ideal reaction for examination of collision-induced energy transfer phenomena. For the fluence dependence experiments, the laser was operated on the P(20) 944.19 cm-1 transition of the 10.6-μm band. CaF2 flats and KCl windows were used to attenuate the beam for the lower fluence data. For the wavelength data, the laser was operated on the P(6) through P(34) lines of the 10.6-pm band (corresponding to the maximum of the R through the maximum of the P branch of the ν4(C-

elementary reactions [1,2].

images can be converted into a OH temperature field [1,2].

zero combinations of low-frequency modes can accept the energy, and so the lifetime of the initially excited mode may be sufficiently long to allow mode-selective chemistry. This was already demonstrated for the outcome of the gas-phase reaction of H atoms with singly deuterated water (HOD) that can be controlled through laser excitation of specific HOD vibrational modes [10] and it is also illustrated in Figure 5 for a molecular case where one can take profit of the C - Cl bond being stronger than C - Br one.

Fig. 5. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level population of the reaction product.

From the few attempts at IR mode-selective desorption of molecules from surfaces, that have been reported, perhaps the most successfull was the experimental findings of Liu et al [11], where the authors first created an absorbed layer of about 15% H atoms and 85% D atoms on an Si(111) surface. They then irradiated the surface with a free-electron laser tuned to the 4,8 μm Si-H stretching mode. They found that almost all desorbing atoms were in the form of H2 and less than 5% of desorbing molecules were HD or D2. This result rules out any local heating mechanism, which would produce a statistical mixture (2% H2, 26% HD and 72% D2). Largely as a result of the layer-molecular beam experiments of Y. T. Lee et al [12] the interpretation of the IR-MPD is now almost entirely clarified. The key to the understanding was the determination, under collision-free conditions, of the translational energy distribution of the photofragments as a function of laser intensity and, separately, laser fluence. It is found, indeed, that a suitable tailored statistical (RRKM) theory of unimolecular dissociation can explain most of the observations. All the evidence from these experiments suggests that mode selective molecular dissociation is only possible to achieve using a fast (ps) intense laser pulse.

Thermodynamic laws can be used to determine the equilibrium state of chemical reaction systems like the H2/O2 combustion (2H2 + O2= 2H2O). If chemical reactions are fast compared to all other physical processes (molecular diffusion, heat conduction and flow) thermodynamics alone allows the local description of even complex systems [13]. However, in most cases, chemistry occurs on time scales which are comparable with those of molecular transport. Thus, chemical kinetics information is required, e.g rate coefficients

zero combinations of low-frequency modes can accept the energy, and so the lifetime of the initially excited mode may be sufficiently long to allow mode-selective chemistry. This was already demonstrated for the outcome of the gas-phase reaction of H atoms with singly deuterated water (HOD) that can be controlled through laser excitation of specific HOD vibrational modes [10] and it is also illustrated in Figure 5 for a molecular case where one can

Fig. 5. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level

From the few attempts at IR mode-selective desorption of molecules from surfaces, that have been reported, perhaps the most successfull was the experimental findings of Liu et al [11], where the authors first created an absorbed layer of about 15% H atoms and 85% D atoms on an Si(111) surface. They then irradiated the surface with a free-electron laser tuned to the 4,8 μm Si-H stretching mode. They found that almost all desorbing atoms were in the form of H2 and less than 5% of desorbing molecules were HD or D2. This result rules out any local heating mechanism, which would produce a statistical mixture (2% H2, 26% HD and 72% D2). Largely as a result of the layer-molecular beam experiments of Y. T. Lee et al [12] the interpretation of the IR-MPD is now almost entirely clarified. The key to the understanding was the determination, under collision-free conditions, of the translational energy distribution of the photofragments as a function of laser intensity and, separately, laser fluence. It is found, indeed, that a suitable tailored statistical (RRKM) theory of unimolecular dissociation can explain most of the observations. All the evidence from these experiments suggests that mode selective molecular dissociation is only possible to achieve using a fast

Thermodynamic laws can be used to determine the equilibrium state of chemical reaction systems like the H2/O2 combustion (2H2 + O2= 2H2O). If chemical reactions are fast compared to all other physical processes (molecular diffusion, heat conduction and flow) thermodynamics alone allows the local description of even complex systems [13]. However, in most cases, chemistry occurs on time scales which are comparable with those of molecular transport. Thus, chemical kinetics information is required, e.g rate coefficients

take profit of the C - Cl bond being stronger than C - Br one.

population of the reaction product.

(ps) intense laser pulse.

k(T) of the individual elementary reactions which occur on the molecular level have to be known. For example, the above mentioned H2/O2 combustion reaction consists of 38 elementary reactions involving a variety of reactive intermediates like H, O atoms and OH radicals [14]. Laser pump-and-probe techniques, which combine pulsed laser photolysis for reactive species generation with time-resolved laser-induced fluorescence (LIF) detection for reaction products, have paved the way for detailed studies of the molecular dynamics of the elementary reactions [1,2].

The experimental possibilities for studying processes in technical combustion devices have expanded a lot in recent years as a result of the development of various pulsed high-power laser sources which provide high temporal, spectral and spatial resolution. Laser spectroscopic methods are important for non-intensive measurements in systems where complex chemical kinetics are coupled with transport processes. One of the key factors for improving the performance of many technical combustion devices is an optimum control of the ignition process. Optimized reproducible ignition ensures an efficient and safe operation. Actually, experimental studies on CO2 laser-induced thermal ignition of CH3OH/O2 mixtures have been performed. In a quartz cell equipped with SrF2 windows, CH3OH/O2 mixtures are ignited using a cw CO2 laser in the pulsed mode. The coincidence of the 9P(12) CO2 laser line in the (001)-(020) band with the R(12) CO stretch fundamental band of the methanol molecule at 9.6 μm allows controlled heating and ignition of the mixture. OH radicals formed during flame propagation were excited in the (v' = 3, v\* = 0) vibrational band of the OH (A2Σ+ - X2Π) transition around 248 nm using two tunable KrF excimer lasers (laser wavelength tunable in the range 247.9 - 248.9 nm with a bandwidth of typically 0.5 cm-1). The time delay between the two excimer laser pulses is 100 ns, in order to separate the signals induced by them. The fluorescence is collected using achromatic UV lens. Reflection filters are used to spectrally isolate the (v' = 3, v'' = 2) fluorescence band of the OH radical for detection. Fluorescence is detected by gated image-intensified CCD cameras. Excitation of two different optical transitions starting from the N'' = 8 and N'' = 11 rotational levels of the OH (X2Π - v'' = 0) vibrational state, allowed the measurement of spatially corresponding LIF image pairs. Assuming a Boltzmann distribution for the population of the OH (X2Π - v'' = 0) rotational states, the ratio of the two OH fluorescence images can be converted into a OH temperature field [1,2].

Interest in infrared laser-induced chemical reactions centered on the quest for mode selectivity was sparked by work on laser isotope separation in the 1970s. In much of this work it is common to find a strongly increasing yield with fluence. However, it is much less common to find an increase with pressure. The most dramatic example of an increasing yield with pressure is the IR laser-induced reaction of isomerization of methyl isocyanid to acetonitrile. [15]. It was then demonstrated that absorption and dissociation can be significantly enhanced through collisions and such reaction exhibits a sharp threshold pressure above which nearly complete isomerization occurs in a single pulse. Thus, the laser-initiated isomerization of methyl isocyanide is an ideal reaction for examination of collision-induced energy transfer phenomena. For the fluence dependence experiments, the laser was operated on the P(20) 944.19 cm-1 transition of the 10.6-μm band. CaF2 flats and KCl windows were used to attenuate the beam for the lower fluence data. For the wavelength data, the laser was operated on the P(6) through P(34) lines of the 10.6-pm band (corresponding to the maximum of the R through the maximum of the P branch of the ν4(C-

Infrared Lasers in Nanoscale Science 337

features one single peak centered at 1042.2 cm−1. This is consistent with a cyclic structure in which all three methanol molecules are equivalent. Higher cluster spectra are characterized

LIF detection is also used for various ultrasensitive techniques by probing reagents that are either autofluorescing or tagged with a fluorescent dye molecule. Applying different microscopic techniques with tight spatial and spectral filtering, various groups have directly visualized a variety of single fluorescent dye molecules (rhodamines and coumarins), dissolved in liquids by using coherent one- and two-photon excitation. Photophysical parameters and photobleaching play a crucial role for the accuracy of single-molecule detection by LIF and for the high sensitivity of fluorescence spectroscopy. Key properties for a fluorescent dye are its absorption coefficient, fluorescence- and photobleaching quantum yield. Photobleaching is a dynamic irreversible process in which fluorescent molecules undergo photoinduced chemical destruction upon absorption of light, thus losing their ability to fluoresce. Thus, for every absorption process there is a certain fixed probability φ<sup>b</sup> of the molecule to be bleached. The probability P to survive n absorption cycles and become

P (survival of n absorptions) = (1- φb)n φ<sup>b</sup> This corresponds to the geometric distribution, which is the discrete counterpart of the exponential distribution. Due to this exponential nature of the photodestruction process with the standard deviation (1- φb)/φb , the relative fluctuation of the number of detected photons due to a single molecule transit can be as high as 100% [18]. The mean number of

μ = (1- φb)/φb = 1/φ<sup>b</sup> Photobleaching is the ultimate limit of fluorescence-based single-molecule spectroscopy,

Unfortunatelly, the total number of absorbed molecules cannot be measured directly and precisely under single-molecule-detection, making it impossible to determine φb. However, the above definition can be expressed in kinetic terms. If a dye solution is illuminated, it is possible to measure the number of irreversibly photobleached molecules as a decrease in the dye concentration c(t) with time t. Under appropriate conditions the rate of this decrease is proportional to c(t), and so photobleaching reaction can be treated as a quasi-unimolecular

dc(t)/dt = -kb c(t) → c(t) = c(0) e-kbt The rate constant kb is dependent on cw laser irradiance, and the study of this dependence can lead to the evaluation of φb. The photostability of many organic dyes in organic solvents

Pursuing the goal of single-molecule spectroscopy where ultra-low concentrations of the fluorophore in water (< 10 pM) are used, we should focus on photoreactions occuring under these conditions. In single-molecule spectroscopy, photostability and fluorescence

by single peaks gradually blue-shifted with respect to the trimer line [17].

bleached in the (n+1)th cycle is given by:

reaction:

is higher than in water.

survived absorption cycles μ is equal to the standard deviation:

and the quantum yield of photobleaching is defined by the ratio:

φb = number of photobleached molecules/total number of absorbed molecules

N) stretch of methyl isocyanide) with an average energy per pulse of 1.05 J at the sample. Analysis for reaction was performed by monitoring the 2165 cm-1 ν2(C=N) stretch with a grating spectrophotometer. To determine the relationship between fluence and threshold pressure, the fluence was kept constant while the pressure was varied so as to bracket the threshold. Then the process was repeated with a new fluence. The results indicate a linear variation of the fluence with the inverse of the pressure. Analysis of the fluence dependence of the threshold pressure indicates that the inverse of the threshold pressure is directly proportional to the average number of photons absorbed per molecule. This balance between incident fluence (or average number of photons absorbed) and threshold pressure can be understood in terms of the usual model of increasing yield with fluence. Hence, if the yield is increased owing to an increase in fluence, the threshold pressure should exhibit a concomitant decrease, as is observed. The multiphoton absorption spectrum as well as the wavelength dependence of the threshold pressure reflects the structure of the linear absorption spectrum [1].

Interest in using infrared laser radiation for studying charge transfer processes at surfaces relies on the possibility of exciting vibrational modes of adsorbate molecules. It is wellknown that vibrational excitation is very important in promoting endoergic gas-phase chemical reactions, as well as in controlling chemical processes occurring in adsorbed layers. The use of IR lasers for initiating gas-surface reactions when the gas consists of polyatomic molecules allows us to put, at the gas-surface interface, a large amount of energy due to IR multiphoton absorption. As is known from gas-phase experiments, at rather moderate for IR C02 laser energy fluences of about 1 J/cm2, it is possible to excite to high vibrationally excited states (up to energy levels E > 1 eV) practically all irradiated molecule. Therefore, in spite of a rather small value of C02 laser quantum (0.1-0.12 eV) and longer pulse duration (>100 ns), one can induce effective charge transfer.

The use of a pulsed TEA C02 laser allowed us to apply the time-of-flight (TOF) technique for the detection of ion signal and, as a result, to distinguish between molecular negative ions and electron emission. A TEA C02 laser line tunable in the range 9-11 μm was used for the excitation of molecules at the Ba surface. The laser beam was directed to the Ba surface perpendicular to the SF6 beam direction via a ZnSe window in the HVC and a KBr window in the UHVC [16]. The negative molecular ion signal was shown to be very sensitive to the SF6 molecular absorption (to the exciting C02 laser frequency). Thus, enhancement factors of 10 or 4 were found for 10P(20) versus 10R(20) and 10P(16) versus 10P(22) lines, respectively. This support the vibrational selectivity (vibrational enhancement) of the SF6 + Ba gas-surface IR laser-photoinduced ionization process [16].

Regarding applications in cluster spectroscopy, some experiments have been performed, in particular infrared photodissociation by crossing a continuous supersonic molecular beam of small methanol clusters with the radiation of a pulsed CO2 laser [17] . Subsequent scattering by a secondary He beam disperses the cluster beam and allows the off-axis detection of selected cluster species, undisturbed by ionizer fragmentation artifacts. In the region of the ν8 C-O stretching vibration, the dependence of IR photon absorption on laser frequency and fluence is investigated as a function of cluster size [17]. The predissociation spectrum of the dimer shows two distinct peaks at 1026.5 and 1051.6 cm−1 which correspond to the excitation of the two non-equivalent monomers in the dimer. The trimer spectrum

N) stretch of methyl isocyanide) with an average energy per pulse of 1.05 J at the sample. Analysis for reaction was performed by monitoring the 2165 cm-1 ν2(C=N) stretch with a grating spectrophotometer. To determine the relationship between fluence and threshold pressure, the fluence was kept constant while the pressure was varied so as to bracket the threshold. Then the process was repeated with a new fluence. The results indicate a linear variation of the fluence with the inverse of the pressure. Analysis of the fluence dependence of the threshold pressure indicates that the inverse of the threshold pressure is directly proportional to the average number of photons absorbed per molecule. This balance between incident fluence (or average number of photons absorbed) and threshold pressure can be understood in terms of the usual model of increasing yield with fluence. Hence, if the yield is increased owing to an increase in fluence, the threshold pressure should exhibit a concomitant decrease, as is observed. The multiphoton absorption spectrum as well as the wavelength dependence of the threshold pressure reflects the structure of the linear

Interest in using infrared laser radiation for studying charge transfer processes at surfaces relies on the possibility of exciting vibrational modes of adsorbate molecules. It is wellknown that vibrational excitation is very important in promoting endoergic gas-phase chemical reactions, as well as in controlling chemical processes occurring in adsorbed layers. The use of IR lasers for initiating gas-surface reactions when the gas consists of polyatomic molecules allows us to put, at the gas-surface interface, a large amount of energy due to IR multiphoton absorption. As is known from gas-phase experiments, at rather moderate for IR C02 laser energy fluences of about 1 J/cm2, it is possible to excite to high vibrationally excited states (up to energy levels E > 1 eV) practically all irradiated molecule. Therefore, in spite of a rather small value of C02 laser quantum (0.1-0.12 eV) and longer pulse duration

The use of a pulsed TEA C02 laser allowed us to apply the time-of-flight (TOF) technique for the detection of ion signal and, as a result, to distinguish between molecular negative ions and electron emission. A TEA C02 laser line tunable in the range 9-11 μm was used for the excitation of molecules at the Ba surface. The laser beam was directed to the Ba surface perpendicular to the SF6 beam direction via a ZnSe window in the HVC and a KBr window in the UHVC [16]. The negative molecular ion signal was shown to be very sensitive to the SF6 molecular absorption (to the exciting C02 laser frequency). Thus, enhancement factors of 10 or 4 were found for 10P(20) versus 10R(20) and 10P(16) versus 10P(22) lines, respectively. This support the vibrational selectivity (vibrational enhancement) of the SF6 + Ba gas-surface

Regarding applications in cluster spectroscopy, some experiments have been performed, in particular infrared photodissociation by crossing a continuous supersonic molecular beam of small methanol clusters with the radiation of a pulsed CO2 laser [17] . Subsequent scattering by a secondary He beam disperses the cluster beam and allows the off-axis detection of selected cluster species, undisturbed by ionizer fragmentation artifacts. In the region of the ν8 C-O stretching vibration, the dependence of IR photon absorption on laser frequency and fluence is investigated as a function of cluster size [17]. The predissociation spectrum of the dimer shows two distinct peaks at 1026.5 and 1051.6 cm−1 which correspond to the excitation of the two non-equivalent monomers in the dimer. The trimer spectrum

absorption spectrum [1].

(>100 ns), one can induce effective charge transfer.

IR laser-photoinduced ionization process [16].

features one single peak centered at 1042.2 cm−1. This is consistent with a cyclic structure in which all three methanol molecules are equivalent. Higher cluster spectra are characterized by single peaks gradually blue-shifted with respect to the trimer line [17].

LIF detection is also used for various ultrasensitive techniques by probing reagents that are either autofluorescing or tagged with a fluorescent dye molecule. Applying different microscopic techniques with tight spatial and spectral filtering, various groups have directly visualized a variety of single fluorescent dye molecules (rhodamines and coumarins), dissolved in liquids by using coherent one- and two-photon excitation. Photophysical parameters and photobleaching play a crucial role for the accuracy of single-molecule detection by LIF and for the high sensitivity of fluorescence spectroscopy. Key properties for a fluorescent dye are its absorption coefficient, fluorescence- and photobleaching quantum yield. Photobleaching is a dynamic irreversible process in which fluorescent molecules undergo photoinduced chemical destruction upon absorption of light, thus losing their ability to fluoresce. Thus, for every absorption process there is a certain fixed probability φ<sup>b</sup> of the molecule to be bleached. The probability P to survive n absorption cycles and become bleached in the (n+1)th cycle is given by:

#### P (survival of n absorptions) = (1- φb)n φ<sup>b</sup>

This corresponds to the geometric distribution, which is the discrete counterpart of the exponential distribution. Due to this exponential nature of the photodestruction process with the standard deviation (1- φb)/φb , the relative fluctuation of the number of detected photons due to a single molecule transit can be as high as 100% [18]. The mean number of survived absorption cycles μ is equal to the standard deviation:

$$\mu = (1 - \phi\_{\rm b}) / \phi\_{\rm b} = 1 / \phi\_{\rm b}$$

Photobleaching is the ultimate limit of fluorescence-based single-molecule spectroscopy, and the quantum yield of photobleaching is defined by the ratio:

φb = number of photobleached molecules/total number of absorbed molecules

Unfortunatelly, the total number of absorbed molecules cannot be measured directly and precisely under single-molecule-detection, making it impossible to determine φb. However, the above definition can be expressed in kinetic terms. If a dye solution is illuminated, it is possible to measure the number of irreversibly photobleached molecules as a decrease in the dye concentration c(t) with time t. Under appropriate conditions the rate of this decrease is proportional to c(t), and so photobleaching reaction can be treated as a quasi-unimolecular reaction:

$$\mathbf{\dot{c}} \mathbf{\dot{c}}(\mathbf{t})/\mathbf{\dot{d}t} = \mathbf{\dot{-}} \mathbf{\dot{s}}\_b \mathbf{c}(\mathbf{t}) \quad \rightarrow \quad \mathbf{c}(\mathbf{t}) = \mathbf{c}(0) \text{ } \mathbf{e}^\text{-k} \mathbf{b}^\text{ } \mathbf{t}$$

The rate constant kb is dependent on cw laser irradiance, and the study of this dependence can lead to the evaluation of φb. The photostability of many organic dyes in organic solvents is higher than in water.

Pursuing the goal of single-molecule spectroscopy where ultra-low concentrations of the fluorophore in water (< 10 pM) are used, we should focus on photoreactions occuring under these conditions. In single-molecule spectroscopy, photostability and fluorescence

Infrared Lasers in Nanoscale Science 339

and ability to maintain steep temperature gradients allows for precise control of the nucleation and growth rates favoring the formation of very fine and uniform powders. When reaction occurs in the gas phase, far from polluting walls very pure nano-scale materials may be prepared, and so conditions are created permitting the homogeneous nucleation of particles by condensation from a supersaturated vapor phase. In addition, the

Pulsed laser ablation of materials in aqueous solutions of surfactants can also lead in some cases to the formation of ultrafine particles as in the case of TiO2 crystalline anatase 3 nm nanoparticles, using the third harmonic of a Nd:YAG laser (355 nm) operating at 10 Hz [19]. Laser-induced production of silicon nanoparticles has been mostly based on the global

SiH4 (g) + nhν → Si(s) + 2H2 (g) Silane strongly absorbs in the ν4 band at 10.5 μm resonant with the P(20) line of the 00º1-10º0 transition at 944.19 cm-1 of the CO2 laser [20]. Experiments have been performed in high vacuum with a pulsed TEA CO2 laser at fluences that varied between 0.5 J/cm2 and 150 J/cm2 (using different converging focal distance infrared lenses). The shape and size of the nanoparticles is then examined by electron microscopy (SEM and TEM) and Atomic Force Microscopy (AFM), and their structure by X-ray diffraction [21,22]. Figure 7 schematically describes the experimental set-up for laser pyrolysis (chemical decomposition by heat in the absence of oxygen). Silane gas is laminarilly flowing through the center of the laser pyrolysis reactor, surrounded by another laminar flow of helium. The focalized laser pulse (Δt = 100 ns, *I* = 30-40 mJ/pulse) decompose silane into silicon and hydrogen atoms which then recombine to form molecular hydrogen. Since a nozzle is placed close to the reaction zone, the silicon atoms and small silicon clusters are extracted in the majority helium supersonic expansion (like an ultra-fast cooler at a rate of 109 Ks-1), giving rise by condensation to larger clusters and nanoparticles. Their formation is processing in bursts of some nanoseconds, according to the laser pulses characteristics. The nanoparticles size selection is assured by a synchronized chopper with the laser pulses, and the size distribution is measured in-situ by time-of-flight mass spectrometry (TOFMS). It is also possible to deposit these silicon

laser processing is cleaner.

nanocrystals on a removable target mica substrate [21,22].

Fig. 7. Laser pirolysis set-up for size-selected nanoparticles production [21,22].

reaction

saturation of the fluorophores impose limitations on the achievable fluorescence flow and the resulting signal-to-background ratio.

The rate constants for excitation from a state *i* to a state *f* are proportional to the irradiance *I* [W/cm2] and to the absorption cross section Qif(λ) [cm2] at a wavelength λ:

$$\mathbf{k}\_{\rm{Tif}}(\boldsymbol{\chi}) = \mathbf{I} \, \mathbf{Q}\_{\rm{if}}(\boldsymbol{\chi}) \, \boldsymbol{\chi}$$

where = γ/(hc), being *c* the velocity of light in vacuum and *h* the Planck constant.

Fluorescence saturation follows from the fact that a molecule cannot be in an electronically excited state and in the ground state at the same time; i.e. a single molecule can emit only a limited number of fluorescence photons in a certain time interval. Thus, the saturation characteristics of the fluorescent flows are determined by ground state depletion due to the finite excited state lifetimes of the S1 and T1 states (Figure 6).

Fig. 6. Electronic Energy Diagram of a dye molecule with 5 electronic levels: S - singlet states; T - triplet states

The main reasons for using in life sciences near-infrared and infrared dyes as infrared fluorophores are threefold (although they photobleach more readily than dyes emitting in the visible):


Most of these dyes are cyanines, and they display a common problem which is their stability in biological fluids (composed mainly of water). They tend to aggregate, and so contribute to the quenching of emission. One method of preventing such aggregation is to isolate the dyes by encapsulation (in a nanobubble or a liposome) or to use chemical functionalization.

#### **3. Nanoparticles and carbon nanotechnology**

Given the importance of nanoscale particles in present technology, size distribution is a fundamental aspect as a quality control parameter. The production of nanoparticles using laser-induced gas-phase reactions techniques assures in general narrow size distributions contrarily to chemical techniques (such as precipitation or sol-gel processing) or to usual vapor-phase methods (furnace-heated vapor or arc-plasma), while the low reaction volume

saturation of the fluorophores impose limitations on the achievable fluorescence flow and

The rate constants for excitation from a state *i* to a state *f* are proportional to the irradiance *I*

kTif(λ) = I Qif(λ) γ

Fluorescence saturation follows from the fact that a molecule cannot be in an electronically excited state and in the ground state at the same time; i.e. a single molecule can emit only a limited number of fluorescence photons in a certain time interval. Thus, the saturation characteristics of the fluorescent flows are determined by ground state depletion due to the

Fig. 6. Electronic Energy Diagram of a dye molecule with 5 electronic levels: S - singlet

The main reasons for using in life sciences near-infrared and infrared dyes as infrared fluorophores are threefold (although they photobleach more readily than dyes emitting in


Most of these dyes are cyanines, and they display a common problem which is their stability in biological fluids (composed mainly of water). They tend to aggregate, and so contribute to the quenching of emission. One method of preventing such aggregation is to isolate the dyes by encapsulation (in a nanobubble or a liposome) or to use chemical functionalization.

Given the importance of nanoscale particles in present technology, size distribution is a fundamental aspect as a quality control parameter. The production of nanoparticles using laser-induced gas-phase reactions techniques assures in general narrow size distributions contrarily to chemical techniques (such as precipitation or sol-gel processing) or to usual vapor-phase methods (furnace-heated vapor or arc-plasma), while the low reaction volume

often limited by the auto-fluorescence background, is significantly improved. - the laser wavelength also produce reduced scattering in the biological tissue, and thus increase both the penetration depth and the efficiency of collection of emission. - available low-cost and compact NIR and IR diode lasers (e.g. 650 nm, 800 nm, 970 nm,

[W/cm2] and to the absorption cross section Qif(λ) [cm2] at a wavelength λ:

finite excited state lifetimes of the S1 and T1 states (Figure 6).

etc...) can be used as excitation sources for these dyes.

**3. Nanoparticles and carbon nanotechnology** 

where = γ/(hc), being *c* the velocity of light in vacuum and *h* the Planck constant.

the resulting signal-to-background ratio.

states; T - triplet states

the visible):

and ability to maintain steep temperature gradients allows for precise control of the nucleation and growth rates favoring the formation of very fine and uniform powders. When reaction occurs in the gas phase, far from polluting walls very pure nano-scale materials may be prepared, and so conditions are created permitting the homogeneous nucleation of particles by condensation from a supersaturated vapor phase. In addition, the laser processing is cleaner.

Pulsed laser ablation of materials in aqueous solutions of surfactants can also lead in some cases to the formation of ultrafine particles as in the case of TiO2 crystalline anatase 3 nm nanoparticles, using the third harmonic of a Nd:YAG laser (355 nm) operating at 10 Hz [19].

Laser-induced production of silicon nanoparticles has been mostly based on the global reaction

$$\text{SiH}\_4\text{ (g)} + \text{nhv} \rightarrow \text{Si(s)} + 2\text{H}\_2\text{(g)}$$

Silane strongly absorbs in the ν4 band at 10.5 μm resonant with the P(20) line of the 00º1-10º0 transition at 944.19 cm-1 of the CO2 laser [20]. Experiments have been performed in high vacuum with a pulsed TEA CO2 laser at fluences that varied between 0.5 J/cm2 and 150 J/cm2 (using different converging focal distance infrared lenses). The shape and size of the nanoparticles is then examined by electron microscopy (SEM and TEM) and Atomic Force Microscopy (AFM), and their structure by X-ray diffraction [21,22]. Figure 7 schematically describes the experimental set-up for laser pyrolysis (chemical decomposition by heat in the absence of oxygen). Silane gas is laminarilly flowing through the center of the laser pyrolysis reactor, surrounded by another laminar flow of helium. The focalized laser pulse (Δt = 100 ns, *I* = 30-40 mJ/pulse) decompose silane into silicon and hydrogen atoms which then recombine to form molecular hydrogen. Since a nozzle is placed close to the reaction zone, the silicon atoms and small silicon clusters are extracted in the majority helium supersonic expansion (like an ultra-fast cooler at a rate of 109 Ks-1), giving rise by condensation to larger clusters and nanoparticles. Their formation is processing in bursts of some nanoseconds, according to the laser pulses characteristics. The nanoparticles size selection is assured by a synchronized chopper with the laser pulses, and the size distribution is measured in-situ by time-of-flight mass spectrometry (TOFMS). It is also possible to deposit these silicon nanocrystals on a removable target mica substrate [21,22].

Fig. 7. Laser pirolysis set-up for size-selected nanoparticles production [21,22].

Infrared Lasers in Nanoscale Science 341

IR-active, graphite-like *E*1*<sup>u</sup>*mode (also known as the *G* band) originating from the *sp*2 hybridized carbon. The absorbance peak at 1200 cm−1, is a disorder-induced one phonon absorbance band (D band-from the *sp*3-hybridized carbon), which has been also observed in neutron irradiated diamonds [24]. This lattice mode arises from the disruption of the translational symmetry of the diamond lattice. In general, the presence of CH*<sup>x</sup>* groups (evidenced in the IR active bands in the range of 3000 cm−1) and non-conjugated carboxylic carbonyl groups (peak around 1725 cm−1) can benefit a number of bio-sensing applications

Fig. 9. Typical infrared absorbance spectrum obtained on 60 nm multiwall carbon

Production of single-wall carbon nanotubes (SWNT) by the laser-ablation technique using graphite, pitch and coke as carbonaceous feedstock materials has been reported [26]. This has been done with a 250W continuous-wave CO2-laser at a wavelength of 10.6 *μ*m and varying the nature and the concentration of the metal catalyst, the type and pressure of the buffergas as well as the laser conditions. The amount of SWNT material obtained is much higher when using graphite as a precursor than in the case of coke and carbonaceous feedstock [26]. Laser-assisted chemical vapour deposition (LCVD) for the formation and growth of carbon nanotubes has also been performed using a medium-power continuous-wave CO2 laser to

acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown [27]. Electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron)

Using a CO2 laser perpendicularly directed onto a silicon substrate, sensitized mixtures of iron pentacarbonyl vapour and acetylene were pyrolyzed in a flow reactor. The method involves the heating of both the gas phase and the substrate by IR radiation. The carbon

by offering a simple route to nanotube functionalization [25].

irradiate a sensitized mixture of Fe(CO)5 vapour and

that seems to catalyze the nanotube growth [27].

nanotubes [25]

The kinetic energy of the clusters is small (less than 0.4 eV per atom for 4 nm size particles, which corresponds to 10% of the bonding energy) and so a Low Energy Cluster Beam Deposition (LECBD) is taking place, without changing the cluster properties in gas phase. Due to the geometry of the system, a distribution of nanoparticle sizes also appears on the substrate, and it can be guaranteed that all the sizes correspond to the same air exposition history. By varying some experimental parameters (pressure, flow, laser power, delay between laser pulse and chopper slit) it is possible to control the size distribution of the silicon nanoparticles deposited onto the substrate, for ultramicroscopic analysis (Figure 8).

Fig. 8. AFM image of size-selected silicon nanoparticles together with their size distributions measured by TOFMS and AFM[21,22].

The advantage of using a TEA CO2 laser in the pyrolysis becomes clear for the fine tuning wavelength adjust, in order to optimize the production process of several other types of nanoparticles [22].

Several synthesis reactors geometries based on the vaporization of a target (graphite/metal catalyst pellet) inside a oven at a fixed temperature (above 1000 K) by continuous CO2 laser beam (λ = 10.6 μm) have been developed to produce several types of carbon nanotubes. The laser power can be varied from 100 W to 1600 W and the temperature of the target is measured with an optical pyrometer. In general a inert gas flow carries away the solid particles formed in the laser ablation process which are then collected on a filter [23].

A typical mid-IR to near-IR absorbance spectrum taken on uniformly dispersed, purified CNTs (grown by CVD-mostly MWNT) at room temperature is displayed in Figure 9 [24]. In the measured IR absorbance spectrum, a prime intensity peak is seen at 1584 cm−1. This is an

The kinetic energy of the clusters is small (less than 0.4 eV per atom for 4 nm size particles, which corresponds to 10% of the bonding energy) and so a Low Energy Cluster Beam Deposition (LECBD) is taking place, without changing the cluster properties in gas phase. Due to the geometry of the system, a distribution of nanoparticle sizes also appears on the substrate, and it can be guaranteed that all the sizes correspond to the same air exposition history. By varying some experimental parameters (pressure, flow, laser power, delay between laser pulse and chopper slit) it is possible to control the size distribution of the silicon nanoparticles deposited onto the substrate, for ultramicroscopic analysis (Figure 8).

Fig. 8. AFM image of size-selected silicon nanoparticles together with their size distributions

The advantage of using a TEA CO2 laser in the pyrolysis becomes clear for the fine tuning wavelength adjust, in order to optimize the production process of several other types of

Several synthesis reactors geometries based on the vaporization of a target (graphite/metal catalyst pellet) inside a oven at a fixed temperature (above 1000 K) by continuous CO2 laser beam (λ = 10.6 μm) have been developed to produce several types of carbon nanotubes. The laser power can be varied from 100 W to 1600 W and the temperature of the target is measured with an optical pyrometer. In general a inert gas flow carries away the solid

A typical mid-IR to near-IR absorbance spectrum taken on uniformly dispersed, purified CNTs (grown by CVD-mostly MWNT) at room temperature is displayed in Figure 9 [24]. In the measured IR absorbance spectrum, a prime intensity peak is seen at 1584 cm−1. This is an

particles formed in the laser ablation process which are then collected on a filter [23].

measured by TOFMS and AFM[21,22].

nanoparticles [22].

IR-active, graphite-like *E*1*<sup>u</sup>*mode (also known as the *G* band) originating from the *sp*2 hybridized carbon. The absorbance peak at 1200 cm−1, is a disorder-induced one phonon absorbance band (D band-from the *sp*3-hybridized carbon), which has been also observed in neutron irradiated diamonds [24]. This lattice mode arises from the disruption of the translational symmetry of the diamond lattice. In general, the presence of CH*<sup>x</sup>* groups (evidenced in the IR active bands in the range of 3000 cm−1) and non-conjugated carboxylic carbonyl groups (peak around 1725 cm−1) can benefit a number of bio-sensing applications by offering a simple route to nanotube functionalization [25].

Fig. 9. Typical infrared absorbance spectrum obtained on 60 nm multiwall carbon nanotubes [25]

Production of single-wall carbon nanotubes (SWNT) by the laser-ablation technique using graphite, pitch and coke as carbonaceous feedstock materials has been reported [26]. This has been done with a 250W continuous-wave CO2-laser at a wavelength of 10.6 *μ*m and varying the nature and the concentration of the metal catalyst, the type and pressure of the buffergas as well as the laser conditions. The amount of SWNT material obtained is much higher when using graphite as a precursor than in the case of coke and carbonaceous feedstock [26].

Laser-assisted chemical vapour deposition (LCVD) for the formation and growth of carbon nanotubes has also been performed using a medium-power continuous-wave CO2 laser to irradiate a sensitized mixture of Fe(CO)5 vapour and

acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown [27]. Electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron) that seems to catalyze the nanotube growth [27].

Using a CO2 laser perpendicularly directed onto a silicon substrate, sensitized mixtures of iron pentacarbonyl vapour and acetylene were pyrolyzed in a flow reactor. The method involves the heating of both the gas phase and the substrate by IR radiation. The carbon

Infrared Lasers in Nanoscale Science 343

LCVD technique in general has several prominent advantages including high deposition rates that are favorable for scale-up production of CNTs. In contrast to the standard LCVD system, in which the in situ thermal decarbonylation of Fe(CO)5 is used for obtaining Fe nanoparticles C-LCVD employs the catalytic activity of pre-deposited metal-based nanoparticles. For a better control of CNT growth conditions, it proved to be advantageous to carry out separately the catalyst deposition and carbon nanotube growth. Thus, as a main advantage, C-LCVD allows *ex-situ* prepared metal-based particles with the desired properties and dispersion degree to provide the nucleation conditions for the growth of CNTs [28]. The temperature is measured with a thermocouple positioned behind the substrate. Since the temperature of the area which is irradiated by the laser is expected to be considerably higher than the average temperature of the substrate holder, the temperature inside the laser spot is measured optically with a pyrometer. Under a total gas pressure of about 80 mbar, the temperature in the laser spot, measured with the pyrometer could reach values between 800 and 900 C, while the thermocouple indicates values that are about 100 C lower. Mainly depending on the ethylene concentration, nanotube mean diameters between 10 and 60 nm were found. By increasing ethylene precursor flow rate, not only larger mean diameters of the CNTs were found but also the distribution of the CNT diameters became

Cementite (Fe3C) is of great technological importance for the mechanical properties of steels and iron alloys and for its role as catalyst to produce various hydrocarbons (including olefins, from CO2 and H2) and preferred catalysts for carbon fibers, nanotubes and nanoparticles due to their low nanometric mean sizes and narrow size distributions. Pure Fe3C nanomaterials have also been obtained by the pyrolysis of methyl methacrylate monomer, ethylene (as sensitizer), and iron pentacarbonyl (vapors) in a suitable range of laser intensities (by irradiating the same reactive mixture with a lower intensity radiation, the chemical content of the produced nanoparticles shifts towards mixtures of iron and iron oxides). Such nanopowders exhibited core (Fe3C)–shell polymer-based morphologies and their magnetic properties are likely to display high values for the saturation

Semiconductor photocatalysts have been used in different applications, and the combination of high photocatalytic activity, high stability and the benefit of environmental friendliness makes titanium dioxide the material of choice for such applications. In order to enhance the performance of this material for industrial purposes, coupling of TiO2 with other semiconductors and immobilization of TiO2 on porous materials have been studied as means for improving its photocatalytic activity. Nanocomposites of TiO2 and multiwalled carbon nanotubes were prepared and deposited by sol–gel spin coating on borosilicate substrates and sintered in air at 300 ºC. Further irradiation of the films with different CO2 laser intensities was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12*.*5 W/m2 was used [29]. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is

broader [28].

magnetization.

nanotubes were formed via the catalyzing action of the fine iron particles produced in the same experiment by the decomposition of the organometallic precursor molecules [27]. The reactant gas, a mixture of iron pentacarbonyl vapour (Fe(CO)5), ethylene (C2H4) and acetylene (C2H2), is admitted to the reaction cell through a rectangular nozzle, creating a gas flow close and parallel to the Si substrate and being pumped from the opposite side. The flow of ethylene is directed through a bubbler containing liquid iron pentacarbonyl at room temperature (and 27 Torr vapour pressure). Thus, the ethylene serves as carrier gas for the iron pentacarbonyl. The third gas, acetylene, is supplied by an extra line. Before entering the flow reactor, the gases are mixed in a small mixing vessel. The iron nanoparticles, needed to catalyze the formation of carbon nanotubes from carbon-containing precursors, are obtained by decomposing Fe(CO)5 during the laser-induced reaction. Ethylene gas, introduced into the gaseous atmosphere, serves also as a sensitizer activating the laser reaction and speeding up the Fe(CO)5 dissociation. C2H4 has a resonant absorption at the CO2 laser emission wavelength (10.6 μm) and is characterized by a rather high dissociation energy. Under the present conditions, C2H4 is only expected to collisionally exchange its internal energy with the other precursor molecules that do not absorb the CO2 laser radiation, thus heating the entire gas mixture [27].

The pressure inside the reaction chamber was keptat a constant value of 150 Torr. A flow of argon (500 sccm) was used to avoid contamination of the ZnSe entrance window during irradiation. At the heated surface and interface, iron pentacarbonyl is the first molecule to undergo dissociation, which can proceed until bare iron is obtained. Carbon nanotubes with straight, curved or even branched structures have been identified by ultramicroscopy. While some nanotubes were hollow, many of them were found to be partially or totally filled with nanoparticles most probably being metallic iron [27].

Films of vertically aligned MWCNTs of extremely high packing density were produced by this technique under very clean hydrocarbon supply conditions. Using an open-air pyrolitic LCVD system in which the role of gas-phase reactions are minimized, the growth of highly oriented and aligned single- and multiwall carbon nanotubes have been reported [28]. The

nanotubes were formed via the catalyzing action of the fine iron particles produced in the same experiment by the decomposition of the organometallic precursor molecules [27]. The reactant gas, a mixture of iron pentacarbonyl vapour (Fe(CO)5), ethylene (C2H4) and acetylene (C2H2), is admitted to the reaction cell through a rectangular nozzle, creating a gas flow close and parallel to the Si substrate and being pumped from the opposite side. The flow of ethylene is directed through a bubbler containing liquid iron pentacarbonyl at room temperature (and 27 Torr vapour pressure). Thus, the ethylene serves as carrier gas for the iron pentacarbonyl. The third gas, acetylene, is supplied by an extra line. Before entering the flow reactor, the gases are mixed in a small mixing vessel. The iron nanoparticles, needed to catalyze the formation of carbon nanotubes from carbon-containing precursors, are obtained by decomposing Fe(CO)5 during the laser-induced reaction. Ethylene gas, introduced into the gaseous atmosphere, serves also as a sensitizer activating the laser reaction and speeding up the Fe(CO)5 dissociation. C2H4 has a resonant absorption at the CO2 laser emission wavelength (10.6 μm) and is characterized by a rather high dissociation energy. Under the present conditions, C2H4 is only expected to collisionally exchange its internal energy with the other precursor molecules that do not absorb the CO2 laser radiation, thus heating the

Fig. 10. Experimental set-up for the deposition of carbon nanotubes by LCVD

nanoparticles most probably being metallic iron [27].

The pressure inside the reaction chamber was keptat a constant value of 150 Torr. A flow of argon (500 sccm) was used to avoid contamination of the ZnSe entrance window during irradiation. At the heated surface and interface, iron pentacarbonyl is the first molecule to undergo dissociation, which can proceed until bare iron is obtained. Carbon nanotubes with straight, curved or even branched structures have been identified by ultramicroscopy. While some nanotubes were hollow, many of them were found to be partially or totally filled with

Films of vertically aligned MWCNTs of extremely high packing density were produced by this technique under very clean hydrocarbon supply conditions. Using an open-air pyrolitic LCVD system in which the role of gas-phase reactions are minimized, the growth of highly oriented and aligned single- and multiwall carbon nanotubes have been reported [28]. The

entire gas mixture [27].

Adapted from [27]

LCVD technique in general has several prominent advantages including high deposition rates that are favorable for scale-up production of CNTs. In contrast to the standard LCVD system, in which the in situ thermal decarbonylation of Fe(CO)5 is used for obtaining Fe nanoparticles C-LCVD employs the catalytic activity of pre-deposited metal-based nanoparticles. For a better control of CNT growth conditions, it proved to be advantageous to carry out separately the catalyst deposition and carbon nanotube growth. Thus, as a main advantage, C-LCVD allows *ex-situ* prepared metal-based particles with the desired properties and dispersion degree to provide the nucleation conditions for the growth of CNTs [28]. The temperature is measured with a thermocouple positioned behind the substrate. Since the temperature of the area which is irradiated by the laser is expected to be considerably higher than the average temperature of the substrate holder, the temperature inside the laser spot is measured optically with a pyrometer. Under a total gas pressure of about 80 mbar, the temperature in the laser spot, measured with the pyrometer could reach values between 800 and 900 C, while the thermocouple indicates values that are about 100 C lower. Mainly depending on the ethylene concentration, nanotube mean diameters between 10 and 60 nm were found. By increasing ethylene precursor flow rate, not only larger mean diameters of the CNTs were found but also the distribution of the CNT diameters became broader [28].

Cementite (Fe3C) is of great technological importance for the mechanical properties of steels and iron alloys and for its role as catalyst to produce various hydrocarbons (including olefins, from CO2 and H2) and preferred catalysts for carbon fibers, nanotubes and nanoparticles due to their low nanometric mean sizes and narrow size distributions. Pure Fe3C nanomaterials have also been obtained by the pyrolysis of methyl methacrylate monomer, ethylene (as sensitizer), and iron pentacarbonyl (vapors) in a suitable range of laser intensities (by irradiating the same reactive mixture with a lower intensity radiation, the chemical content of the produced nanoparticles shifts towards mixtures of iron and iron oxides). Such nanopowders exhibited core (Fe3C)–shell polymer-based morphologies and their magnetic properties are likely to display high values for the saturation magnetization.

Semiconductor photocatalysts have been used in different applications, and the combination of high photocatalytic activity, high stability and the benefit of environmental friendliness makes titanium dioxide the material of choice for such applications. In order to enhance the performance of this material for industrial purposes, coupling of TiO2 with other semiconductors and immobilization of TiO2 on porous materials have been studied as means for improving its photocatalytic activity. Nanocomposites of TiO2 and multiwalled carbon nanotubes were prepared and deposited by sol–gel spin coating on borosilicate substrates and sintered in air at 300 ºC. Further irradiation of the films with different CO2 laser intensities was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12*.*5 W/m2 was used [29]. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is

Infrared Lasers in Nanoscale Science 345

matrix improved the photocatalytic activity of TiO2 coatings, as has been demonstrated by the degradation of a stearic acid layer deposited on the films [29]. In addition, higher CO2 laser intensities during the sintering implies enhanced photocatalytic activity of the

Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Incandescence (LII) are two suitable techniques for analyzing small particles. In LII a pulsed laser rapidly heats the particles and by monitoring the rate of decay of the resulting incandescent radiation, one can extract particle size information, as the rate is related to the size of the particle. In order to get information on the particles chemical composition LIBS must be used instead. With it, the pulsed laser is tightly focused on the sample to induce a breakdown (microspark) of the material, and so by monitoring the emission of light from this plasma one can gather information about chemical compositions. Sensor technology is now able to capture light between 200 and 940 nm, a region where all elements emit. Since LIBS data is generated in real-time (response time of 1 second or less), one keep track of rapid changes in the composition of the particles during the actual production run. LIBS is very sensitive, having a resolution in the femtogram region and capable of detecting as few as 100

Irradiating the surface of a solid with a laser, material can be ablated in a controlled way by optimizing intensity and pulse duration of the laser (laser ablation). Depending on the laser wavelength, the ablation is dominated by thermal evaporation (using a CO2 laser) or photochemical processes (using an excimer laser). Laser-spectroscopic diagnostics can distinguish between the two processes. Excitation spectroscopy or resonant two-photon ionization of the sputtered atoms, molecules, clusters, nanoparticles or even microparticles

In addition, the velocity distribution of particles emitted from the surface can be obtained from the Doppler shifts and broadning of the absorption lines, and their internal energy distribution from the intensity ratios of different vibrational-rotational transitions. With a pulsed ablation laser, the measured time delay between ablation pulses and probe laser

nanocomposites.

particles/cm3.

allows their identification (Figure 12).

Fig. 12. Laser ablation from a surface

even improved when high CO2 laser intensities are used during the sintering of the coatings [29].

The high aspect ratio combined with high mechanical and chemical stability of carbon nanotubes could help in enhancing the photocatalytic activity of TiO2. Nanocomposites of TiO2/CNTs were prepared by the addition of –NH2 functionalized, 10 nm outer diameter MWNTs to the TiO2-based sol, in a concentration of 3 mg/ml. After a fine dispersion of the nanotubes was achieved using a high shear processor, the sol was deposited on borosilicate glass substrates by a spin coating technique (2000 rpm, 10 s) [29].

Fig. 11. Raman spectra in the ranges (a) 100-800 and (b) 300-3000 cm-1 of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities [29]

Raman spectra of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities are shown in Figure 11. Non-irradiated films sintered in air at 300◦C have shown no Raman bands, as is clear from Figure 11 (a). Higher sintering temperatures were applied to the coatings inorder to obtain anatase TiO2 phase structure, but oxidation of the MWNTs took place, limiting the study of the composites. Therefore, CO2 laser irradiation was applied to the coatings with different density outputs as an alternative sintering process. This process is very fast, and because it is pulsed (highly spatially limited heated zone), it has not damaged the CNT's structure. Films irradiated with lasers of intensity lower than 12*.*8 Wm−2 have shown similar results to non-irradiated coatings in the range 100–1000 cm−<sup>1</sup> [29]. However, a sharp and intense Raman band at 144 cm−1 corresponding to anatase TiO2 phase was observed when such minimum laser intensity was applied during the sintering of the coatings, suggesting that the local temperature obtained using 12*.*8 Wm−<sup>2</sup> is sufficient to crystallize the coating. Figure 11 (b) shows D and G bands corresponding to the presence of MWNTs at 1319–1328 and 1592–1601 cm−1, respectively. The D band corresponds to defects present in carbonaceous materials. The addition of CNTs to the TiO2

even improved when high CO2 laser intensities are used during the sintering of the

The high aspect ratio combined with high mechanical and chemical stability of carbon nanotubes could help in enhancing the photocatalytic activity of TiO2. Nanocomposites of TiO2/CNTs were prepared by the addition of –NH2 functionalized, 10 nm outer diameter MWNTs to the TiO2-based sol, in a concentration of 3 mg/ml. After a fine dispersion of the nanotubes was achieved using a high shear processor, the sol was deposited on borosilicate

Fig. 11. Raman spectra in the ranges (a) 100-800 and (b) 300-3000 cm-1 of TiO2/MWNT

Raman spectra of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities are shown in Figure 11. Non-irradiated films sintered in air at 300◦C have shown no Raman bands, as is clear from Figure 11 (a). Higher sintering temperatures were applied to the coatings inorder to obtain anatase TiO2 phase structure, but oxidation of the MWNTs took place, limiting the study of the composites. Therefore, CO2 laser irradiation was applied to the coatings with different density outputs as an alternative sintering process. This process is very fast, and because it is pulsed (highly spatially limited heated zone), it has not damaged the CNT's structure. Films irradiated with lasers of intensity lower than 12*.*8 Wm−2 have shown similar results to non-irradiated coatings in the range 100–1000 cm−<sup>1</sup> [29]. However, a sharp and intense Raman band at 144 cm−1 corresponding to anatase TiO2 phase was observed when such minimum laser intensity was applied during the sintering of the coatings, suggesting that the local temperature obtained using 12*.*8 Wm−<sup>2</sup> is sufficient to crystallize the coating. Figure 11 (b) shows D and G bands corresponding to the presence of MWNTs at 1319–1328 and 1592–1601 cm−1, respectively. The D band corresponds to defects present in carbonaceous materials. The addition of CNTs to the TiO2

coatings not irradiated and irradiated with different CO2 laser intensities [29]

glass substrates by a spin coating technique (2000 rpm, 10 s) [29].

coatings [29].

matrix improved the photocatalytic activity of TiO2 coatings, as has been demonstrated by the degradation of a stearic acid layer deposited on the films [29]. In addition, higher CO2 laser intensities during the sintering implies enhanced photocatalytic activity of the nanocomposites.

Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Incandescence (LII) are two suitable techniques for analyzing small particles. In LII a pulsed laser rapidly heats the particles and by monitoring the rate of decay of the resulting incandescent radiation, one can extract particle size information, as the rate is related to the size of the particle. In order to get information on the particles chemical composition LIBS must be used instead. With it, the pulsed laser is tightly focused on the sample to induce a breakdown (microspark) of the material, and so by monitoring the emission of light from this plasma one can gather information about chemical compositions. Sensor technology is now able to capture light between 200 and 940 nm, a region where all elements emit. Since LIBS data is generated in real-time (response time of 1 second or less), one keep track of rapid changes in the composition of the particles during the actual production run. LIBS is very sensitive, having a resolution in the femtogram region and capable of detecting as few as 100 particles/cm3.

Irradiating the surface of a solid with a laser, material can be ablated in a controlled way by optimizing intensity and pulse duration of the laser (laser ablation). Depending on the laser wavelength, the ablation is dominated by thermal evaporation (using a CO2 laser) or photochemical processes (using an excimer laser). Laser-spectroscopic diagnostics can distinguish between the two processes. Excitation spectroscopy or resonant two-photon ionization of the sputtered atoms, molecules, clusters, nanoparticles or even microparticles allows their identification (Figure 12).

Fig. 12. Laser ablation from a surface

In addition, the velocity distribution of particles emitted from the surface can be obtained from the Doppler shifts and broadning of the absorption lines, and their internal energy distribution from the intensity ratios of different vibrational-rotational transitions. With a pulsed ablation laser, the measured time delay between ablation pulses and probe laser

Infrared Lasers in Nanoscale Science 347

absorbed by cancer cells. During the surgery, the light beam is positioned at the tumor site,

A sensitizer in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is when an alkaline solution of sodium hypochlorite and a

ClO-(aq) + H2O2(aq) → O2\*(g) + H+(aq) + Cl-(aq) + OH-(aq) O2\*is excited oxygen - meaning, one or more electrons in the O2 molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It


New types of photosensitizers used in photodynamic therapy, which are based on photon upconverting nanoparticles, have been developed. Such photosensitizers are excitable with infrared irradiation, which has several times larger tissue penetration depth than the currently available ones. Photon upconverting materials convert lower-energy light to higher-energy light through excitation with multiple photons. For instance, such materials would adsorb infrared irradiation and emit visible light to further excite the photosensitizing molecules. Photon upconverting nanoparticles (PUNPs) play here a crucial role. They can be first coated with a porous, thin layer of silica through sol-gel reaction. During the coating process, photosensitizing molecules with high absorbance in the spectral window matching the emission of the PUNPs are doped, so that the resulting silica layer contains a certain amount of these photosensitizing molecules. Finally, an antibody, specific to antigens expressed on the target cell surface, is covalently attached to the silica-coated nanoparticles. When the thus-prepared nanoparticles are irradiated by infrared light, emission from the PUNPs will be absorbed by the photosensitizing molecules coated on their surfaces. Subsequently, excited photosensitizing molecules will interact with surrounding groundstate molecular oxygen, generating singlet oxygen, leading to oxidative damage of the neighboring cells to which the nanoparticles are attached via specific antigen-

PUNPs made from NaYF4:Yb3+,Er3+ have been recognized as one of the most efficient

When excited by an infrared (974 nm) source, strong visible bands appear around 537 nm and 635 nm. Merocyanine 540 (M-540) is used as photosensitizing molecule, and doped into the silica layer during the coating process. M-540 is a molecule that can produce singlet oxygen and other reactive oxygen species, and has been used before in photodynamic therapy as a photosensitizer with a visible light source. Both the emission spectrum of NaYF4:Yb3+,Er3+ nanoparticles and the absorption spectrum of M-540 show a good overlap

which then activates the drug that kills the cancer cells, thus photodynamic therapy.

concentrated solution of hydrogen peroxide are mixed, a reaction occurs:

can do that in more than one way:


antibody binding.

photon upconverting phosphors [30].


transferring energy to another molecule

pulses allows the determination of the velocity distribution. Resonant two-photon ionization in combination with a TOFMS gives the mass spectrum. It is common to observe a broad mass range of clusters. The question is whether these clusters were emitted from the solid or whether they were formed by collisions in the evaporated cloud just after emission. Measurements of the vibrational energy distributions can give an answer. If the mean vibrational energy is much higher than the temperature of the solid, the molecules were formed in the gas phase, where an insufficient number of collisions cannot fully transfer the internal energy of molecules formed by recombination of sputtered atoms into kinetic energy.

Whereas laser ablation of graphite yields thermalized C2 molecules with a rotationalvibrational energy distribution following a Boltzmann distribution at the temperature T of the solid, ablation of electrical insulators, such as AlO, produces AlO molecules with a large kinetic energy (≈1 eV), but a "rotational temperature" of only 500 K.

#### **4. Technological applications**

Photodynamic therapy (PDT) is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic. Photosensitization is a process of transferring the energy of absorbed light. After absorption, the energy is transferred to the (chosen) reactants. This is part of the work of photochemistry in general. In particular this process is commonly employed where reactions require light sources of certain wavelengths that are not readily available. For example, mercury absorbs radiation at 1849 and 2537 angstroms, and the source is often high-intensity mercury lamps. It is a commonly used sensitizer. When mercury vapor is mixed with ethylene, and the compound is irradiated with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.

In order to achieve the selective destruction of the target biological area using PDT while leaving normal tissues untouched, the photosensitizer can be applied locally to the target area. For instance, in the treatment of skin conditions, including acne, psoriasis, and also skin cancers, the photosensitizer can be locally excited by a light source. In the local treatment of internal tissues and cancers, after photosensitizers have been administered intravenously, light can be delivered to the target area using endoscopes and fiber optic catheters. Compared to normal tissues, most types of cancers are especially active in both the uptake and accumulation of photosensitizers agents, which makes cancers especially vulnerable to PDT. Since photosensitizers can also have a high affinity for vascular endothelial cells, PDT can be targetted to the blood carrying vasculature that supplies nutrients to tumours, increasing further the destruction of tumours.

The optimal spectral window for biological tissue penetration of irradiation is around 800 nm to 1 μm. The use of CO2 laser relies on water (the largest constituent of most biological tissues) absorbing strongly at 10.6 μm; this strong absorption leads to a shorter optical penetration depth (≈13 μm), limiting its use to extremely thin tissues. A patient would be given a photo sensitive drug (photofrin) containing cancer killing substances which are

pulses allows the determination of the velocity distribution. Resonant two-photon ionization in combination with a TOFMS gives the mass spectrum. It is common to observe a broad mass range of clusters. The question is whether these clusters were emitted from the solid or whether they were formed by collisions in the evaporated cloud just after emission. Measurements of the vibrational energy distributions can give an answer. If the mean vibrational energy is much higher than the temperature of the solid, the molecules were formed in the gas phase, where an insufficient number of collisions cannot fully transfer the internal energy of molecules formed by recombination of sputtered atoms into kinetic

Whereas laser ablation of graphite yields thermalized C2 molecules with a rotationalvibrational energy distribution following a Boltzmann distribution at the temperature T of the solid, ablation of electrical insulators, such as AlO, produces AlO molecules with a large

Photodynamic therapy (PDT) is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic. Photosensitization is a process of transferring the energy of absorbed light. After absorption, the energy is transferred to the (chosen) reactants. This is part of the work of photochemistry in general. In particular this process is commonly employed where reactions require light sources of certain wavelengths that are not readily available. For example, mercury absorbs radiation at 1849 and 2537 angstroms, and the source is often high-intensity mercury lamps. It is a commonly used sensitizer. When mercury vapor is mixed with ethylene, and the compound is irradiated with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.

In order to achieve the selective destruction of the target biological area using PDT while leaving normal tissues untouched, the photosensitizer can be applied locally to the target area. For instance, in the treatment of skin conditions, including acne, psoriasis, and also skin cancers, the photosensitizer can be locally excited by a light source. In the local treatment of internal tissues and cancers, after photosensitizers have been administered intravenously, light can be delivered to the target area using endoscopes and fiber optic catheters. Compared to normal tissues, most types of cancers are especially active in both the uptake and accumulation of photosensitizers agents, which makes cancers especially vulnerable to PDT. Since photosensitizers can also have a high affinity for vascular endothelial cells, PDT can be targetted to the blood carrying vasculature that supplies

The optimal spectral window for biological tissue penetration of irradiation is around 800 nm to 1 μm. The use of CO2 laser relies on water (the largest constituent of most biological tissues) absorbing strongly at 10.6 μm; this strong absorption leads to a shorter optical penetration depth (≈13 μm), limiting its use to extremely thin tissues. A patient would be given a photo sensitive drug (photofrin) containing cancer killing substances which are

kinetic energy (≈1 eV), but a "rotational temperature" of only 500 K.

nutrients to tumours, increasing further the destruction of tumours.

**4. Technological applications** 

energy.

absorbed by cancer cells. During the surgery, the light beam is positioned at the tumor site, which then activates the drug that kills the cancer cells, thus photodynamic therapy.

A sensitizer in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is when an alkaline solution of sodium hypochlorite and a concentrated solution of hydrogen peroxide are mixed, a reaction occurs:

$$\text{ClO-(aq)} + \text{H}\_2\text{O}\_2(\text{aq}) \rightarrow \text{O}\_2\text{\*(g)} + \text{H}^\*(\text{aq}) + \text{Cl}\cdot(\text{aq}) + \text{OH}\cdot(\text{aq})$$

O2\*is excited oxygen - meaning, one or more electrons in the O2 molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It can do that in more than one way:


New types of photosensitizers used in photodynamic therapy, which are based on photon upconverting nanoparticles, have been developed. Such photosensitizers are excitable with infrared irradiation, which has several times larger tissue penetration depth than the currently available ones. Photon upconverting materials convert lower-energy light to higher-energy light through excitation with multiple photons. For instance, such materials would adsorb infrared irradiation and emit visible light to further excite the photosensitizing molecules. Photon upconverting nanoparticles (PUNPs) play here a crucial role. They can be first coated with a porous, thin layer of silica through sol-gel reaction. During the coating process, photosensitizing molecules with high absorbance in the spectral window matching the emission of the PUNPs are doped, so that the resulting silica layer contains a certain amount of these photosensitizing molecules. Finally, an antibody, specific to antigens expressed on the target cell surface, is covalently attached to the silica-coated nanoparticles. When the thus-prepared nanoparticles are irradiated by infrared light, emission from the PUNPs will be absorbed by the photosensitizing molecules coated on their surfaces. Subsequently, excited photosensitizing molecules will interact with surrounding groundstate molecular oxygen, generating singlet oxygen, leading to oxidative damage of the neighboring cells to which the nanoparticles are attached via specific antigenantibody binding.

PUNPs made from NaYF4:Yb3+,Er3+ have been recognized as one of the most efficient photon upconverting phosphors [30].

When excited by an infrared (974 nm) source, strong visible bands appear around 537 nm and 635 nm. Merocyanine 540 (M-540) is used as photosensitizing molecule, and doped into the silica layer during the coating process. M-540 is a molecule that can produce singlet oxygen and other reactive oxygen species, and has been used before in photodynamic therapy as a photosensitizer with a visible light source. Both the emission spectrum of NaYF4:Yb3+,Er3+ nanoparticles and the absorption spectrum of M-540 show a good overlap

Infrared Lasers in Nanoscale Science 349

signaling plant defence mechanisms [32]. The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption. The absorption of photons of a suitable wavelength and energy by the gas molecules excites them to a higher ro-vibrational state. The absorbed energy is subsequently transferred by intermolecular collisions to translational energy, and thereby to heat. When a gas sample is collected in a closed cell, the heating of the gas molecules will increase the cell pressure. Hence, by modulating the light intensity pressure variations are produced that generate a sound wave, which can be detected with a sensitive microphone. A schematic view of a typical LPAS

experimental system for the detection of volatile molecules is shown in Figure 15.

Fig. 14. Photoluminescence spectra of NaYF4:Yb3+,Tm3+nanoparticles before and after being

Fig. 15. Schematic view of a LPAS set-up for the detection of volatile molecule emission

The photoacoustic signal depends on the number of absorbing molecules present in the gas, the absorption strength of the molecules at a specific light frequency, and the intensity of the light. Then, for trace gas detection, the light source should have a narrow bandwidth and be tuneable (in order to match the specific molecular absorption feature), and it should have a high intensity to ensure a good signal-to-noise ratio. Since the absorption processes of interest involve ro-vibrational transitions, it is normally necessary to work in the IR region. In this spectral range each molecule has its own *fingerprint* absorption spectrum,whose strength can vary rapidly over a short wavelength interval. Specifically, the preferred range for spectroscopic applications lies in the range 3–20 μm. Specifically, CO2 and CO lasers serve as the most frequently used light sources for photoacoustic detection of gases because

coated with Ru(bpy)3-doped silica [31]

from plants.

between the nanoparticles' emission and M-540's absorption [30]. The photon upconverting property of the nanoparticles was not affected by the silica coating, as confirmed by their photoluminescence spectrum. The presence of M-540 in the silica coating could be readily confirmed by the change in color of the nanoparticles, to slightly yellowish.

Photosensitizers drugs, should ideally be specific to the target, highly effective in producing reactive oxygen species (ROS) when exposed to appropriate illumination, and excitable by a wavelength close to the near-infrared region (800 nm to 1 μm),where tissue penetration of the illumination is at a maximum.

Regarding the last desired feature, single photons with infrared wavelengths are usually too weak energetically to generate reactive oxygen species (1O2). Thus, multiphoton excitation would be needed for infrared light to be used as illumination source. It has been reported the synthesis and characterization of a type of nanomaterial capable of generating 1O2 under continuous wave infrared excitation, based on photon upconverting nanoparticles (PUNPs). The results demonstrate that such nanoparticles have great potential for becoming a new type of versatile PDT drugs for photodynamic therapy [31]. Although photon upconverting materials do not directly produce ROS, one utilizes the fact that they adsorb infrared photons and emit visible ones to further excite the photosensitizing molecules, thus indirectly causing the photosensitizing molecules to generate 1O2 under infrared excitation. The design and synthesis of the PUNP-based photosensitizers follow a schema depicted in Figure 13.

Fig. 13. PUNP-based photosensitizer preparation.

The core is a NaYF4:Yb3+,Tm3+ nanoparticle, a photon upconverting material capable of emitting blue light (≈ 477 nm) upon excitation by an infrared light source (≈ 975 nm). The nanoparticle was then coated by a thin layer of tris(bipyridine)ruthenium(II)-doped silica which generates 1O2. The NaYF4:Yb3+,Tm3+ nanoparticles were synthesized by a microemulsion method [31].

The photoluminescence spectra of the NaYF4:Yb3+,Tm3+ nanoparticles before and after beingcoated with Ru(bpy)3-doped silica, under 975 nm excitation, are shown in Figure 14.

One of the most sensitive methods to detect volatile compounds released by the plants is laser photoacoustic spectroscopy (LPAS), which allows the identification of many molecules

between the nanoparticles' emission and M-540's absorption [30]. The photon upconverting property of the nanoparticles was not affected by the silica coating, as confirmed by their photoluminescence spectrum. The presence of M-540 in the silica coating could be readily

Photosensitizers drugs, should ideally be specific to the target, highly effective in producing reactive oxygen species (ROS) when exposed to appropriate illumination, and excitable by a wavelength close to the near-infrared region (800 nm to 1 μm),where tissue penetration of

Regarding the last desired feature, single photons with infrared wavelengths are usually too weak energetically to generate reactive oxygen species (1O2). Thus, multiphoton excitation would be needed for infrared light to be used as illumination source. It has been reported the synthesis and characterization of a type of nanomaterial capable of generating 1O2 under continuous wave infrared excitation, based on photon upconverting nanoparticles (PUNPs). The results demonstrate that such nanoparticles have great potential for becoming a new type of versatile PDT drugs for photodynamic therapy [31]. Although photon upconverting materials do not directly produce ROS, one utilizes the fact that they adsorb infrared photons and emit visible ones to further excite the photosensitizing molecules, thus indirectly causing the photosensitizing molecules to generate 1O2 under infrared excitation. The design and synthesis of the PUNP-based photosensitizers follow a schema depicted in

The core is a NaYF4:Yb3+,Tm3+ nanoparticle, a photon upconverting material capable of emitting blue light (≈ 477 nm) upon excitation by an infrared light source (≈ 975 nm). The nanoparticle was then coated by a thin layer of tris(bipyridine)ruthenium(II)-doped silica which generates 1O2. The NaYF4:Yb3+,Tm3+ nanoparticles were synthesized by a

The photoluminescence spectra of the NaYF4:Yb3+,Tm3+ nanoparticles before and after beingcoated with Ru(bpy)3-doped silica, under 975 nm excitation, are shown in Figure 14. One of the most sensitive methods to detect volatile compounds released by the plants is laser photoacoustic spectroscopy (LPAS), which allows the identification of many molecules

confirmed by the change in color of the nanoparticles, to slightly yellowish.

the illumination is at a maximum.

Fig. 13. PUNP-based photosensitizer preparation.

microemulsion method [31].

Figure 13.

signaling plant defence mechanisms [32]. The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption. The absorption of photons of a suitable wavelength and energy by the gas molecules excites them to a higher ro-vibrational state. The absorbed energy is subsequently transferred by intermolecular collisions to translational energy, and thereby to heat. When a gas sample is collected in a closed cell, the heating of the gas molecules will increase the cell pressure. Hence, by modulating the light intensity pressure variations are produced that generate a sound wave, which can be detected with a sensitive microphone. A schematic view of a typical LPAS experimental system for the detection of volatile molecules is shown in Figure 15.

Fig. 14. Photoluminescence spectra of NaYF4:Yb3+,Tm3+nanoparticles before and after being coated with Ru(bpy)3-doped silica [31]

Fig. 15. Schematic view of a LPAS set-up for the detection of volatile molecule emission from plants.

The photoacoustic signal depends on the number of absorbing molecules present in the gas, the absorption strength of the molecules at a specific light frequency, and the intensity of the light. Then, for trace gas detection, the light source should have a narrow bandwidth and be tuneable (in order to match the specific molecular absorption feature), and it should have a high intensity to ensure a good signal-to-noise ratio. Since the absorption processes of interest involve ro-vibrational transitions, it is normally necessary to work in the IR region. In this spectral range each molecule has its own *fingerprint* absorption spectrum,whose strength can vary rapidly over a short wavelength interval. Specifically, the preferred range for spectroscopic applications lies in the range 3–20 μm. Specifically, CO2 and CO lasers serve as the most frequently used light sources for photoacoustic detection of gases because

Infrared Lasers in Nanoscale Science 351

deliver the laser's energy to heat the bonded cut and are used for controlling the temperature. They also make it possible to bond tissues inside the body. Sutures or stitches are not water tight, and blood or urine can pass through cuts, causing severe infection. Laser-bonded tissues heal faster, with less scarring. Even using today's microsurgery techniques, the treated wounds are open to infection, and the patient is inevitably left with permanent and unsightly scars. The near-infrared light is just the right wavelength to excite vibrations in chemical bonds in the water molecules (via first-overtone excitation in the OH-

Keeping the heat from the laser at exactly the right temperature for optimal wound healing, allows surgeons to seal cuts both on our skin and inside our bodies with less scarring, and less exposure to infection. When the laser begins to overheat and risks burning the tissue, the device reduces laser power, and if the temperature is too low to complete a closure, laser

There is also an enormous potential of a CO2-laser system for rapidly producing polymer microfluidic structures. The common polymer poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. The narrowest produced channel was 85 μm wide. A solvent-assisted thermal bonding method proved to be the most time-efficient one. These systems provide a cost effective alternative to UV-laser systems and they are especially useful in microfluidic prototyping

Furthermore, surface heat treatment in glasses and ceramics, using CO2 lasers, has drawn the attention to several technological applications, such as lab-on-a-chip devices, diffraction gratings and microlenses. Microlens fabrication on a glass surface has been studied mainly due to its importance in optical devices, as fiber coupling and CCD signal enhancement. Using microlens arrays, recorded on the glass surface, can enable the bidimensional codification for product identification. This would allow the production of codes without any residues (like the fine powder generated by laser ablation) and resistance to an aggressive environment, such as sterilization processes. Microlens arrays can be fabricated using a continuous wave CO2 laser, focused on the surface of flat commercial soda-lime

Silicon micromachining is a very important technology in microfabrication and microelectromechanical system (MEMS) industry. Nd:YAG laser has a wavelength of 1.06 μm, which is adsorbed by silicon, and is easily used for direct silicon machining. But the cost is very high. Although CO2 laser is cheap, its wavelength of 10.64 μm is not absorbed by silicon. However, a silicon sample put on the top of a glass, instead of pure silicon, is used for CO2 laser micromachining. The silicon on the top of a glass may absorb the CO2 laser and become able to be etched, even through the wafer. Commercial available air-cooled CO2 laser equipment can be used with a maximum laser power of 30 W. A glass below the silicon changes the absorption of silicon to CO2 laser during machining. The silicon on the top of a glass may be etched by CO2 laser even through the wafer due to the absorption variation. The etching depth increases with the pass number at constant laser power and scanning

stretch manifold); the vibrations quickly turn into heat.

power is increased appropriately.

silicate glass substrates.

speed.

due to the very short cycle time of production.

they provide relatively high CW powers, typically up to 100 W and 20 W respectively, over this wavelength region. LPAS shows a large versatility of applications not only in plant science, but also in other fields, e.g. in environmental chemistry [33]. It has been shown to be a reliable method for the detection of ethylene in several plant physiological processes at parts per trillion concentration levels (e.g. from a cherry tomato under different conditions) [34].

One of the major analytical problems with fruit and vegetable samples is the detection and identification of non-volatile organic compounds present in low concentration levels, as happens for most of the phytoalexins produced by plants. Mass spectrometry is widely used in the analysis of such compounds, providing exact mass identification. However, the difficulty with their unequivocal identification and quantitative detection lies in their volatilization into the gas phase prior to injection into the analyser. This constitutes particular problems for thermally labile samples, as they rapidly decompose upon heating. To circumvent this difficulty a wide range of techniques have been applied for non-volatile compound analysis, including LD (Laser Desorption). Recently, LD methods have been developed in which the volatilization and ionization steps are separated, providing higher sample sensitivity. In particular, REMPI-TOFMS is considered to be one of the most powerful methods for trace component analysis in complex matrices [2]. The high selectivity of REMPI-TOFMS stems from the combination of the mass-selective detection with the resonant ionization process, i.e. the ionization is achieved by absorption of two or more laser photons through a resonant, intermediate state. This condition provides a second selectivity to the technique, namely laser wavelength-selective ionization. In addition, it shows an easy control of the molecular fragmentation by the laser intensity and the possibility of simultaneous analysis of different components present in a matrix.

As an example, it is possible to perform fast and direct analysis of non-volatile compounds in fruit and vegetables, particularly *trans*-resveratrol in grapes and vine leaves. The method is based on the combination of LD followed by REMPI and TOFMS detection, often identified by its sum of acronyms, i.e. LD-REMPI-TOFMS [35]. *Trans*-Resveratrol is an antioxidant compound naturally produced in a huge number of plants, including grapes. Analysis of *trans*-resveratrol is generally carried out by high-performance liquid chromatography. Its analysis in grapes and wines requires the use of pre-concentration prior to analysis and/or multi-solvent extraction techniques, due to the complexity of the matrices and to the low concentration of the analyte. The extraction methods generally employed are liquid extraction with organic solvents or solid-phase extraction. It is generally accepted that the sample preparation is the limiting step in *trans*-resveratrol analysis, not only because of the need for costly and time-consuming operations, but also because of the error sources introduced during this operation. These error sources can largely be overcome when applying the method of LD-REMPI-TOFMS. The experimental set-up used in this analysis method basically consists of two independent high vacuum chambers; the first chamber is used for both laser desorption and laser post-ionization of the samples, and the second chamber for TOFMS [2].

Some other relevant technological applications of infrared lasers will be generally described here in the following paragraphs.

Katzir was the first researcher to apply the carbon dioxide laser, coupled to optical fibers made from silver halide, for wound closure under a tight temperature control. The fibers

they provide relatively high CW powers, typically up to 100 W and 20 W respectively, over this wavelength region. LPAS shows a large versatility of applications not only in plant science, but also in other fields, e.g. in environmental chemistry [33]. It has been shown to be a reliable method for the detection of ethylene in several plant physiological processes at parts per trillion concentration levels (e.g. from a cherry tomato under different conditions) [34]. One of the major analytical problems with fruit and vegetable samples is the detection and identification of non-volatile organic compounds present in low concentration levels, as happens for most of the phytoalexins produced by plants. Mass spectrometry is widely used in the analysis of such compounds, providing exact mass identification. However, the difficulty with their unequivocal identification and quantitative detection lies in their volatilization into the gas phase prior to injection into the analyser. This constitutes particular problems for thermally labile samples, as they rapidly decompose upon heating. To circumvent this difficulty a wide range of techniques have been applied for non-volatile compound analysis, including LD (Laser Desorption). Recently, LD methods have been developed in which the volatilization and ionization steps are separated, providing higher sample sensitivity. In particular, REMPI-TOFMS is considered to be one of the most powerful methods for trace component analysis in complex matrices [2]. The high selectivity of REMPI-TOFMS stems from the combination of the mass-selective detection with the resonant ionization process, i.e. the ionization is achieved by absorption of two or more laser photons through a resonant, intermediate state. This condition provides a second selectivity to the technique, namely laser wavelength-selective ionization. In addition, it shows an easy control of the molecular fragmentation by the laser intensity and the possibility of

simultaneous analysis of different components present in a matrix.

chamber for TOFMS [2].

here in the following paragraphs.

As an example, it is possible to perform fast and direct analysis of non-volatile compounds in fruit and vegetables, particularly *trans*-resveratrol in grapes and vine leaves. The method is based on the combination of LD followed by REMPI and TOFMS detection, often identified by its sum of acronyms, i.e. LD-REMPI-TOFMS [35]. *Trans*-Resveratrol is an antioxidant compound naturally produced in a huge number of plants, including grapes. Analysis of *trans*-resveratrol is generally carried out by high-performance liquid chromatography. Its analysis in grapes and wines requires the use of pre-concentration prior to analysis and/or multi-solvent extraction techniques, due to the complexity of the matrices and to the low concentration of the analyte. The extraction methods generally employed are liquid extraction with organic solvents or solid-phase extraction. It is generally accepted that the sample preparation is the limiting step in *trans*-resveratrol analysis, not only because of the need for costly and time-consuming operations, but also because of the error sources introduced during this operation. These error sources can largely be overcome when applying the method of LD-REMPI-TOFMS. The experimental set-up used in this analysis method basically consists of two independent high vacuum chambers; the first chamber is used for both laser desorption and laser post-ionization of the samples, and the second

Some other relevant technological applications of infrared lasers will be generally described

Katzir was the first researcher to apply the carbon dioxide laser, coupled to optical fibers made from silver halide, for wound closure under a tight temperature control. The fibers deliver the laser's energy to heat the bonded cut and are used for controlling the temperature. They also make it possible to bond tissues inside the body. Sutures or stitches are not water tight, and blood or urine can pass through cuts, causing severe infection. Laser-bonded tissues heal faster, with less scarring. Even using today's microsurgery techniques, the treated wounds are open to infection, and the patient is inevitably left with permanent and unsightly scars. The near-infrared light is just the right wavelength to excite vibrations in chemical bonds in the water molecules (via first-overtone excitation in the OHstretch manifold); the vibrations quickly turn into heat.

Keeping the heat from the laser at exactly the right temperature for optimal wound healing, allows surgeons to seal cuts both on our skin and inside our bodies with less scarring, and less exposure to infection. When the laser begins to overheat and risks burning the tissue, the device reduces laser power, and if the temperature is too low to complete a closure, laser power is increased appropriately.

There is also an enormous potential of a CO2-laser system for rapidly producing polymer microfluidic structures. The common polymer poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. The narrowest produced channel was 85 μm wide. A solvent-assisted thermal bonding method proved to be the most time-efficient one. These systems provide a cost effective alternative to UV-laser systems and they are especially useful in microfluidic prototyping due to the very short cycle time of production.

Furthermore, surface heat treatment in glasses and ceramics, using CO2 lasers, has drawn the attention to several technological applications, such as lab-on-a-chip devices, diffraction gratings and microlenses. Microlens fabrication on a glass surface has been studied mainly due to its importance in optical devices, as fiber coupling and CCD signal enhancement. Using microlens arrays, recorded on the glass surface, can enable the bidimensional codification for product identification. This would allow the production of codes without any residues (like the fine powder generated by laser ablation) and resistance to an aggressive environment, such as sterilization processes. Microlens arrays can be fabricated using a continuous wave CO2 laser, focused on the surface of flat commercial soda-lime silicate glass substrates.

Silicon micromachining is a very important technology in microfabrication and microelectromechanical system (MEMS) industry. Nd:YAG laser has a wavelength of 1.06 μm, which is adsorbed by silicon, and is easily used for direct silicon machining. But the cost is very high. Although CO2 laser is cheap, its wavelength of 10.64 μm is not absorbed by silicon. However, a silicon sample put on the top of a glass, instead of pure silicon, is used for CO2 laser micromachining. The silicon on the top of a glass may absorb the CO2 laser and become able to be etched, even through the wafer. Commercial available air-cooled CO2 laser equipment can be used with a maximum laser power of 30 W. A glass below the silicon changes the absorption of silicon to CO2 laser during machining. The silicon on the top of a glass may be etched by CO2 laser even through the wafer due to the absorption variation. The etching depth increases with the pass number at constant laser power and scanning speed.

Infrared Lasers in Nanoscale Science 353

nanoparticle production, surgery and biomedicine, nanoanalysis, nanomaterials processing

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4319.

Fuente, *Nanotechnology* 12 (2001) 147.

Several techniques are available that allow the size distribution of an aerosol to be determined in real time, but the determination of chemical composition, which has traditionally been done by impaction methods, is slow and yields only an average composition of the ensemble of particles in a given size range. It is essential, therefore, that new techniques be developed to allow the characterization of both the physical properties and chemical composition of aerosols, and that these operate on a time-scale that allows changes in the aerosol composition to be determined in real time. Several variants of the aerosol TOFMS (ATOFMS) instrument have been described in the literature [2]. The principle of the most sophisticated instrument reported to date employs two laser systems; first, a tuneable IR laser (OPO) is used to desorb material selectively from the particle, and then a second (VUV) laser is used to ionize the molecules that are produced [2]. With this approach, greater control over the particle ablation and ionization steps is possible, and by using low IR laser energy for the first evaporation step it is possible to depth profile heterogeneously mixed aerosol particles. Molecular information can be obtained by tuning the laser energy to just above the threshold required for desorption.

#### **5. Conclusions**

Several applications have been demonstrated for CO2 lasers, despite it is impossible to use photoelectric emission to detect this radiation (photon energy of about 0.1 eV is only about five times room temperature, and cryogenically cooled photoconductors are necessary to achieve fast low-level detection) and little engineering has been done in the mid-infrared region.

All applications mentioned require a stable, single-frequency source of radiation. A kilowatt of radiation at 10 microns, focused down to its diffraction limit, is a power density of 1 gigawatt per square cm. Because most materials absorb at 10 microns, considerable interest has been shown in CO2 lasers in many applications, and any problem which requires controlled surface heating or burning might find a potential solution with the CO2 laser.

The MTL3-GT CO2 laser ability to combine the laser pulsed mode with tune-ability introduces new perspectives to perform different experiments, which require a suitable, reliable and user-friendly procedure. With the method described in this work, many experiments can be performed in real time with simultaneous control of power*/*energy and wavelength, and taking advantage of the full laser power for each selected wavelength. One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behavior was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used. The observation of the energy line variation in real time is very important for understanding the behavior of the laser under certain external conditions and also to make sure that internal mechanisms are also error free.

It was demonstrated that CO2 laser applications in the fields of nanoscience and nanotechnology are very promising, with particular relevance on spectroscopy, photodynamic kinetics and photodynamic therapy, ultra-pure and size-selected nanoparticle production, surgery and biomedicine, nanoanalysis, nanomaterials processing and composite nano-engineered catalysts.

#### **6. References**

352 CO2 Laser – Optimisation and Application

Several techniques are available that allow the size distribution of an aerosol to be determined in real time, but the determination of chemical composition, which has traditionally been done by impaction methods, is slow and yields only an average composition of the ensemble of particles in a given size range. It is essential, therefore, that new techniques be developed to allow the characterization of both the physical properties and chemical composition of aerosols, and that these operate on a time-scale that allows changes in the aerosol composition to be determined in real time. Several variants of the aerosol TOFMS (ATOFMS) instrument have been described in the literature [2]. The principle of the most sophisticated instrument reported to date employs two laser systems; first, a tuneable IR laser (OPO) is used to desorb material selectively from the particle, and then a second (VUV) laser is used to ionize the molecules that are produced [2]. With this approach, greater control over the particle ablation and ionization steps is possible, and by using low IR laser energy for the first evaporation step it is possible to depth profile heterogeneously mixed aerosol particles. Molecular information can be obtained by tuning the laser energy to

Several applications have been demonstrated for CO2 lasers, despite it is impossible to use photoelectric emission to detect this radiation (photon energy of about 0.1 eV is only about five times room temperature, and cryogenically cooled photoconductors are necessary to achieve fast low-level detection) and little engineering has been done in the mid-infrared

All applications mentioned require a stable, single-frequency source of radiation. A kilowatt of radiation at 10 microns, focused down to its diffraction limit, is a power density of 1 gigawatt per square cm. Because most materials absorb at 10 microns, considerable interest has been shown in CO2 lasers in many applications, and any problem which requires controlled surface heating or burning might find a potential solution with the CO2 laser.

The MTL3-GT CO2 laser ability to combine the laser pulsed mode with tune-ability introduces new perspectives to perform different experiments, which require a suitable, reliable and user-friendly procedure. With the method described in this work, many experiments can be performed in real time with simultaneous control of power*/*energy and wavelength, and taking advantage of the full laser power for each selected wavelength. One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behavior was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used. The observation of the energy line variation in real time is very important for understanding the behavior of the laser under certain external conditions and also to make

It was demonstrated that CO2 laser applications in the fields of nanoscience and nanotechnology are very promising, with particular relevance on spectroscopy, photodynamic kinetics and photodynamic therapy, ultra-pure and size-selected

just above the threshold required for desorption.

sure that internal mechanisms are also error free.

**5. Conclusions** 

region.


**Part 4** 

**Medical Applications**


## **Part 4**

**Medical Applications**

354 CO2 Laser – Optimisation and Application

[32] F. J. M. Harren and J. Reuss, *Photoacoustic Spectroscopy* in Encyclopedia of Applied

[36] C. Montero, J. M. Orea, M. Soledad Muñoz, R. F. M. Lobo, A. González Ureña, *Applied* 

[33] M. W. Sigrist, A. Bohren, T. Lerber, M. Nagel, M. Romann, *Anal. Sci*., 17 (2001) S511.

[35] J. M. Orea, C. Montero, J. B. Jiménez, A. G. Ureña, *Anal Chem*., 73 (2001) 5921.

Physics, vol 19, G L Trigg (ed.) , VCH, Weinheim,1997.

[34] De Vries, F J M Harren and J Reuss, *Biol. Technol.*, 6 (1995) 275.

*Phys. B*, 71 (2000) 601.

**14** 

 *Korea* 

**Clinical Application of CO2 Laser** 

The carbon dioxide (CO2) laser was first introduced in 1964 by Patel and has been extensively used in the next two decades as an incision tool in increasingly wide areas, such as neurosurgery, dermatology and plastic surgery, otorhinolaryngology, ophthalmology, gynecology, and general surgery. In 1984, its reliability resulted in its approval by the U.S. Food and Drug Administration, and thus, medical use of lasers became more prevalent. Currently, the CO2 laser is considered an indispensable piece of diagnostic and therapeutic

The CO2 laser produces a beam of infrared light with the principal wavelength bands centering at 10,600 nanometers. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation. It is easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch,

CO2 lasers are attracting attention as cutting tools. They are able to seal lymphatic and blood vessels less than 0.5-mm wide and can reduce intraoperative bleeding and the occurrence of postoperative swelling. CO2 lasers emit a longer wavelength than those transmitted by other types of lasers. Their penetration depth of 0.03 mm is very safe. Coagulation in small blood vessels, as well as sealing of lymphatic and small peripheral nerves, have been reported in

The CO2 laser also offers more comfort to patients by reducing intraoperative bleeding and postoperative edema, facilitating the process of wound healing after surgery. The boundaries between the tissues receiving heat damage and the surrounding intact tissue are very well defined. A CO2 laser can evaporate through the surrounding tissue without physical force, sealing the vessel and minimizing bleeding; thus, it is useful when a bloodless view is required during surgery. Moreover, wounds can be treated in a sterile

Regarding its disadvantages, the equipment is expensive, operators require time to become familiar with it, and the sophisticated operation is technically difficult. Therefore, more repetitions are required to gain the necessary experience and practice. In addition, there is a risk of fire if the laser is used improperly. It can also damage the cornea; thus, eye protection is needed for the surgeon and the patient. Because the gas discharged from the vaporization of tissue contains an excess of CO2 or virus particles, it can be harmful to the human body.

giving rise to Q-switched peak powers up to gigawatts (GW) of peak power.

experimental studies using CO2 lasers; this sealing alleviates postoperative pain.

manner because of high-temperature evaporation of tissue lesions.

**1. Introduction** 

equipment.

Hyeong-Seok Oh and Jin-Sung Kim

*Wooridul Spine Hospital, Seoul* 

### **Clinical Application of CO2 Laser**

Hyeong-Seok Oh and Jin-Sung Kim *Wooridul Spine Hospital, Seoul Korea* 

#### **1. Introduction**

The carbon dioxide (CO2) laser was first introduced in 1964 by Patel and has been extensively used in the next two decades as an incision tool in increasingly wide areas, such as neurosurgery, dermatology and plastic surgery, otorhinolaryngology, ophthalmology, gynecology, and general surgery. In 1984, its reliability resulted in its approval by the U.S. Food and Drug Administration, and thus, medical use of lasers became more prevalent. Currently, the CO2 laser is considered an indispensable piece of diagnostic and therapeutic equipment.

The CO2 laser produces a beam of infrared light with the principal wavelength bands centering at 10,600 nanometers. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation. It is easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers up to gigawatts (GW) of peak power.

CO2 lasers are attracting attention as cutting tools. They are able to seal lymphatic and blood vessels less than 0.5-mm wide and can reduce intraoperative bleeding and the occurrence of postoperative swelling. CO2 lasers emit a longer wavelength than those transmitted by other types of lasers. Their penetration depth of 0.03 mm is very safe. Coagulation in small blood vessels, as well as sealing of lymphatic and small peripheral nerves, have been reported in experimental studies using CO2 lasers; this sealing alleviates postoperative pain.

The CO2 laser also offers more comfort to patients by reducing intraoperative bleeding and postoperative edema, facilitating the process of wound healing after surgery. The boundaries between the tissues receiving heat damage and the surrounding intact tissue are very well defined. A CO2 laser can evaporate through the surrounding tissue without physical force, sealing the vessel and minimizing bleeding; thus, it is useful when a bloodless view is required during surgery. Moreover, wounds can be treated in a sterile manner because of high-temperature evaporation of tissue lesions.

Regarding its disadvantages, the equipment is expensive, operators require time to become familiar with it, and the sophisticated operation is technically difficult. Therefore, more repetitions are required to gain the necessary experience and practice. In addition, there is a risk of fire if the laser is used improperly. It can also damage the cornea; thus, eye protection is needed for the surgeon and the patient. Because the gas discharged from the vaporization of tissue contains an excess of CO2 or virus particles, it can be harmful to the human body.

Clinical Application of CO2 Laser 359

surgery (Nerubay, Caspi et al. 1997; Hellinger 1999; Houck 2006). Nerubay et al. reported that 50 patients with low back and radicular pains were successfully treated by percutaneous laser nucleolysis using a CO2 laser (Nerubay, Caspi et al. 1997), and successful vaporization of the disk was accomplished in animal models (Stein, Sedlacek et al. 1990).

Considering the similarity between the disk and the meniscus (Whipple, Caspari et al. 1984), we cite studies on the effect of the CO2 laser on the meniscus. According to these research results, there was a considerable proliferation of cells resembling chondrocytes after 2 weeks of the CO2 laser treatment and there was definitely an increase in the production of ground substance and immature collagen fibers after 4 weeks; the collagen had become well reorganized into a logical orientation, resembling the normal architecture of fibrocartilage,

These animal and clinical studies strongly support the claim that CO2 lasers can safely and feasibly be used for the removal of protruded disks and discal cysts. Moreover, the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of the

Laser removes disk material by vaporization (Stein, Sedlacek et al. 1990) and consequently lowers intradiskal pressure (Gropper, Robertson et al. 1984). In spine surgery, the use of a laser has advantages over scalpel use in terms of precision; the ability to be used on delicate tissues; minimal tissue manipulation; and less bleeding, swelling, and trauma (Jeon, Lee et al. 2007). It is especially useful in the small spaces involved in herniated disks (Kim, Choi et al. 2009). Therefore, a laser is an effective tool for performing a minimally invasive spinal surgery with percutaneous and open spinal procedures (Ahn, Lee et al. 2005; Lee, Ahn et al. 2006; Lee, Ahn et al. 2006; Jeon, Lee et al. 2007; Lee, Ahn et al. 2008; Kim, Choi et al. 2009;

In the Wooridul Hospital, CO2 laser-equipped surgical microscopes have been used for open lumbar microdiscectomy since December 1991 (Fig. 1). These microscopes coaxially align the invisible CO2 laser beam with a visible helium-neon laser beam and can focus exactly on and evaporate the target disk material by the commonly used 20- to 30-W single-pulse mode laser. Therefore, we aimed to determine whether a CO2 laser-equipped surgical microscope

Lee et al. (Lee and Lee 2011) reported that the CO2 laser-assisted microdiscectomy could be an effective alternative to conventional microdiscectomy techniques. Because the CO2 laser enabled effective removal of extraforaminal lumbar disk herniation(EFLDH) via a narrow extraforaminal operative corridor without excessive loss of the facet joint and/or the par interarticularis, a thorough decompression of the extraforaminal and/or the foraminal zone was achieved while preserving spinal stability (Fig. 2). Thirty-one patients exhibited a marked reduction in leg pain immediately after the surgery. No patient complained of persistent severe leg pain in the perioperative period. In the present study, reherniation occurred in 1 patient (3.6%) at the 1-year follow-up. The CO2 laser is also believed to decrease reherniation after discectomy owing to laser-induced metaplasia. (Kim, Choi et al.

after 10 weeks (Benjamin, Qin et al. 1995).

**2.2.1 Disk herniation** 

Kim and Lee 2009).

2009)

is a useful tool for microdiscectomy.

discal cyst and, if needed, easy vaporization of disk material.

#### **2. Clinical application in neurosurgery**

The CO2 laser is most widely used in the field of neurosurgery for removal and evaporation of tumors located in difficult surgical fields, such as the base of the skull, ventricles, brainstem, and spinal cord.

#### **2.1 Brain tumor surgery**

The CO2 laser has been used in brain microsurgery after Steller et al. (Stellar, Polanyi et al. 1970) had first successfully used it in removing a recurrent glioma in 1969. The most ideal treatment of a brain tumor is minimizing damage to the normal brain tissue and removing only the tumor area. To overcome the surgical difficulty of avoiding damage to the brain tissue, a special instrument was developed. Theoretically, lasers have several advantages. First, although the surgical field is narrow, it makes surgery possible. Other small-sized surgical approaches are facilitated to minimize injury to normal brain tissue. Second, brain retraction is minimized, thus causing less damage to normal brain tissue. Third, laser beam minimizes injury to surrounding tissues and enables removal of a tumor with less thermal injury. Fourth, lasers have a coagulating property that lessens bleeding of the surgical field. Fifth, operation time is shortened (Tew and Tobler 1983; Krishnamurthy and Powers 1994).

The CO2 laser is the main instrument used in brain surgery. It has the advantage of rapidly removing separated tumor cells and exact irradiation of target cells by a microsurgical technique where the CO2 laser is installed with a microscope. However, as energy cannot pass through an optical fiber, it is inconvenient to use the equipment. It has limited function in bleeding control, as control of bleeding is not possible in a vessel with a diameter 0.5 mm, necessitating the use of the equipment in conjunction with other equipments for severe bleeding management (Heppner 1978; Ascher and Heppner 1984; Deruty, Pelissou-Guyotat et al. 1993).

The CO2 laser is most widely used in the field of neurosurgery, and it is mainly used in the removal of tumors by evaporation where surgical approach of the tumor site is difficult. It is common opinion that the CO2 laser is most effective with skull base, ventricular, brainstem, and spinal cord tumors (Powers, Cush et al. 1991; Origitano and Reichman 1993). In particular, it is most effective in removing a meningioma that is relatively hard or has less vascular distribution to be calcified. In addition, it is suitable for removing a low-grade glioma that is relatively rigid (Deruty, Pelissou-Guyotat et al. 1993).

The Nd:YAG laser has the advantage that energy can be passed to thinner fiberoptic cables and excellent clotting function is possible at a 3-mm vessel. Therefore, it has been reported as a valid technique of removing brain tumors having greater vascular distribution and cerebral vascular malformation (Beck 1980). The combolaser has been developed in recent years by Fasano et al. (Glasscock, Jackson et al. 1981) and has been applied in surgery. It is composed of CO2 and Nd:YAG lasers, combining the advantages of both. It works by first emitting Nd:YAG energy to the tumor for clotting, followed by tumor removal by evaporation using the CO2 laser (Beck 1980; Glasscock, Jackson et al. 1981).

#### **2.2 Spine surgery**

Since the first trial of Nd:YAG in a lumbar disk surgery in 1986 (Choy, Case et al. 1987), there have been many reports about the usefulness of different kinds of lasers in disk

The CO2 laser is most widely used in the field of neurosurgery for removal and evaporation of tumors located in difficult surgical fields, such as the base of the skull, ventricles,

The CO2 laser has been used in brain microsurgery after Steller et al. (Stellar, Polanyi et al. 1970) had first successfully used it in removing a recurrent glioma in 1969. The most ideal treatment of a brain tumor is minimizing damage to the normal brain tissue and removing only the tumor area. To overcome the surgical difficulty of avoiding damage to the brain tissue, a special instrument was developed. Theoretically, lasers have several advantages. First, although the surgical field is narrow, it makes surgery possible. Other small-sized surgical approaches are facilitated to minimize injury to normal brain tissue. Second, brain retraction is minimized, thus causing less damage to normal brain tissue. Third, laser beam minimizes injury to surrounding tissues and enables removal of a tumor with less thermal injury. Fourth, lasers have a coagulating property that lessens bleeding of the surgical field. Fifth, operation time is shortened (Tew and Tobler 1983; Krishnamurthy and Powers 1994). The CO2 laser is the main instrument used in brain surgery. It has the advantage of rapidly removing separated tumor cells and exact irradiation of target cells by a microsurgical technique where the CO2 laser is installed with a microscope. However, as energy cannot pass through an optical fiber, it is inconvenient to use the equipment. It has limited function in bleeding control, as control of bleeding is not possible in a vessel with a diameter 0.5 mm, necessitating the use of the equipment in conjunction with other equipments for severe bleeding management (Heppner 1978; Ascher and Heppner 1984; Deruty, Pelissou-Guyotat

The CO2 laser is most widely used in the field of neurosurgery, and it is mainly used in the removal of tumors by evaporation where surgical approach of the tumor site is difficult. It is common opinion that the CO2 laser is most effective with skull base, ventricular, brainstem, and spinal cord tumors (Powers, Cush et al. 1991; Origitano and Reichman 1993). In particular, it is most effective in removing a meningioma that is relatively hard or has less vascular distribution to be calcified. In addition, it is suitable for removing a low-grade

The Nd:YAG laser has the advantage that energy can be passed to thinner fiberoptic cables and excellent clotting function is possible at a 3-mm vessel. Therefore, it has been reported as a valid technique of removing brain tumors having greater vascular distribution and cerebral vascular malformation (Beck 1980). The combolaser has been developed in recent years by Fasano et al. (Glasscock, Jackson et al. 1981) and has been applied in surgery. It is composed of CO2 and Nd:YAG lasers, combining the advantages of both. It works by first emitting Nd:YAG energy to the tumor for clotting, followed by tumor removal by

Since the first trial of Nd:YAG in a lumbar disk surgery in 1986 (Choy, Case et al. 1987), there have been many reports about the usefulness of different kinds of lasers in disk

glioma that is relatively rigid (Deruty, Pelissou-Guyotat et al. 1993).

evaporation using the CO2 laser (Beck 1980; Glasscock, Jackson et al. 1981).

**2. Clinical application in neurosurgery** 

brainstem, and spinal cord.

**2.1 Brain tumor surgery** 

et al. 1993).

**2.2 Spine surgery** 

surgery (Nerubay, Caspi et al. 1997; Hellinger 1999; Houck 2006). Nerubay et al. reported that 50 patients with low back and radicular pains were successfully treated by percutaneous laser nucleolysis using a CO2 laser (Nerubay, Caspi et al. 1997), and successful vaporization of the disk was accomplished in animal models (Stein, Sedlacek et al. 1990).

Considering the similarity between the disk and the meniscus (Whipple, Caspari et al. 1984), we cite studies on the effect of the CO2 laser on the meniscus. According to these research results, there was a considerable proliferation of cells resembling chondrocytes after 2 weeks of the CO2 laser treatment and there was definitely an increase in the production of ground substance and immature collagen fibers after 4 weeks; the collagen had become well reorganized into a logical orientation, resembling the normal architecture of fibrocartilage, after 10 weeks (Benjamin, Qin et al. 1995).

These animal and clinical studies strongly support the claim that CO2 lasers can safely and feasibly be used for the removal of protruded disks and discal cysts. Moreover, the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of the discal cyst and, if needed, easy vaporization of disk material.

#### **2.2.1 Disk herniation**

Laser removes disk material by vaporization (Stein, Sedlacek et al. 1990) and consequently lowers intradiskal pressure (Gropper, Robertson et al. 1984). In spine surgery, the use of a laser has advantages over scalpel use in terms of precision; the ability to be used on delicate tissues; minimal tissue manipulation; and less bleeding, swelling, and trauma (Jeon, Lee et al. 2007). It is especially useful in the small spaces involved in herniated disks (Kim, Choi et al. 2009). Therefore, a laser is an effective tool for performing a minimally invasive spinal surgery with percutaneous and open spinal procedures (Ahn, Lee et al. 2005; Lee, Ahn et al. 2006; Lee, Ahn et al. 2006; Jeon, Lee et al. 2007; Lee, Ahn et al. 2008; Kim, Choi et al. 2009; Kim and Lee 2009).

In the Wooridul Hospital, CO2 laser-equipped surgical microscopes have been used for open lumbar microdiscectomy since December 1991 (Fig. 1). These microscopes coaxially align the invisible CO2 laser beam with a visible helium-neon laser beam and can focus exactly on and evaporate the target disk material by the commonly used 20- to 30-W single-pulse mode laser. Therefore, we aimed to determine whether a CO2 laser-equipped surgical microscope is a useful tool for microdiscectomy.

Lee et al. (Lee and Lee 2011) reported that the CO2 laser-assisted microdiscectomy could be an effective alternative to conventional microdiscectomy techniques. Because the CO2 laser enabled effective removal of extraforaminal lumbar disk herniation(EFLDH) via a narrow extraforaminal operative corridor without excessive loss of the facet joint and/or the par interarticularis, a thorough decompression of the extraforaminal and/or the foraminal zone was achieved while preserving spinal stability (Fig. 2). Thirty-one patients exhibited a marked reduction in leg pain immediately after the surgery. No patient complained of persistent severe leg pain in the perioperative period. In the present study, reherniation occurred in 1 patient (3.6%) at the 1-year follow-up. The CO2 laser is also believed to decrease reherniation after discectomy owing to laser-induced metaplasia. (Kim, Choi et al. 2009)

Clinical Application of CO2 Laser 361

Owing to the steep learning curve of PELD (Lee and Lee 2008), the modified

In the study by Kims et al (Kim 2010), 21 cases of rLDH, which caused the same symptoms and signs as those of virgin lumbar disk herniations, were excised successfully with

The author used CO2 laser during modified lumbar microdiscectomy and reported that using the technique, surgeons can focus the laser beam exactly on the target adhesion scar for adhesiolysis and vaporization and then quickly and easily dissect the adhesion scar tissue. In his results, no approach-related or CO2 laser-related complications developed. In our opinion, the reason that no incidental durotomy occurred in our series is the precise and

Fig. 3. A. Operative view showing granulation tissue and recurrent lumbar disc herniation located ventromedially to the L5 nerve root (black asterisk) B. The small tip of the CO2 laser

Fig. 4. Operative view presenting easily access narrow ventral part of nerve root using CO2 laser where blunt scalpel couldn't access, with a slight gentle retraction of nerve root

microdiscectomy is still more popularity.

modified microdiscectomy using a CO2 laser.

gentle dissection using the CO2 laser (Fig. 3 A.B).

A. B.

could be seen on the protruded disc (black arrow).

Fig. 1. Photograph of a CO2 laser-equipped surgical microscope.

Fig. 2. Intraoperative photomicrographs depicting CO2 laser-assisted microdiscectomy for EFLDH at the L5/S1 level. Left. Photomicrograph taken after exposure of the L5 dorsal root ganglion (A:upper border of the sacral ala; D:herniated disc; F: the lateral L5-S1 facet joint, G:the L5 dorsal root ganglion; and T:the lower border of the L5 transverse process). Right: EFLDH being removed by CO2 laser with gentle retraction of L5 dorsal root ganglion. Note the red He-Ne beam in the surgical field.

#### **2.2.2 Recurrent disk herniation**

There are various surgical treatments for recurrent lumbar disk herniation (rLDH), including revision microdiscectomy, lumbar fusion with or without instrumentation (Choi, Lee et al. 2008), and recently, some minimally invasive methods, such as percutaneous endoscopic lumbar discectomy (PELD) (Ahn, Lee et al. 2004), have also been developed. They noted that favorable pain relief was achieved in most patients through this procedure.

Fig. 1. Photograph of a CO2 laser-equipped surgical microscope.

the red He-Ne beam in the surgical field.

**2.2.2 Recurrent disk herniation** 

Fig. 2. Intraoperative photomicrographs depicting CO2 laser-assisted microdiscectomy for EFLDH at the L5/S1 level. Left. Photomicrograph taken after exposure of the L5 dorsal root ganglion (A:upper border of the sacral ala; D:herniated disc; F: the lateral L5-S1 facet joint, G:the L5 dorsal root ganglion; and T:the lower border of the L5 transverse process). Right: EFLDH being removed by CO2 laser with gentle retraction of L5 dorsal root ganglion. Note

There are various surgical treatments for recurrent lumbar disk herniation (rLDH), including revision microdiscectomy, lumbar fusion with or without instrumentation (Choi, Lee et al. 2008), and recently, some minimally invasive methods, such as percutaneous endoscopic lumbar discectomy (PELD) (Ahn, Lee et al. 2004), have also been developed. They noted that favorable pain relief was achieved in most patients through this procedure. Owing to the steep learning curve of PELD (Lee and Lee 2008), the modified microdiscectomy is still more popularity.

In the study by Kims et al (Kim 2010), 21 cases of rLDH, which caused the same symptoms and signs as those of virgin lumbar disk herniations, were excised successfully with modified microdiscectomy using a CO2 laser.

The author used CO2 laser during modified lumbar microdiscectomy and reported that using the technique, surgeons can focus the laser beam exactly on the target adhesion scar for adhesiolysis and vaporization and then quickly and easily dissect the adhesion scar tissue. In his results, no approach-related or CO2 laser-related complications developed. In our opinion, the reason that no incidental durotomy occurred in our series is the precise and gentle dissection using the CO2 laser (Fig. 3 A.B).

Fig. 3. A. Operative view showing granulation tissue and recurrent lumbar disc herniation located ventromedially to the L5 nerve root (black asterisk) B. The small tip of the CO2 laser could be seen on the protruded disc (black arrow).

Fig. 4. Operative view presenting easily access narrow ventral part of nerve root using CO2 laser where blunt scalpel couldn't access, with a slight gentle retraction of nerve root

Clinical Application of CO2 Laser 363

Direct anterior decompression by corpectomy followed by fusion should be the proper choice of surgical treatment of this multi-level OPLL than indirect decompression by posterior

The rationale of preferring the anterior approach is based on evidence that the compressive elements are located anterior to the spinal cord in 75% of cases, and therapeutic benefit can be obtained by directly approaching these lesions(Cusick 1991). The degree of cervical myelopathy caused by OPLL is also reported to be influenced not only by static compression from the ossification mass, but also by abnormal intervertebral mobility at the

Despite these theoretical advantages, anterior corpectomy has been reported to be fraught with iatrogenic deterioration of the neurological state, and complications such as spinal fluid fistula or graft problem. Naturally, it will be more technically demanding if the OPLL is involved at multiple cervical levels, and treatment success will depend heavily on a less

In Lee at al report, the authors concluded that direct anterior cervical corpectomy using the CO2 laser resulted in a better recovery of neurological deficit, and adequate decompression of the spinal canal and maintenance of cervical regional lordosis at the operated level for patients with multilevel cervical OPLL. Assuming the surgeon can employ safe anterior microsurgical tools combined CO2 laser and decompression method, proceeding with direct decompressive corpectomy rather than indirect, inadequate laminoplasty is recommended if

They expected that a focused laser beam could vaporize the OPLL and even produce a positive effect. A 5-W pulse, single-pulse mode laser was sufficient to vaporize a thinned OPLL or an osteophyte, as it is known to penetrate only the outer table of the bone(Neblett

Fig. 6. An illustration showing the tope view (A) and side view(B) of the surgical technique

microdissector (d), which is held between the OPLL. (p) and the dura to avoid laser-induced

used to remove the densely adhered OPLL using the CO2 laser (L) with the angled

laminoplasy.

traumatic manipulation.

1992).(Fig.6)

damage to the dura.

responsible level(s)(Onari, Akiyama et al. 2001).

the patient's preoperative status is appropriate.

Because an epidural or perineural scar tissue may hinder the dissection using the modified microdiscectomy, increasing the risk of incidental durotomy or iatrogenic neural injury, the CO2 laser can help surgeons make more precise and safe dissections of the scar tissue than when using a blunt scalpel. Calcifications around recurrent disk fragments are often seen, which may also hinder surgeons to dissect safely. However, with the aid of the CO2 laser, surgeons can evaporate the calcified portion of the disk without excessive retraction of the nerve root via a narrow operative corridor (Lee, Ahn et al. 2008). Moreover, with a slight gentle retraction of the nerve root, surgeons can easily access the narrow ventral part of the nerve root using the CO2 laser, where a blunt scalpel could not (Fig. 4).

#### **2.2.3 Discal cyst**

Many kinds of surgical methods have been introduced for the treatment of discal cysts. Most discal cysts reported have been treated by open surgical excision (Chiba, Toyama et al. 2001; Lee, Lee et al. 2006) or with some direct intervention, such as computed tomography-guided aspiration and steroid injection (Kang, Liu et al. 2008). Recently, it was reported that a discal cyst was treated with a minimally invasive technique using PELD (Min 2006).

Kim et al. (Kim and Lee 2009) reported that the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of a discal cyst and, if needed, easy vaporization of disk material. In his study, 14 cases of discal cyst that caused the same symptoms and signs as those of lumbar disk herniations were excised successfully by open surgery using a CO2 laser.

After the intraoperative removal of the discal cyst, the authors found the communication hole between the cyst and the protruded disk. They then used the heat energy produced by CO2 lasering and removed the pulled-out disk fragment, if any existed, after pushing into the disk space with a right-angled probe (Fig. 5 A.B).

Fig. 5. (A) Distinct communication-like hole between the cyst and intervertebral disc (white arrow) and (B) small tip of laser on the protruded disc (white arrow)

#### **2.2.4 Cervical ossification of ligamentum flavum (OPLL)**

The choice of a surgical approach for multi-level cervical OPLL is still a controversial issue.

Because an epidural or perineural scar tissue may hinder the dissection using the modified microdiscectomy, increasing the risk of incidental durotomy or iatrogenic neural injury, the CO2 laser can help surgeons make more precise and safe dissections of the scar tissue than when using a blunt scalpel. Calcifications around recurrent disk fragments are often seen, which may also hinder surgeons to dissect safely. However, with the aid of the CO2 laser, surgeons can evaporate the calcified portion of the disk without excessive retraction of the nerve root via a narrow operative corridor (Lee, Ahn et al. 2008). Moreover, with a slight gentle retraction of the nerve root, surgeons can easily access the narrow ventral part of the

Many kinds of surgical methods have been introduced for the treatment of discal cysts. Most discal cysts reported have been treated by open surgical excision (Chiba, Toyama et al. 2001; Lee, Lee et al. 2006) or with some direct intervention, such as computed tomography-guided aspiration and steroid injection (Kang, Liu et al. 2008). Recently, it was reported that a discal cyst was treated with a minimally invasive technique using

Kim et al. (Kim and Lee 2009) reported that the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of a discal cyst and, if needed, easy vaporization of disk material. In his study, 14 cases of discal cyst that caused the same symptoms and signs as those of lumbar disk herniations were excised successfully by open

After the intraoperative removal of the discal cyst, the authors found the communication hole between the cyst and the protruded disk. They then used the heat energy produced by CO2 lasering and removed the pulled-out disk fragment, if any existed, after pushing into

Fig. 5. (A) Distinct communication-like hole between the cyst and intervertebral disc (white

The choice of a surgical approach for multi-level cervical OPLL is still a controversial issue.

arrow) and (B) small tip of laser on the protruded disc (white arrow)

**2.2.4 Cervical ossification of ligamentum flavum (OPLL)** 

nerve root using the CO2 laser, where a blunt scalpel could not (Fig. 4).

**2.2.3 Discal cyst** 

PELD (Min 2006).

surgery using a CO2 laser.

the disk space with a right-angled probe (Fig. 5 A.B).

Direct anterior decompression by corpectomy followed by fusion should be the proper choice of surgical treatment of this multi-level OPLL than indirect decompression by posterior laminoplasy.

The rationale of preferring the anterior approach is based on evidence that the compressive elements are located anterior to the spinal cord in 75% of cases, and therapeutic benefit can be obtained by directly approaching these lesions(Cusick 1991). The degree of cervical myelopathy caused by OPLL is also reported to be influenced not only by static compression from the ossification mass, but also by abnormal intervertebral mobility at the responsible level(s)(Onari, Akiyama et al. 2001).

Despite these theoretical advantages, anterior corpectomy has been reported to be fraught with iatrogenic deterioration of the neurological state, and complications such as spinal fluid fistula or graft problem. Naturally, it will be more technically demanding if the OPLL is involved at multiple cervical levels, and treatment success will depend heavily on a less traumatic manipulation.

In Lee at al report, the authors concluded that direct anterior cervical corpectomy using the CO2 laser resulted in a better recovery of neurological deficit, and adequate decompression of the spinal canal and maintenance of cervical regional lordosis at the operated level for patients with multilevel cervical OPLL. Assuming the surgeon can employ safe anterior microsurgical tools combined CO2 laser and decompression method, proceeding with direct decompressive corpectomy rather than indirect, inadequate laminoplasty is recommended if the patient's preoperative status is appropriate.

They expected that a focused laser beam could vaporize the OPLL and even produce a positive effect. A 5-W pulse, single-pulse mode laser was sufficient to vaporize a thinned OPLL or an osteophyte, as it is known to penetrate only the outer table of the bone(Neblett 1992).(Fig.6)

Fig. 6. An illustration showing the tope view (A) and side view(B) of the surgical technique used to remove the densely adhered OPLL using the CO2 laser (L) with the angled microdissector (d), which is held between the OPLL. (p) and the dura to avoid laser-induced damage to the dura.

Clinical Application of CO2 Laser 365

After destructive treatment by CO2 laser at skin tumor was universal, it was known that aging skin was reformed as causing shrinkage in the recovery process after resurfacing. Laser resurfacing has been used as representative of treatment of aging of the skin in spite of

There was effort to reduce the inconvenience of resurfacing by conventional laser using for rejuvenation and minimized the downtime. In 1991 Dr Shimon Dckhouse developed intense pulsed light(IPL) emiting to single pulse from multiple optical energy and introduced advantage for various clinical effect. So it was introduced the concept of non-ablative

Since introduction of the infrared wavelength range of equipment in 2006, AFR concept of a number of devices are being launched as merging ablation and FP using CO2 and Er:YAG laser. Recently, laser equipment of IR, CO2, and Er:YAG are coexisting. The concept of FP having advantage of being safely usable of high powered energy is proliferated broadly to

CO2 laser is used to treatment of antiaging as removing by ablation of aging tissue and

Though resurfacing using existing CO2 laser has many discomfort as ablation of total skin, CO2 fractional laser is focus to treat only fine territory partially. So it is enable to treat safely epidermis and dermis though more high energy than existing treatment is transferred. Because the wound can be restored quickly and easily from surrounding normal skin

**3.1.2 The development and change of fractional photothermolysis(FP)** 

accelerating to regeneration of dermis by transmission of thermal stimulus.

2. The period of CO2 laser resurfacing

long downtime from early 1990.

3. Introduction of NAR

rejuvenation(NAR).

all territory of laser.

**3.2 Treatment principle of CO2 fractional laser** 

though injured at epidermis and dermis by laser (Fig. 8.9).

Fig. 8. The basic concept of Fractional Photothermolysis

#### **2.2.5 Complication**

Previously, a case of major vessel injury involving perforation of the iliac artery during CO2 laser-assisted lumbar microdiscectomy, caused by prolonged irradiation of the CO2 laser into the deep anterior disc space, has been reported.(Jeon, Lee et al. 2007) Avoiding point focusing of the CO 2 laser on the surface of the anterior annulus, as well as injecting a small amount of saline at the bottom of the intradiscal space during laser ablation, can prevent the occurrence of such a complication. (Jeon, Lee et al. 2007)

And during this procedure, keeping the surgical field moistened was important to minimize the risk of inadvertent injury, as water can absorb CO2 laser energy immediately (Choi, Lee et al. 2005). (Fig.7)

Fig. 7. Preoperative axial MRI at the L5-S1 level. White arrow indicates the direction of the carbon dioxide laser beam. A, External iliac artery. B, Internal iliac artery. C, Common iliac vein. D, Herniated disc fragment.

### **3. Clinical application in dermatology (Jung 2008)**

#### **3.1 Evolution of rejuvenation using lasers. (Alexiades-Armenakas, Dover et al. 2008; Jih and Kimyai-Asadi 2008)**

It is described about the evolution of CO2 fractional lasers utilized in the treatment of aging on a typical appliance, equipmental characteristics, clinical utilization in the future development direction,

#### **3.1.1 Historical background of fractional photothermolysis(FP)**

1. The period of introduction of concept of SPTL

Since Dr Rox Anderson represented the concept of selective photothermolysis(SPTL) In 1983, specific treatment methods of selectively targeting chromophore like melanin and hemoglobin have been developed clinically.

Previously, a case of major vessel injury involving perforation of the iliac artery during CO2 laser-assisted lumbar microdiscectomy, caused by prolonged irradiation of the CO2 laser into the deep anterior disc space, has been reported.(Jeon, Lee et al. 2007) Avoiding point focusing of the CO 2 laser on the surface of the anterior annulus, as well as injecting a small amount of saline at the bottom of the intradiscal space during laser ablation, can prevent the

And during this procedure, keeping the surgical field moistened was important to minimize the risk of inadvertent injury, as water can absorb CO2 laser energy immediately (Choi, Lee

Fig. 7. Preoperative axial MRI at the L5-S1 level. White arrow indicates the direction of the carbon dioxide laser beam. A, External iliac artery. B, Internal iliac artery. C, Common iliac

**3.1 Evolution of rejuvenation using lasers. (Alexiades-Armenakas, Dover et al. 2008;** 

It is described about the evolution of CO2 fractional lasers utilized in the treatment of aging on a typical appliance, equipmental characteristics, clinical utilization in the future

Since Dr Rox Anderson represented the concept of selective photothermolysis(SPTL) In 1983, specific treatment methods of selectively targeting chromophore like melanin and

occurrence of such a complication. (Jeon, Lee et al. 2007)

**2.2.5 Complication** 

et al. 2005). (Fig.7)

vein. D, Herniated disc fragment.

**Jih and Kimyai-Asadi 2008)** 

development direction,

**3. Clinical application in dermatology (Jung 2008)** 

**3.1.1 Historical background of fractional photothermolysis(FP)** 

1. The period of introduction of concept of SPTL

hemoglobin have been developed clinically.

2. The period of CO2 laser resurfacing

After destructive treatment by CO2 laser at skin tumor was universal, it was known that aging skin was reformed as causing shrinkage in the recovery process after resurfacing. Laser resurfacing has been used as representative of treatment of aging of the skin in spite of long downtime from early 1990.

3. Introduction of NAR

There was effort to reduce the inconvenience of resurfacing by conventional laser using for rejuvenation and minimized the downtime. In 1991 Dr Shimon Dckhouse developed intense pulsed light(IPL) emiting to single pulse from multiple optical energy and introduced advantage for various clinical effect. So it was introduced the concept of non-ablative rejuvenation(NAR).

#### **3.1.2 The development and change of fractional photothermolysis(FP)**

Since introduction of the infrared wavelength range of equipment in 2006, AFR concept of a number of devices are being launched as merging ablation and FP using CO2 and Er:YAG laser. Recently, laser equipment of IR, CO2, and Er:YAG are coexisting. The concept of FP having advantage of being safely usable of high powered energy is proliferated broadly to all territory of laser.

#### **3.2 Treatment principle of CO2 fractional laser**

CO2 laser is used to treatment of antiaging as removing by ablation of aging tissue and accelerating to regeneration of dermis by transmission of thermal stimulus.

Though resurfacing using existing CO2 laser has many discomfort as ablation of total skin, CO2 fractional laser is focus to treat only fine territory partially. So it is enable to treat safely epidermis and dermis though more high energy than existing treatment is transferred. Because the wound can be restored quickly and easily from surrounding normal skin though injured at epidermis and dermis by laser (Fig. 8.9).

Fig. 8. The basic concept of Fractional Photothermolysis

Clinical Application of CO2 Laser 367

Fig. 10. The main difference between fractional infrared and fractional CO2 laser.

Fig. 11. The wound healing process of fractional CO2 laser treatment

The water absorption rate is in the order of Er: YAG, CO2, and IR (Fig. 12)

CO2 and Er:YAG lasers have higher water absorption rates compared to IR (1064~1600nm) equipment under the same condition (Fig. 12), and most of the energy disappears during the ablation and vaporization process. Therefore, they have less lateral heat diffusion to surrounding tissues when the laser is irradiated and can minimize heat accumulation inside the dermis. In other words, IR equipment accumulates relatively more heat in dermis

**3.2.1.1 Difference in tissue reaction according to wavelength** 

Infrared (Er:Glass, 1,400-1,600nm)

CO2: 10,640 nm Er:YAG: 2,940 nm

tissues.

1. Water absorption

2. Lateral Heat Diffusion

Fig. 9. The basic concept of Fractional CO2 resurfacing: advantages.

You shall awaken warning to attach a laser cover at an operating room entrance as you use a CO2 laser if you enforce an operation.

An enough exhaust device shall install because a lot of extensions occur, and you disturb an operation visual field, and you pollute air when you vaporize an organization.

#### **3.2.1 Differences between the fractional infrared laser (1064~1600nm) and the CO2 fractional laser (10,600nm)**

CO2 laser ablates tissue, such as the epidermis and dermis, resulting in tissue damage. This outcome differs completely from the Fraxal tissue reaction, which occurs when a cut is treated by the existing infrared laser (Fig. 10). However, no comparative study has been conducted on the recovery process of infrared rays (IR) and CO2 laser. We know that CO2 laser damages the dermo-epidermal junction, causing severe inflammation at the outset, which in turn results in edema and erythema reactions. We also understand that the lesions damaged by CO2 laser ablation is first filled with keratonocyte within 48 hours and replaced by dermis through the remodeling process, a process that can be continued even after three months(Hantash, Bedi et al. 2007) (Fig. 11).

Fig. 9. The basic concept of Fractional CO2 resurfacing: advantages.

CO2 laser if you enforce an operation.

months(Hantash, Bedi et al. 2007) (Fig. 11).

**fractional laser (10,600nm)** 

You shall awaken warning to attach a laser cover at an operating room entrance as you use a

An enough exhaust device shall install because a lot of extensions occur, and you disturb an

CO2 laser ablates tissue, such as the epidermis and dermis, resulting in tissue damage. This outcome differs completely from the Fraxal tissue reaction, which occurs when a cut is treated by the existing infrared laser (Fig. 10). However, no comparative study has been conducted on the recovery process of infrared rays (IR) and CO2 laser. We know that CO2 laser damages the dermo-epidermal junction, causing severe inflammation at the outset, which in turn results in edema and erythema reactions. We also understand that the lesions damaged by CO2 laser ablation is first filled with keratonocyte within 48 hours and replaced by dermis through the remodeling process, a process that can be continued even after three

**3.2.1 Differences between the fractional infrared laser (1064~1600nm) and the CO2** 

operation visual field, and you pollute air when you vaporize an organization.

Fig. 10. The main difference between fractional infrared and fractional CO2 laser.

Fig. 11. The wound healing process of fractional CO2 laser treatment

### **3.2.1.1 Difference in tissue reaction according to wavelength**

Infrared (Er:Glass, 1,400-1,600nm) CO2: 10,640 nm Er:YAG: 2,940 nm

1. Water absorption

The water absorption rate is in the order of Er: YAG, CO2, and IR (Fig. 12)

2. Lateral Heat Diffusion

CO2 and Er:YAG lasers have higher water absorption rates compared to IR (1064~1600nm) equipment under the same condition (Fig. 12), and most of the energy disappears during the ablation and vaporization process. Therefore, they have less lateral heat diffusion to surrounding tissues when the laser is irradiated and can minimize heat accumulation inside the dermis. In other words, IR equipment accumulates relatively more heat in dermis tissues.

Clinical Application of CO2 Laser 369

The potential of erythema and pigmentation is related to the damage level of the epidermis and dermis-epidermis joint area as well as the inflammatory reaction due to heat stimulation on the dermis. The risk decreases as fractional treatment convergence is decreased, with the proper convergence of fractional treatment being 20%. Since the risk increases in proportion to the level of heat damage, the CO2 fractional laser can theoretically reduce epidermis damage and dermis heat damage because it has less lateral heat diffusion compared to the IR laser, which in turn will reduce the risk of erythema and pigmentation if

Theoretically, the CO2 fractional laser minimizes pain during procedures. Since most of the heat is lost during the tissue ablation process performed after laser irradiation, it has less lateral heat diffusion, which in turn reduces heat accumulation in tissues compared to IR equipment. Therefore, we expect less pain if we use the CO2 fractional laser. In addition, short pulse duration, smaller spot size, and shorter irradiation time on the skin can reduce pain. Therefore, it is theoretically possible to actualize the CO2 fractional laser with very little pain. However, this laser can damage the epidermis and dermis joint, which will cause

Since the CO2 laser is a continuous wave laser, output is written in watts. Most CO2 lasers can control parameters such as watt and pulse duration independently. Therefore, the same amount of energy (J) can be irradiated while the depth is controlled by watt level and the coagulation range (heat damage range) can be controlled by pulse duration (Fig. 13). These are very important strong points of the CO2 laser, differing from IR equipment, which changes power and pulse duration according to the J (energy) level. That is, IR equipments cannot independently control factors since power and pulse duration is simultaneously

5. Risk of erythema and pigmentation

6. Level of Pain

it can produce a quality laser beam and be irradiated.

**3.2.1.2 Can parameter be independently controlled?** 

increased as J (energy) is increased.

J (Energy) = Watt (Power)\* Time (pulse duration)

Fig. 13. Parameter controllability

more severe initial inflammation reaction and edema and burn feeling.

Fig. 12. Water absorption curve.

3. Shrinkage (Rosenberg, Brito et al. 1999; Fitzpatrick, Rostan et al. 2000; Jun, Harris et al. 2003; Zelickson, Kist et al. 2004; Hantash, Bedi et al. 2007)

Shrinkage is a tissue reaction related to the tightening effect that occurs due to tissue shortening after laser irradiation. So far, it is known to progress in the collagen denaturation zone (reversible thermal damage zone) (Fig. 10). Even though no comparative studies have been conducted on this process, IR equipment accumulates more heat inside dermis tissues under the same condition, which is why we think shrinkage will happen more with IR equipment. However, it is hard to estimate if more shrinkage is directly related to the rejuvenation effect, such as tissue tightening or lifting.

4. Penetration Depth

IR can penetrate 1.0~1.5 mm maximum. However, it is believed that the Er:YAG or CO2 lasers penetrate less than the IR laser since they absorb more water. According to recent studies, the CO2 laser can penetrate as deeply as the IR laser if the wattage is increased, the size of the beam is reduced, and the quality beam is irradiated vertically on skin. Due to the CO2 laser's characteristics, if the laser is repeatedly stacked on the same area, tissues can be constantly vaporized so that the laser can penetrate deeply enough. However, repetitive and deep penetration performed several times can cause unwanted excessive heat stimulation, leading to a higher chance of side effects. Therefore, it is right to consider the depth of single beam irradiation as a standard.

3. Shrinkage (Rosenberg, Brito et al. 1999; Fitzpatrick, Rostan et al. 2000; Jun, Harris et al.

Shrinkage is a tissue reaction related to the tightening effect that occurs due to tissue shortening after laser irradiation. So far, it is known to progress in the collagen denaturation zone (reversible thermal damage zone) (Fig. 10). Even though no comparative studies have been conducted on this process, IR equipment accumulates more heat inside dermis tissues under the same condition, which is why we think shrinkage will happen more with IR equipment. However, it is hard to estimate if more shrinkage is directly related to the

IR can penetrate 1.0~1.5 mm maximum. However, it is believed that the Er:YAG or CO2 lasers penetrate less than the IR laser since they absorb more water. According to recent studies, the CO2 laser can penetrate as deeply as the IR laser if the wattage is increased, the size of the beam is reduced, and the quality beam is irradiated vertically on skin. Due to the CO2 laser's characteristics, if the laser is repeatedly stacked on the same area, tissues can be constantly vaporized so that the laser can penetrate deeply enough. However, repetitive and deep penetration performed several times can cause unwanted excessive heat stimulation, leading to a higher chance of side effects. Therefore, it is right to consider the depth of single

2003; Zelickson, Kist et al. 2004; Hantash, Bedi et al. 2007)

rejuvenation effect, such as tissue tightening or lifting.

Fig. 12. Water absorption curve.

4. Penetration Depth

beam irradiation as a standard.

5. Risk of erythema and pigmentation

The potential of erythema and pigmentation is related to the damage level of the epidermis and dermis-epidermis joint area as well as the inflammatory reaction due to heat stimulation on the dermis. The risk decreases as fractional treatment convergence is decreased, with the proper convergence of fractional treatment being 20%. Since the risk increases in proportion to the level of heat damage, the CO2 fractional laser can theoretically reduce epidermis damage and dermis heat damage because it has less lateral heat diffusion compared to the IR laser, which in turn will reduce the risk of erythema and pigmentation if it can produce a quality laser beam and be irradiated.

6. Level of Pain

Theoretically, the CO2 fractional laser minimizes pain during procedures. Since most of the heat is lost during the tissue ablation process performed after laser irradiation, it has less lateral heat diffusion, which in turn reduces heat accumulation in tissues compared to IR equipment. Therefore, we expect less pain if we use the CO2 fractional laser. In addition, short pulse duration, smaller spot size, and shorter irradiation time on the skin can reduce pain. Therefore, it is theoretically possible to actualize the CO2 fractional laser with very little pain. However, this laser can damage the epidermis and dermis joint, which will cause more severe initial inflammation reaction and edema and burn feeling.

#### **3.2.1.2 Can parameter be independently controlled?**

Since the CO2 laser is a continuous wave laser, output is written in watts. Most CO2 lasers can control parameters such as watt and pulse duration independently. Therefore, the same amount of energy (J) can be irradiated while the depth is controlled by watt level and the coagulation range (heat damage range) can be controlled by pulse duration (Fig. 13). These are very important strong points of the CO2 laser, differing from IR equipment, which changes power and pulse duration according to the J (energy) level. That is, IR equipments cannot independently control factors since power and pulse duration is simultaneously increased as J (energy) is increased.

J (Energy) = Watt (Power)\* Time (pulse duration) Fig. 13. Parameter controllability

Clinical Application of CO2 Laser 371

After Fraxal acquired a patent on moving method irradiation, the equipment produced later on mostly adapted the stamp method. This method can be classified into two divisions. The most common method is micro lens array (the beam goes through the lens and is simultaneously irradiated after being divided into several fine beams, like LUX1540 and Affirm) that treats the parts by stamping microbeams. The other one is also a stamp method, but uses the scanner method rather than lens array and irradiates beams in order. The scanner method can be classified in two manners. The sequential type irradiates beams sequentially adjacently. Alternative type irradiates one line and skips the close line and irradiates the next line. Random type irradiates with no order. Theoretically, the sequential type includes a high possibility of heat accumulation due to adjacent irradiation. The random type can be a safer treatment since it minimizes heat accumulation. However, it is

Density is the portion of treated area where the laser beam is irradiated. Density is considered to be low when there are many normal tissues left around the area. If density is too low, it is safe but can be ineffective. However, when density is high, there are less normal tissues, which is meaningless since fractional technology's strength is safe and fast recovery. Therefore, we need to determine the density within proper range, considering the purpose of the treatment as well as the characteristics of the equipment. If more than two passes of treatment is done right after the first pass, density increases in proportion. However, it can weaken its safety as it causes excessive heat accumulation and repetitive irradiation on the same area. Therefore, it is safer to obtain the intended density from the

We can observe various equipment characteristics, such as spot size, pitch, pitch control possibility and range, controllable range of irradiation time, watt range, scanning method scanning range, and scanning shape. However, the most important evaluation factor should be: "How superior is each beam's characteristic?" In fact, the users are not fully aware of CO2 laser equipment's characteristics. For example, if this equipment includes a scanner with various modes with an inconsistent quality beam, inconsistent irradiation direction or penetration depth, it can be a low-priced CO2 laser with a scanner. It is too much to expect this type of equipment to irradiate the quality beam uniformly on skin with detailed control over lateral heat diffusion and to penetrate into the skin with the depth one wants and

(*Longitudinally excited, Transversely excited, Gas dynamic, Waveguide laser*)

2. Stamp method: micro lens array and scanner type

first pass rather than acquiring it from repetitive passes.

not clear if it has clinical significance.

**3.2.2.4 Characteristics of the beam** 

obtain satisfactory results safely.

● Type of resonator and beam

● Individual control of pulse duration

Table 1. Factors Affecting the Beam Quality of CO2 Laser

● Durability of resonator

● Beam window ● Calibration ● Auto-detection

**3.2.2.3 Density** 

Due to these characteristics, the CO2 fractional laser can treat very deep layers and offers a better recovery process after treatment as it allows high power (deep penetration) and short pulse duration (minimal lateral heat diffusion to the surrounding area), creates a very narrow vertical ablation zone, and forms a limited lateral heat diffusion zone (Fig. 14).

Fig. 14. Vertical and horizontal view of Fractional CO2 laser.

#### **3.2.2 Basic considerations for the fractional laser**

#### **3.2.2.1 Spot size**

The spot size of the laser refers to the diameter of irradiated beam. As the spot gets bigger, the re-epithelization process takes longer, which in turn cases a longer downtime. In fact, if the spot size is 140um, re-epithelization of the dermo-epidermal junction takes less than 36 hours. It takes two to four days for 300*μ*m, three to five days for 500*μ*m, and five to ten days for over 1.25mm. Therefore, it is important to make the spot small to facilitate safe treatment and fewer inconveniences. Since there is a limit to the minimum spot size that can be actualized physically, it is impossible to reduce the size below a certain point. If the spot is too small, a lot more lasers should be irradiated on a certain area to get converge that is required for proper treatment. In that case, we cannot exclude excessive heat accumulation, which is why we cannot say a smaller spot size is always advantageous.

#### **3.2.2.2 Laser beam irradiation methods**

There are various methods of irradiating several spots on a certain area. They can be classified into the moving method and the stamp method according to the type of equipment being utilized.

1. Moving method

The moving method is used for Fraxal repair that has a function that can realize regular spot converge by controlling beam irradiation speed regardless of moving speed and eCO2 equipment that has fixed beam irradiation speed by moving the handpiece constantly.

Due to these characteristics, the CO2 fractional laser can treat very deep layers and offers a better recovery process after treatment as it allows high power (deep penetration) and short pulse duration (minimal lateral heat diffusion to the surrounding area), creates a very narrow vertical ablation zone, and forms a limited lateral heat diffusion zone (Fig. 14).

The spot size of the laser refers to the diameter of irradiated beam. As the spot gets bigger, the re-epithelization process takes longer, which in turn cases a longer downtime. In fact, if the spot size is 140um, re-epithelization of the dermo-epidermal junction takes less than 36 hours. It takes two to four days for 300*μ*m, three to five days for 500*μ*m, and five to ten days for over 1.25mm. Therefore, it is important to make the spot small to facilitate safe treatment and fewer inconveniences. Since there is a limit to the minimum spot size that can be actualized physically, it is impossible to reduce the size below a certain point. If the spot is too small, a lot more lasers should be irradiated on a certain area to get converge that is required for proper treatment. In that case, we cannot exclude excessive heat accumulation,

There are various methods of irradiating several spots on a certain area. They can be classified into the moving method and the stamp method according to the type of

The moving method is used for Fraxal repair that has a function that can realize regular spot converge by controlling beam irradiation speed regardless of moving speed and eCO2 equipment that has fixed beam irradiation speed by moving the handpiece constantly.

Fig. 14. Vertical and horizontal view of Fractional CO2 laser.

which is why we cannot say a smaller spot size is always advantageous.

**3.2.2 Basic considerations for the fractional laser** 

**3.2.2.2 Laser beam irradiation methods** 

equipment being utilized.

1. Moving method

**3.2.2.1 Spot size** 

2. Stamp method: micro lens array and scanner type

After Fraxal acquired a patent on moving method irradiation, the equipment produced later on mostly adapted the stamp method. This method can be classified into two divisions. The most common method is micro lens array (the beam goes through the lens and is simultaneously irradiated after being divided into several fine beams, like LUX1540 and Affirm) that treats the parts by stamping microbeams. The other one is also a stamp method, but uses the scanner method rather than lens array and irradiates beams in order. The scanner method can be classified in two manners. The sequential type irradiates beams sequentially adjacently. Alternative type irradiates one line and skips the close line and irradiates the next line. Random type irradiates with no order. Theoretically, the sequential type includes a high possibility of heat accumulation due to adjacent irradiation. The random type can be a safer treatment since it minimizes heat accumulation. However, it is not clear if it has clinical significance.

#### **3.2.2.3 Density**

Density is the portion of treated area where the laser beam is irradiated. Density is considered to be low when there are many normal tissues left around the area. If density is too low, it is safe but can be ineffective. However, when density is high, there are less normal tissues, which is meaningless since fractional technology's strength is safe and fast recovery. Therefore, we need to determine the density within proper range, considering the purpose of the treatment as well as the characteristics of the equipment. If more than two passes of treatment is done right after the first pass, density increases in proportion. However, it can weaken its safety as it causes excessive heat accumulation and repetitive irradiation on the same area. Therefore, it is safer to obtain the intended density from the first pass rather than acquiring it from repetitive passes.

#### **3.2.2.4 Characteristics of the beam**

We can observe various equipment characteristics, such as spot size, pitch, pitch control possibility and range, controllable range of irradiation time, watt range, scanning method scanning range, and scanning shape. However, the most important evaluation factor should be: "How superior is each beam's characteristic?" In fact, the users are not fully aware of CO2 laser equipment's characteristics. For example, if this equipment includes a scanner with various modes with an inconsistent quality beam, inconsistent irradiation direction or penetration depth, it can be a low-priced CO2 laser with a scanner. It is too much to expect this type of equipment to irradiate the quality beam uniformly on skin with detailed control over lateral heat diffusion and to penetrate into the skin with the depth one wants and obtain satisfactory results safely.


Table 1. Factors Affecting the Beam Quality of CO2 Laser

<sup>●</sup> Type of resonator and beam (*Longitudinally excited, Transversely excited, Gas dynamic, Waveguide laser*)

Clinical Application of CO2 Laser 373

because CO2 laser can seal the lymphatics located at cutting plane and directly seal at peripheral vessel small sized less than 0.5mm, you can support patient to more comfort by accelerating wound healing process and by decreasing intraoperative bleeding and

This CO2 laser has definite advantage that it is very clear between the tissue which has thermal injury and no damaged tissue located at surrounding area, and it enable to cut tissue by not putting to physical stress to surrounding tissue. It has profit at the procedure by need of bloodless surgical field because of minimizing bleeding. And it enable to treat

In palatoplasty and pharyngeal flap operation, compared to conventional method, complication caused by intra- and postoperative bleeding was decreased and there was no difference between CO2 using and conventional method in aspect of wound healing. And

In patients of shortening of frenulum, CO2 laser is available to frenotomy (Yoon CH 1998) In blepharoplasty using CO2 laser, it was first reported by Baker in 1984(Baker, Muenzler et al. 1984). Using CO2 laser, blepharoplasty was processed safely and more enhanced to decrease bleeding, operation time, edema, time to heal. Mittelman et al (Mittelman and Apfelberg 1990) evaluated as safe method in spite of having risks of eyeball injury,

Extramammary Paget's disease is eczema like disease accompanied mainly by itching of anus and genitalia. It was found very much among a middle age or patients of prime of manhood with the past history which a treatment of skin clinic was failed in during long periods. About this disease, invasive method was surgical excision and topical ointment using 5-fluorouracil, radiotherapy. Recently it is the trend that an interest of the treatment

In wound healing, Low-powered CO2 laser helps to induce the synthesis of DNA by give effects to permeability of cell membrane, and to activate fibroblast and condrocyte, and to accelerate to absorp the hematom and to remove necrotic tissue, and to help to healing

In acne scar, dermabrasion using CO2 laser minimizing the common complication by conventional method and is enable to control the depth of peeling by depth of acne scar. So is enable to avoid to complication of hypertrophic scar and keloid etc.. It is available to process peeling safely and deeply. So recovery toward a daily life is fast as managements after operations is convenient because of decreasing of postoperative edema and pain by

CO2 laser is infrared having 10,600 nm wavelength. It has vibrational energy and enable to control to transfer through the mirror. The diameter of focus is about 1mm by

antiseptically to wound as vaporize the tissue by high temperature.

CO2 laser has merits to decrease hospitalization (Song IC 1998).

breakaway from its course, fire at operation room, burn injury of skin.

using CO2 laser and Nd:YAG laer is rising (Ewing 1991; Yoon ES 2000)

In atropic scar, pulsed CO2 laser having high power was utilized.

process of bone and cartilage (Tsai, Huang et al. 1997).

disconnecting at nerve terminals (Yoon ES 1998).

**5.1 Laryngeal microsurgery using CO2 laser** 

**5. Clinical application in otorhinolaryngology** 

postoperative edema, contusion.

Therefore, when comparing CO2 fractional laser performance, the most important factor that shows the biggest deviation is quality of beam. The following are the major factors that determine beam quality (Table 1).

#### **3.3 Clinical application of CO2 fractional laser**

Laser induced regeneration by vaporizing aging tissue at the epidermis and dermis. It can effectively lead to regrowth and remodeling in the process, so it can be applied broadly to a variety of issues (scar, pigmentation, texture so on) at epidermis and dermis and it was also effective to improve the aging skin because laser can treated from 0.2mm in depth to 1mm or more at dermis.

However, it is limited to case reflected in the previously mentioned characteristics of the CO2 when the CO2 laser beam to penetrate the organization is elaborately controlled.

Because it can not be told that there was less possibility of erythema and pigmentation if not uncontrolled beam.

By such advantage, CO2 fractional laser can be utilized to various indications of epidermis and dermis. (Table 2)

Since it was initially introduced in 2006, CO2 fractional laser has been made up a large development and evolution for a short period of time, and future developments are expected as follows. (Table 3)



Table 3. Future Development of CO2 Fractional Laser

#### **4. Clinical application in plastic surgery**

You decrease a pain after operations by a laser in case of constancy as the way that used a CO2 laser compares to the way that used a knife as you seal the peripheral nerve edge. And

Therefore, when comparing CO2 fractional laser performance, the most important factor that shows the biggest deviation is quality of beam. The following are the major factors that

Laser induced regeneration by vaporizing aging tissue at the epidermis and dermis. It can effectively lead to regrowth and remodeling in the process, so it can be applied broadly to a variety of issues (scar, pigmentation, texture so on) at epidermis and dermis and it was also effective to improve the aging skin because laser can treated from 0.2mm in depth to 1mm

However, it is limited to case reflected in the previously mentioned characteristics of the CO2 when the CO2 laser beam to penetrate the organization is elaborately controlled.

Because it can not be told that there was less possibility of erythema and pigmentation if not

By such advantage, CO2 fractional laser can be utilized to various indications of epidermis

Since it was initially introduced in 2006, CO2 fractional laser has been made up a large development and evolution for a short period of time, and future developments are

● Variable parameter ● Surface cooling for epidermal preservation

You decrease a pain after operations by a laser in case of constancy as the way that used a CO2 laser compares to the way that used a knife as you seal the peripheral nerve edge. And

determine beam quality (Table 1).

or more at dermis.

uncontrolled beam.

and dermis. (Table 2)

expected as follows. (Table 3)

2. Surface and deeper structure

1. Superficial problem

3. Dermal problem

● Wrinkles

**3.3 Clinical application of CO2 fractional laser** 

● Pigmentary lesions ● Pore

● Pigmentary lesions: refractory PIH, cloasma, tattoo

● Texture & aging skin ● Laxity(hand, neck)

● Scar ● Striae distensa

Table 2. Clinical Indications of CO2 Fractional Laser Treatment

● Increasing dermal shrinkage ● Deeper penetration ● Pattern of beam ● Quality of beam

● Controllable, smaller spot ● Powerful

Table 3. Future Development of CO2 Fractional Laser

**4. Clinical application in plastic surgery** 

because CO2 laser can seal the lymphatics located at cutting plane and directly seal at peripheral vessel small sized less than 0.5mm, you can support patient to more comfort by accelerating wound healing process and by decreasing intraoperative bleeding and postoperative edema, contusion.

This CO2 laser has definite advantage that it is very clear between the tissue which has thermal injury and no damaged tissue located at surrounding area, and it enable to cut tissue by not putting to physical stress to surrounding tissue. It has profit at the procedure by need of bloodless surgical field because of minimizing bleeding. And it enable to treat antiseptically to wound as vaporize the tissue by high temperature.

In palatoplasty and pharyngeal flap operation, compared to conventional method, complication caused by intra- and postoperative bleeding was decreased and there was no difference between CO2 using and conventional method in aspect of wound healing. And CO2 laser has merits to decrease hospitalization (Song IC 1998).

In patients of shortening of frenulum, CO2 laser is available to frenotomy (Yoon CH 1998)

In blepharoplasty using CO2 laser, it was first reported by Baker in 1984(Baker, Muenzler et al. 1984). Using CO2 laser, blepharoplasty was processed safely and more enhanced to decrease bleeding, operation time, edema, time to heal. Mittelman et al (Mittelman and Apfelberg 1990) evaluated as safe method in spite of having risks of eyeball injury, breakaway from its course, fire at operation room, burn injury of skin.

Extramammary Paget's disease is eczema like disease accompanied mainly by itching of anus and genitalia. It was found very much among a middle age or patients of prime of manhood with the past history which a treatment of skin clinic was failed in during long periods. About this disease, invasive method was surgical excision and topical ointment using 5-fluorouracil, radiotherapy. Recently it is the trend that an interest of the treatment using CO2 laser and Nd:YAG laer is rising (Ewing 1991; Yoon ES 2000)

In wound healing, Low-powered CO2 laser helps to induce the synthesis of DNA by give effects to permeability of cell membrane, and to activate fibroblast and condrocyte, and to accelerate to absorp the hematom and to remove necrotic tissue, and to help to healing process of bone and cartilage (Tsai, Huang et al. 1997).

In atropic scar, pulsed CO2 laser having high power was utilized.

In acne scar, dermabrasion using CO2 laser minimizing the common complication by conventional method and is enable to control the depth of peeling by depth of acne scar. So is enable to avoid to complication of hypertrophic scar and keloid etc.. It is available to process peeling safely and deeply. So recovery toward a daily life is fast as managements after operations is convenient because of decreasing of postoperative edema and pain by disconnecting at nerve terminals (Yoon ES 1998).

#### **5. Clinical application in otorhinolaryngology**

#### **5.1 Laryngeal microsurgery using CO2 laser**

CO2 laser is infrared having 10,600 nm wavelength. It has vibrational energy and enable to control to transfer through the mirror. The diameter of focus is about 1mm by

Clinical Application of CO2 Laser 375

CO2 laser has been developed until nowadays. At present, CO2 laser is considered as essential instrument in medicine. In future, we think that CO2 laser may be more developed

Thank to corresponding Dr. Jun-Sung Kim for helping to support academic basis. And thank to Dr. Sang-Ho Lee for permitting the chance to writing in Wooridul Hospital. Also thank to Dr. Chan-Woo Jung at Leaders dermatologic clinics in South Korea for supporting dermatologic description and figure. Special thank to Mrs. Park Min for supporting

Ahn, Y., S. H. Lee, et al. (2004). "Percutaneous endoscopic lumbar discectomy for recurrent

Ahn, Y., S. H. Lee, et al. (2005). "Percutaneous endoscopic cervical discectomy: clinical outcome and radiographic changes." *Photomed Laser Surg* 23(4): 362-368. Alexiades-Armenakas, M. R., J. S. Dover, et al. (2008). "The spectrum of laser skin

Ascher, P. W. and F. Heppner (1984). "CO2-Laser in neurosurgery." *Neurosurg Rev* 7(2-3):

Baker, S. S., W. S. Muenzler, et al. (1984). "Carbon dioxide laser blepharoplasty."

Beck, O. J. (1980). "The use of the Nd-YAG and the CO2 laser in neurosurgery." *Neurosurg* 

Benjamin, M., S. Qin, et al. (1995). "Fibrocartilage associated with human tendons and their

Chiba, K., Y. Toyama, et al. (2001). "Intraspinal cyst communicating with the intervertebral disc in the lumbar spine: discal cyst." *Spine (Phila Pa 1976)* 26(19): 2112-2118. Choi, K. B., D. Y. Lee, et al. (2008). "Contralateral reherniation after open lumbar

Choi, S., S. H. Lee, et al. (2005). "Factors affecting prognosis of patients who underwent

Cusick, J. F. (1991). "Pathophysiology and treatment of cervical spondylotic myelopathy."

Deruty, R., I. Pelissou-Guyotat, et al. (1993). "Routine use of the CO2 laser technique for resection of cerebral tumours." *Acta Neurochir (Wien)* 123(1-2): 43-45.

microdiscectomy : a comparison with ipsilateral reherniation." *J Korean Neurosurg* 

corpectomy and fusion for treatment of cervical ossification of the posterior longitudinal ligament: analysis of 47 patients.*" J Spinal Disord Tech* 18(4): 309-314. Choy, D. S., R. B. Case, et al. (1987). "Percutaneous laser nucleolysis of lumbar disks." *N Engl* 

consecutive cases." *Spine (Phila Pa 1976)* 29(16): E326-332.

*Dermatol* 58(5): 719-737; quiz 738-740.

*Ophthalmology* 91(3): 238-244.

pulleys." *J Anat* 187 ( Pt 3): 625-633.

disc herniation: surgical technique, outcome, and prognostic factors of 43

resurfacing: nonablative, fractional, and ablative laser resurfacing." *J Am Acad* 

**6. Conclusion** 

**8. References** 

**7. Acknowledgment** 

dedication of this manuscript.

123-133.

*Rev* 3(4): 261-266.

*Soc* 44(5): 320-326.

*J Med* 317(12): 771-772.

*Clin Neurosurg* 37: 661-681.

for achieving the goal and more generalized.

handpiece and about 200-250μm by micromanipulator from focus distance of 400mm. It was used by controlling the focus distance by using purpose, and it enable to cut like common scalpel. If defocused beam is emitted from more longer distance than focus distance, tissue coagulation was available as decreasing per unit of energy (Ossoff, Coleman et al. 1994).

Laryngeal microsurgery using CO2 laser has many advantages more than conventional surgical method. CO2 laser enables the surgeon to remove the exact desired pathology through controlling of power and emission time, which is necessary to preserve normal function. In situations where it is necessary to remove the pathology of a tubed organ, direct contact of the excised area and disturbance of the surgical field can be avoided. A bloodless operation is possible by the unique ability of the CO2 laser to cauterize even arteries and veins as small as 0.5mm in diameter. Safety margins can be secured more easily when removing a tumor. A local inflammatory response is extremely small after an operation, and there is a little hyperplasia of a granulation tissue and scar formation during healing processes. In case of a huge tumor, CO2 laser can help reduce the amount of tissue removed, which in turn can assist in maintenance of laryngeal function (Hall 1971; Norris and Mullarky 1982).

Possible anesthesia methods for laser microlaryngosurgery are intubation and nonintubated anesthesia etc. Rarely it is possible by Jet ventilation or tracheostomy. In cases of intubated anesthesia, protective tubes such as laser-Shield, Bivoan, and Malincrodt tubes can be used. The cuff of the intubation tube must be protected with a cottonoid soaked in a solution of normal saline during an operation. Non intubated anesthesia may be used when the visual field during an operation is obstructed by an anesthesia tube. This is achieved by removing the tube during surgery at 100% O2 saturation, and quickly reinserting the tube after 2-3 minutes when the O2 saturation begins to fall. Tumors located posterior to the larynx may be removed by inserting a laryngoscope behind the intubation tube (Fried 1984; Shapshay, Beamis et al. 1989).

 When using a laser for laryngomicrosurgery, laser power should be set between 1 to 10 watts according the type of tissue to be removed or type of surgery. The most frequently used setting is a microscope magnified 16 to 25 times, laser power of 2 watts, and focus of 250 to 400*μ*m. If the target tissue is too hard and blood vessels are too small, super pulse or ultrapulse methods should be used with a smaller focus (Ossoff, Coleman et al. 1994).

Carbonization of cancer pathology can be visualized more clearly during an operation with a continuous mode laser and can help discriminate between normal and cancer cells. A laryngeal forceps or suction tube should be used to pull the target tissue before using a laser to cut more cleanly while reducing carbonization or thermal injury.

CO2 laser can be used in a variety of laryngeal diseases. First, benign laryngeal diseases such as laryngeal nodule, Reinke's edema, laryngeal cyst, granuloma, papilloma, angioma, septum and so on. Secondly, pre-cancerous pathology like white keratosis. Thirdly, it is partly applied in a treatment of malignant laryngeal pathology.

CO2 laser can also be used to prevent asphyxia in various laryngeal diseases causing airway obstruction such as laryngeal and tracheal stenosis.

#### **6. Conclusion**

374 CO2 Laser – Optimisation and Application

handpiece and about 200-250μm by micromanipulator from focus distance of 400mm. It was used by controlling the focus distance by using purpose, and it enable to cut like common scalpel. If defocused beam is emitted from more longer distance than focus distance, tissue coagulation was available as decreasing per unit of energy (Ossoff,

Laryngeal microsurgery using CO2 laser has many advantages more than conventional surgical method. CO2 laser enables the surgeon to remove the exact desired pathology through controlling of power and emission time, which is necessary to preserve normal function. In situations where it is necessary to remove the pathology of a tubed organ, direct contact of the excised area and disturbance of the surgical field can be avoided. A bloodless operation is possible by the unique ability of the CO2 laser to cauterize even arteries and veins as small as 0.5mm in diameter. Safety margins can be secured more easily when removing a tumor. A local inflammatory response is extremely small after an operation, and there is a little hyperplasia of a granulation tissue and scar formation during healing processes. In case of a huge tumor, CO2 laser can help reduce the amount of tissue removed, which in turn can assist in maintenance of laryngeal function (Hall 1971; Norris and

Possible anesthesia methods for laser microlaryngosurgery are intubation and nonintubated anesthesia etc. Rarely it is possible by Jet ventilation or tracheostomy. In cases of intubated anesthesia, protective tubes such as laser-Shield, Bivoan, and Malincrodt tubes can be used. The cuff of the intubation tube must be protected with a cottonoid soaked in a solution of normal saline during an operation. Non intubated anesthesia may be used when the visual field during an operation is obstructed by an anesthesia tube. This is achieved by removing the tube during surgery at 100% O2 saturation, and quickly reinserting the tube after 2-3 minutes when the O2 saturation begins to fall. Tumors located posterior to the larynx may be removed by inserting a laryngoscope behind the intubation tube (Fried 1984;

 When using a laser for laryngomicrosurgery, laser power should be set between 1 to 10 watts according the type of tissue to be removed or type of surgery. The most frequently used setting is a microscope magnified 16 to 25 times, laser power of 2 watts, and focus of 250 to 400*μ*m. If the target tissue is too hard and blood vessels are too small, super pulse or ultrapulse methods should be used with a smaller focus (Ossoff, Coleman et al. 1994).

Carbonization of cancer pathology can be visualized more clearly during an operation with a continuous mode laser and can help discriminate between normal and cancer cells. A laryngeal forceps or suction tube should be used to pull the target tissue before using a laser

CO2 laser can be used in a variety of laryngeal diseases. First, benign laryngeal diseases such as laryngeal nodule, Reinke's edema, laryngeal cyst, granuloma, papilloma, angioma, septum and so on. Secondly, pre-cancerous pathology like white keratosis. Thirdly, it is

CO2 laser can also be used to prevent asphyxia in various laryngeal diseases causing airway

to cut more cleanly while reducing carbonization or thermal injury.

partly applied in a treatment of malignant laryngeal pathology.

obstruction such as laryngeal and tracheal stenosis.

Coleman et al. 1994).

Mullarky 1982).

Shapshay, Beamis et al. 1989).

CO2 laser has been developed until nowadays. At present, CO2 laser is considered as essential instrument in medicine. In future, we think that CO2 laser may be more developed for achieving the goal and more generalized.

#### **7. Acknowledgment**

Thank to corresponding Dr. Jun-Sung Kim for helping to support academic basis. And thank to Dr. Sang-Ho Lee for permitting the chance to writing in Wooridul Hospital. Also thank to Dr. Chan-Woo Jung at Leaders dermatologic clinics in South Korea for supporting dermatologic description and figure. Special thank to Mrs. Park Min for supporting dedication of this manuscript.

#### **8. References**


Clinical Application of CO2 Laser 377

Lee, D. Y. and S. H. Lee (2011). "Carbon dioxide (CO2) laser-assisted microdiscectomy for

Lee, H. K., D. H. Lee, et al. (2006). "Discal cyst of the lumbar spine: MR imaging features."

Lee, S. H., Y. Ahn, et al. (2006). "Immediate pain improvement is a useful predictor of long-

Lee, S. H., Y. Ahn, et al. (2008). "Laser-assisted anterior cervical corpectomy versus posterior

Mittelman, H. and D. B. Apfelberg (1990). "Carbon dioxide laser blepharoplasty--advantages

Neblett, C. R. (1992). *Laser and the cervical spine*. Philadelphia, Lippincott Williams & Wilkins. Nerubay, J., I. Caspi, et al. (1997). "Percutaneous laser nucleolysis of the intervertebral lumbar disc. An experimental study." *Clin Orthop Relat Res*(337): 42-44. Norris, C. W. and M. B. Mullarky (1982). "Experimental skin incision made with the carbon

Onari, K., N. Akiyama, et al. (2001). "Long-term follow-up results of anterior interbody

Origitano, T. C. and O. H. Reichman (1993). "Photodynamic therapy for intracranial

Ossoff, R. H., J. A. Coleman, et al. (1994). "Clinical applications of lasers in otolaryngology--

Powers, S. K., S. S. Cush, et al. (1991). "Stereotactic intratumoral photodynamic therapy for

Rosenberg, G. J., M. A. Brito, Jr., et al. (1999). "Long-term histologic effects of the CO2 laser."

Shapshay, S. M., J. F. Beamis, Jr., et al. (1989). "Total cervical tracheal stenosis: treatment by laser, dilation, and stenting." *Ann Otol Rhinol Laryngol* 98(11): 890-895. Song IC, P. W., Kil MS (1998). "Laser assisted palatoplasty and pharyngeal flap." J Korean

Stein, E., T. Sedlacek, et al. (1990). "Acute and chronic effects of bone ablation with a pulsed

Stellar, S., T. G. Polanyi, et al. (1970). "Experimental studies with the carbon dioxide laser as

Tew, J. M., Jr. and W. D. Tobler (1983). "The laser: history, biophysics, and neurosurgical

Tsai, C. L., L. L. Huang, et al. (1997). "Effect of CO2 laser on healing of cultured meniscus."

longitudinal ligament." *Spine (Phila Pa 1976)* 26(5): 488-493.

head and neck surgery." *Lasers Surg Med* 15(3): 217-248.

*Plast Reconstr Surg* 104(7): 2239-2244; discussion 2245-2236.

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applications." *Clin Neurosurg* 31: 506-549.

a neurosurgical instrument." *Med Biol Eng* 8(6): 549-558.

fusion applied for cervical myelopathy due to ossification of the posterior

neoplasms: development of an image-based computer-assisted protocol for photodynamic therapy of intracranial neoplasms." *Neurosurgery* 32(4): 587-595;

recurrent malignant brain tumors." *Neurosurgery* 29(5): 688-695; discussion 695-686.

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and disadvantages." *Ann Plast Surg* 24(1): 1-6.

dioxide laser." *Laryngoscope* 92(4): 416-419.

discussion 595-586.

*Soc Plast Reconstr Surg* 25: 252.

*Lasers Surg Med* 20(2): 172-178.

531-535.

*Clin Imaging* 30(5): 326-330.

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laminoplasty for cervical myelopathic patients with multilevel ossification of the


Ewing, T. L. (1991). "Paget's disease of the vulva treated by combined surgery and laser."

Fitzpatrick, R. E., E. F. Rostan, et al. (2000). "Collagen tightening induced by carbon dioxide

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Kim, J. S., G. Choi, et al. (2009). "Removal of a discal cyst using a percutaneous endoscopic interlaminar approach: a case report." *Photomed Laser Surg* 27(2): 365-369. Kim, J. S. and S. H. Lee (2009). "Carbon dioxide (CO2) laser-assisted ablation of lumbar

Krishnamurthy, S. and S. K. Powers (1994). "Lasers in neurosurgery." *Lasers Surg Med* 15(2):

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207.

222-225.

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83-87.

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*Med* 73(6): 864-870.


**15** 

*USA* 

**CO2 Laser:** 

*1University Of California San Francisco,* 

*3Lutheran Medical Center/UCSF,* 

**Evidence Based Applications in Dentistry** 

Pinalben Viraparia1,2, Joel M. White1 and Ram M. Vaderhobli1,3

Ever since Kumar Patel introduced lasers in 1960s', researchers have been looking into its possible applications in the field of dentistry. Researchers have investigated the effects of laser radiation on teeth, bone, pulp and oral mucosal tissues (Taylor, Shklar, & Roeber, 1965). CO2 lasers have been used extensively in medical field and the first laser to be approved by FDA for dental application was Nd:YAG (Neodymium-Yttrium-Aluminum-Garnet) in 1990s. Since then many types of lasers including CO2, Er:YAG (Erbium-Yttrium-Aluminum-Garnet), Diode, Er Cr:YSGG (Erbium-Chromium-Yttrium-Scallium-Gallium-Garnet) have been approved for dental use. FDA approved Er:YAG for dental hard tissue in 1997 and has approved other types of lasers for soft and hard tissue procedures in many area of dentistry. Many authors have reported the use of Carbon Dioxide (CO2) lasers for soft tissue applications in dentistry (Pick & Pecaro, 1987a; White et al., 1998). The Food and Drug Administration (FDA) granted clearance for marketing CO2 lasers for soft tissue procedures such as frenectomy, gingivectomy, biopsies, and removal of benign and malignant lesions because CO2 laser energy is well absorbed by water. Specific indications for use in dentistry include apthous ulcer treatment, coagulation of extraction sites, sulcular debridement and intraoral soft tissue surgeries such as ablating, incising, and excising (U.S. FDA 510(k)

In this chapter, we will discuss basic design, tissue interactions, evidence based clinical

The growth of CO2 laser applications in Dentistry has grown substantially with its wavelength bands ranging from 9.4 and 10.6 micrometers. The laser medium consists of water or air cooled gas discharge (Carbon dioxide, nitrogen, hydrogen, xenon, helium) that helps in producing a beam of infrared light by activating the footswitch. The original CO2 lasers were continuous wave or interrupted pulse durations of about 0.5 sec to 50 msec with non contact delivery and large beam diameters up to 1mm and larger. Because, the delivery

**1. Introduction** 

marketing clearance) as shown in Table 1.

applications, and future of dental applications of CO2 lasers.

**2. Basic equipment design & tissue interactions of CO2 laser** 

*2 Lutheran Medical Center-Advanced Education in General Dentistry,* 

*Dept. Preventive and Restorative Dental Sciences, UCSF* 


### **CO2 Laser: Evidence Based Applications in Dentistry**

Pinalben Viraparia1,2, Joel M. White1 and Ram M. Vaderhobli1,3

*1University Of California San Francisco, 2 Lutheran Medical Center-Advanced Education in General Dentistry, 3Lutheran Medical Center/UCSF, Dept. Preventive and Restorative Dental Sciences, UCSF USA* 

#### **1. Introduction**

378 CO2 Laser – Optimisation and Application

Whipple, T. L., R. B. Caspari, et al. (1984). "Laser subtotal meniscectomy in rabbits." *Lasers* 

Yoon CH, R. Y., Park HS, Kim HJ (1998). "Comparative study between using CO2-laser and classic method in frenulotomy." *J Korean Soc Plast Reconstr Surg* 25: 1475. Yoon ES, C. J., Han SK, Kim WK (2000). "Treatment of extramammary Paget's disease using

Yoon ES, K. S., Ahn DS, Park SH (1998). "CO2 laser resurfacing of acne scar." *J Korean Soc* 

Zelickson, B. D., D. Kist, et al. (2004). "Histological and ultrastructural evaluation of the

effects of a radiofrequency-based nonablative dermal remodeling device: a pilot

the CO2 laser." *J Korean Soc Plast Reconstr Surg* 27: 169.

*Surg Med* 3(4): 297-304.

*Aesth Plast Reconstr Surg* 4: 381.

study." *Arch Dermatol* 140(2): 204-209.

Ever since Kumar Patel introduced lasers in 1960s', researchers have been looking into its possible applications in the field of dentistry. Researchers have investigated the effects of laser radiation on teeth, bone, pulp and oral mucosal tissues (Taylor, Shklar, & Roeber, 1965). CO2 lasers have been used extensively in medical field and the first laser to be approved by FDA for dental application was Nd:YAG (Neodymium-Yttrium-Aluminum-Garnet) in 1990s. Since then many types of lasers including CO2, Er:YAG (Erbium-Yttrium-Aluminum-Garnet), Diode, Er Cr:YSGG (Erbium-Chromium-Yttrium-Scallium-Gallium-Garnet) have been approved for dental use. FDA approved Er:YAG for dental hard tissue in 1997 and has approved other types of lasers for soft and hard tissue procedures in many area of dentistry.

Many authors have reported the use of Carbon Dioxide (CO2) lasers for soft tissue applications in dentistry (Pick & Pecaro, 1987a; White et al., 1998). The Food and Drug Administration (FDA) granted clearance for marketing CO2 lasers for soft tissue procedures such as frenectomy, gingivectomy, biopsies, and removal of benign and malignant lesions because CO2 laser energy is well absorbed by water. Specific indications for use in dentistry include apthous ulcer treatment, coagulation of extraction sites, sulcular debridement and intraoral soft tissue surgeries such as ablating, incising, and excising (U.S. FDA 510(k) marketing clearance) as shown in Table 1.

In this chapter, we will discuss basic design, tissue interactions, evidence based clinical applications, and future of dental applications of CO2 lasers.

#### **2. Basic equipment design & tissue interactions of CO2 laser**

The growth of CO2 laser applications in Dentistry has grown substantially with its wavelength bands ranging from 9.4 and 10.6 micrometers. The laser medium consists of water or air cooled gas discharge (Carbon dioxide, nitrogen, hydrogen, xenon, helium) that helps in producing a beam of infrared light by activating the footswitch. The original CO2 lasers were continuous wave or interrupted pulse durations of about 0.5 sec to 50 msec with non contact delivery and large beam diameters up to 1mm and larger. Because, the delivery

CO2 Laser: Evidence Based Applications in Dentistry 381

Fig. 1. CO2 laser hand piece with different tips marketed by GPT Dental

Numerous studies have been done pre and post FDA approval to improve the technique and provide best practice guidelines for the clinicians. A report published by American Dental Association (ADA) in 2001 describing the challenges in the future of oral health care mentioned the role of laser applications. The report specifically mentioned that more clinical research and technical developments in CO2 laser delivery systems will promise to expand its clinical applications beyond soft tissue procedures (Seldin, 2001). Although CO2 laser 10.6micron wavelength is absorbed by water and even though 9.3micron is absorbed in hydroxyapetite, it is primarily a soft tissue laser (Convissar & Goldstein, 2003). Even before CO2 laser received FDA approval for soft tissue procedures in 1990's, many studies have looked at its hard tissue applications in 1980's. Table 2. Lists CO2 laser application in

One of the earlier case series by Pick and colleagues reported soft tissue procedures using Sharplan 743 CO2 and Xanar Ambulase lasers. In a Clinical trial 250 patients were treated for conditions ranging from, gingival hyperplasia, benign and malignant lesions (along with conventional surgery), incisional & excisional biopsy, red-white lesions, and haemorrhagic

**3. Evidence based clinical applications** 

various dental procedures (Sulewski JG).

**3.1 Soft tissue procedures** 

mode is non-contact this results in lack of tactile sensation to the operator. Previous studies with these continuous wave CO2 lasers showed a variety of structural and ultrasonic changes of the hard tooth structure. These included cracking, flaking, crater formation, charring, melting, and recrystallizaton due to the highly efficient absorption of CO2 wavelengths by the apatite mineral of hard tissues (Boehm, Rich, Webster, & Janke, 1977; J. D. B. Featherstone & D. G. A. Nelson, 1987; McCormack, 1995; Stern & Sognnaes, 1964; Stern, Vahl, & Sognnaes, 1972). All dental tissues have different absorption coefficient for various wavelength depending on water, blood, pigment, and mineral content. For example, Nd:YAG and Diode lasers are absorbed by dark pigments making them ideal for soft tissue procedures. Tissue component that maximally absorbs CO2 wavelength is water followed by apatite (Gouw-Soares et al., 2004). Because of this CO2 lasers have been proven to be the gold standard for intra-oral soft tissue applications for decades. Thermal effects and various parameters settings of CO2 lasers have also been studied extensively (Leighty, Pogrel, Goodis, & White, 1991; Malmström, McCormack, Fried, & Featherstone, 2001). These studies indicated that application of CO2 laser created unacceptable thermal damage to adjacent tissue. Because of these reasons early CO2 laser system had been limited by their continuous wave operations and delivery system constraints. Lasers parameters such as power, repetition rate, average power and highest peak power play a role in surgical and collateral effects. Studies have concluded that high repetition rate, high peak power and lower average power yield favourable clinical results (Wilder-Smith, Dang, & Kurosaki, 1997).


Table 1. Examples of CO2 lasers available in market for dental use

Due to the lack of tactile sensation, their use in hard-tissue applications is not favorable. With new technologies, dental laser manufacturers now claim to have shorter pulse durations (as short as 150 microsecond pulse duration) with beam diameters of as small as 100 microns. This allows for cooling of tissues between pulses and results in minimal thermal damage. These lasers are now marketed for soft tissue intraoral procedures as described earlier. The laser is usually equipped with various hand pieces and tips of differing diameter for tissue ablation as shown in Figure 1. The hand piece is usually the size of a dental drill and the spot size that is emitted from these hand pieces allows for greater accuracy resulting in minimal damage to the surrounding tissues. As these lasers operate the best in pulse or super pulse infrared mode, they are able to remove precise amount of tissues with each pulse emission.

mode is non-contact this results in lack of tactile sensation to the operator. Previous studies with these continuous wave CO2 lasers showed a variety of structural and ultrasonic changes of the hard tooth structure. These included cracking, flaking, crater formation, charring, melting, and recrystallizaton due to the highly efficient absorption of CO2 wavelengths by the apatite mineral of hard tissues (Boehm, Rich, Webster, & Janke, 1977; J. D. B. Featherstone & D. G. A. Nelson, 1987; McCormack, 1995; Stern & Sognnaes, 1964; Stern, Vahl, & Sognnaes, 1972). All dental tissues have different absorption coefficient for various wavelength depending on water, blood, pigment, and mineral content. For example, Nd:YAG and Diode lasers are absorbed by dark pigments making them ideal for soft tissue procedures. Tissue component that maximally absorbs CO2 wavelength is water followed by apatite (Gouw-Soares et al., 2004). Because of this CO2 lasers have been proven to be the gold standard for intra-oral soft tissue applications for decades. Thermal effects and various parameters settings of CO2 lasers have also been studied extensively (Leighty, Pogrel, Goodis, & White, 1991; Malmström, McCormack, Fried, & Featherstone, 2001). These studies indicated that application of CO2 laser created unacceptable thermal damage to adjacent tissue. Because of these reasons early CO2 laser system had been limited by their continuous wave operations and delivery system constraints. Lasers parameters such as power, repetition rate, average power and highest peak power play a role in surgical and collateral effects. Studies have concluded that high repetition rate, high peak power and lower average power yield favourable clinical results (Wilder-Smith, Dang, & Kurosaki, 1997).

> Most Absorption

N/A 9.6 Appetite Hard tissue No N/A 9.3 Appetite Hard Tissue No Smart CO2 10.6 Water Soft Tissue Yes CO2DENTA 10.6 Water Soft Tissue Yes

Due to the lack of tactile sensation, their use in hard-tissue applications is not favorable. With new technologies, dental laser manufacturers now claim to have shorter pulse durations (as short as 150 microsecond pulse duration) with beam diameters of as small as 100 microns. This allows for cooling of tissues between pulses and results in minimal thermal damage. These lasers are now marketed for soft tissue intraoral procedures as described earlier. The laser is usually equipped with various hand pieces and tips of differing diameter for tissue ablation as shown in Figure 1. The hand piece is usually the size of a dental drill and the spot size that is emitted from these hand pieces allows for greater accuracy resulting in minimal damage to the surrounding tissues. As these lasers operate the best in pulse or super pulse infrared mode, they are able to remove precise amount of

Recommended Use

10.6 Water Soft Tissue Yes

FDA Approval

Device Trade Name

Opus 20 Dental laser system

tissues with each pulse emission.

Wavelength (micro mm)

Table 1. Examples of CO2 lasers available in market for dental use

Fig. 1. CO2 laser hand piece with different tips marketed by GPT Dental

#### **3. Evidence based clinical applications**

Numerous studies have been done pre and post FDA approval to improve the technique and provide best practice guidelines for the clinicians. A report published by American Dental Association (ADA) in 2001 describing the challenges in the future of oral health care mentioned the role of laser applications. The report specifically mentioned that more clinical research and technical developments in CO2 laser delivery systems will promise to expand its clinical applications beyond soft tissue procedures (Seldin, 2001). Although CO2 laser 10.6micron wavelength is absorbed by water and even though 9.3micron is absorbed in hydroxyapetite, it is primarily a soft tissue laser (Convissar & Goldstein, 2003). Even before CO2 laser received FDA approval for soft tissue procedures in 1990's, many studies have looked at its hard tissue applications in 1980's. Table 2. Lists CO2 laser application in various dental procedures (Sulewski JG).

#### **3.1 Soft tissue procedures**

One of the earlier case series by Pick and colleagues reported soft tissue procedures using Sharplan 743 CO2 and Xanar Ambulase lasers. In a Clinical trial 250 patients were treated for conditions ranging from, gingival hyperplasia, benign and malignant lesions (along with conventional surgery), incisional & excisional biopsy, red-white lesions, and haemorrhagic

CO2 Laser: Evidence Based Applications in Dentistry 383

Hard tissue applications of continuous wave CO2 lasers have been limited due to thermal damage, charring effect and resultant rough tooth surface. Due to its high absorption overlap with phosphate in enamel apetite crystal, all radiation is absorbed in thin enamel (<10 um). This makes heat transfer as the main way of energy transport leading to thermal damage to pulp (Wigdor et al., 1995). Conversely studies have shown that high reflectivity (9%-50%) at 9.3- and 9.6um wavelengths may pose a safety concern and it requires accurate knowledge of radiation dose while doing treatment (Fried, Glena, Featherstone, & Seka, 1997). In 1990's TEA (Transversely excited atmospheric pressure) 10.6um pulsed (0.1-2usec) CO2 laser had the best reported success in ablating dental hard tissue. However, high plasma induction with TEA CO2 laser posed problems with decreased ablation efficiency and damage due to shock wave rendering it unacceptable for clinical use (Wigdor et al., 1995). Since the 9.6micrometer CO2 laser wavelength is highly absorbed in appetite crystals, it presents a future potential for its applications in cavity preparation, apicectomies and other hard tissue procedures. In vitro study using Scanning Electron Microscopic (SEM) images reported cleaner dentinal surface with fusion and recrystallized dentine following apicectomy and root treatment with pulsed TEA 9.6micrometer CO2 laser (Gouw-Soares et al., 2004). Contrasting results were reported in a more recent SEM analysis study. The study compared the marginal permeability and dentinal surface texture following apicectomy performed with burs and CO2 laser. Authors attributed rougher surface and less favourable marginal fit following CO2 laser treatment to the use of continuous wave mode, no cooling agent and less experience of the operator with CO2 laser (Lustosa-Pereira et al., 2011).

Advantages of using CO2 laser for periodontal procedures have been accepted by American Academy of Periodontology in its position paper. Its ability to provide dry surgical field and haemostasis has been proven useful in periodontal surgical procedures. Additionally CO2 laser use has shown mixed results when used for periodontal pocket debridement in addition to mechanical debridement, pocket reduction, attachment gain, decreased microorganisms, and guided tissue regeneration cases (Convissar & Goldstein, 2003; Matthews, 2010; Wigdor et al., 1995). Porcine mandible study evaluating efficacy of newer micropulse 10.6um CO2 laser showed clinically acceptable results in coagulation, incision depth and width, time required to perform procedure, with minimal hard tissue damage on accidental exposure but surface melting with direct exposure to laser (Vaderhobli, White, Le, Ho, & Jordan, 2010). Other studies have also reported thermal side effects like dentin cracking, carbonization, and melting following CO2 laser use on root surfaces (Matthews,

Preventive uses of CO2 laser have been well researched. Literature suggests that 9.3 and 9.6um wavelength (pulse width <100usec) at a specified pulse rate has higher efficiency than 10.6um in heating dentin/enamel surface leading to desired crystallization and fusion of surface layer for sealing effect (McCormack, Fried, Featherstone, Glena, & Seka, 1995; Wigdor et al., 1995). A case report by Dederich, suggested to use 15W for 0.2sec duration to

**3.2 Endodontics (apicoectomy, root canal debridement)** 

**3.3 Periodontal procedures** 

**3.4 Other restorative uses** 

2010).

and coagulation disorders. They concluded that CO2 laser provided bloodless field, less post-operative discomfort, tissue coagulation, and better accessibility in some areas of oral cavity compared to scalpel surgery (Pick & Pecaro, 1987b). The advantages compared to scalpel wounds also included site-specific wound sterilization; minimal swelling and scarring but slower healing; reduced necessity for suturing; decreased incidence of mechanical trauma; shorter operative time; favorable patient acceptance; decreased use of local anesthesia; and little or no postoperative pain (Pick & Powell, 1993; White et al., 2002; Wigdor et al., 1995). Literature also reported increased levels of hyaluronic acid in laser wounds compared to scalpel wounds, a chemical that plays a key role in wound repair (Pogrel, Pham, Guntenhoner, & Stern, 1993). With increased use of CO2 lasers clinically, adjacent tissue interaction and damage has been an issue. Studies have reported on chemical and thermal interaction of CO2 lasers with surrounding tissue. In vitro study using 9.3 micrometers Duolase CO2 laser (Medical Optics Inc.) investigated variations in incision depth and width, collateral damage, and bone charring using continuous mode (1-9W average power; 1-10W peak power; 0.5-500Hz; 1, 20, 200miliseconds), superpulse (1-7W average power; 20W peak power; 170-1170hz; 300microseconds) and optipulse (0.72-1.20W average power; 60-100W peak power; 10-40Hz; 300microseconds) mode with various parameter combinations. They concluded that superpulse and optipulse mode with lower average powers and higher peak powers created narrow and deep cuts. Also, almost no charring was noticed with optipulse mode. Optipulse mode reduced the collateral damage by the factor of 10 compared to continuous mode (Wilder-Smith et al., 1997).


Table 2. CO2 Laser soft tissue applications

and coagulation disorders. They concluded that CO2 laser provided bloodless field, less post-operative discomfort, tissue coagulation, and better accessibility in some areas of oral cavity compared to scalpel surgery (Pick & Pecaro, 1987b). The advantages compared to scalpel wounds also included site-specific wound sterilization; minimal swelling and scarring but slower healing; reduced necessity for suturing; decreased incidence of mechanical trauma; shorter operative time; favorable patient acceptance; decreased use of local anesthesia; and little or no postoperative pain (Pick & Powell, 1993; White et al., 2002; Wigdor et al., 1995). Literature also reported increased levels of hyaluronic acid in laser wounds compared to scalpel wounds, a chemical that plays a key role in wound repair (Pogrel, Pham, Guntenhoner, & Stern, 1993). With increased use of CO2 lasers clinically, adjacent tissue interaction and damage has been an issue. Studies have reported on chemical and thermal interaction of CO2 lasers with surrounding tissue. In vitro study using 9.3 micrometers Duolase CO2 laser (Medical Optics Inc.) investigated variations in incision depth and width, collateral damage, and bone charring using continuous mode (1-9W average power; 1-10W peak power; 0.5-500Hz; 1, 20, 200miliseconds), superpulse (1-7W average power; 20W peak power; 170-1170hz; 300microseconds) and optipulse (0.72-1.20W average power; 60-100W peak power; 10-40Hz; 300microseconds) mode with various parameter combinations. They concluded that superpulse and optipulse mode with lower average powers and higher peak powers created narrow and deep cuts. Also, almost no charring was noticed with optipulse mode. Optipulse mode reduced the collateral damage

by the factor of 10 compared to continuous mode (Wilder-Smith et al., 1997).

Oral Medicine Aphthous ulcer treatment, Biopsies (incisional/excisional),

procedures Crown lengthening (soft tissue only), Tissue retraction for impression

Abscess incision and drainage, Hemostatic assistance, Fibroma removal, Oral papillectomy, Exposure of nonerupted or partially erupted teeth, Implant recovery, Lesion (tumor) removal, Vestibuloplasty, Frenectomy, Frenotomy, Operculectomy, coagulation

Leukoplakia

Sulcular debridement, Gingival excision/incision, laser assisted new attachment procedure, Gingivectomy/gingivoplasty

Pulpotomy, Pulpotomy, as an adjunct to root canal therapy and retreatment cases, Removal of filling material such as gutta-percha or resin

resin curing, teeth whitening agent activation, caries detection, pit and fissure sealants, enamel treatment to increase caries resistance, enamel etching for resin bonding procedures, caries removal, tissue ablation

Area of Dentistry Procedure

Oral & maxillofacial surgery

Pre-prosthetic

Periodontal procedures

Endodontics

Restorative uses

Table 2. CO2 Laser soft tissue applications

#### **3.2 Endodontics (apicoectomy, root canal debridement)**

Hard tissue applications of continuous wave CO2 lasers have been limited due to thermal damage, charring effect and resultant rough tooth surface. Due to its high absorption overlap with phosphate in enamel apetite crystal, all radiation is absorbed in thin enamel (<10 um). This makes heat transfer as the main way of energy transport leading to thermal damage to pulp (Wigdor et al., 1995). Conversely studies have shown that high reflectivity (9%-50%) at 9.3- and 9.6um wavelengths may pose a safety concern and it requires accurate knowledge of radiation dose while doing treatment (Fried, Glena, Featherstone, & Seka, 1997). In 1990's TEA (Transversely excited atmospheric pressure) 10.6um pulsed (0.1-2usec) CO2 laser had the best reported success in ablating dental hard tissue. However, high plasma induction with TEA CO2 laser posed problems with decreased ablation efficiency and damage due to shock wave rendering it unacceptable for clinical use (Wigdor et al., 1995). Since the 9.6micrometer CO2 laser wavelength is highly absorbed in appetite crystals, it presents a future potential for its applications in cavity preparation, apicectomies and other hard tissue procedures. In vitro study using Scanning Electron Microscopic (SEM) images reported cleaner dentinal surface with fusion and recrystallized dentine following apicectomy and root treatment with pulsed TEA 9.6micrometer CO2 laser (Gouw-Soares et al., 2004). Contrasting results were reported in a more recent SEM analysis study. The study compared the marginal permeability and dentinal surface texture following apicectomy performed with burs and CO2 laser. Authors attributed rougher surface and less favourable marginal fit following CO2 laser treatment to the use of continuous wave mode, no cooling agent and less experience of the operator with CO2 laser (Lustosa-Pereira et al., 2011).

#### **3.3 Periodontal procedures**

Advantages of using CO2 laser for periodontal procedures have been accepted by American Academy of Periodontology in its position paper. Its ability to provide dry surgical field and haemostasis has been proven useful in periodontal surgical procedures. Additionally CO2 laser use has shown mixed results when used for periodontal pocket debridement in addition to mechanical debridement, pocket reduction, attachment gain, decreased microorganisms, and guided tissue regeneration cases (Convissar & Goldstein, 2003; Matthews, 2010; Wigdor et al., 1995). Porcine mandible study evaluating efficacy of newer micropulse 10.6um CO2 laser showed clinically acceptable results in coagulation, incision depth and width, time required to perform procedure, with minimal hard tissue damage on accidental exposure but surface melting with direct exposure to laser (Vaderhobli, White, Le, Ho, & Jordan, 2010). Other studies have also reported thermal side effects like dentin cracking, carbonization, and melting following CO2 laser use on root surfaces (Matthews, 2010).

#### **3.4 Other restorative uses**

Preventive uses of CO2 laser have been well researched. Literature suggests that 9.3 and 9.6um wavelength (pulse width <100usec) at a specified pulse rate has higher efficiency than 10.6um in heating dentin/enamel surface leading to desired crystallization and fusion of surface layer for sealing effect (McCormack, Fried, Featherstone, Glena, & Seka, 1995; Wigdor et al., 1995). A case report by Dederich, suggested to use 15W for 0.2sec duration to

CO2 Laser: Evidence Based Applications in Dentistry 385

Fried, D., Glena, R. E., Featherstone, J. D., & Seka, W. (1997). Permanent and transient

Gouw-Soares, S., Stabholz, A., Lage-Marques, J. L., Zezell, D. M., Groth, E. B., & Eduardo, C.

Leighty, S. M., Pogrel, M. A., Goodis, H. E., & White, J. M. (1991). Thermal effects of the carbon dioxide laser on teeth. Lasers in the life sciences, 4(2), 93-102. Luk, K., Tam, L., & Hubert, M. (2004). Effect of light energy on peroxide tooth bleaching. J

Lustosa-Pereira, A. C., Pozza, D. H., Cunha, A., Dedavid, B. A., Duarte-de Moraes, J. F., &

Malmström, H. S., McCormack, S. M., Fried, D., & Featherstone, J. D. B. (2001). Effect of CO2

Matthews, D. C. (2010). Seeing the Light--the truth about soft tissue lasers and nonsurgical

McCormack, S. M. (1995). Scanning electron microscope observations of CO2 laser effects on

McCormack, S. M., Fried, D., Featherstone, J. D., Glena, R. E., & Seka, W. (1995). Scanning

Melcer, J. (1986). Latest treatment in dentistry by means of the CO2 laser beam. Lasers Surg

N/A. (1998). Laser-assisted bleaching: an update. ADA Council on Scientific Affairs. J Am

Obata, A., Tsumura, T., Niwa, K., Ashizawa, Y., Deguchi, T., & Ito, M. (1999). Super pulse CO2 laser for bracket bonding and debonding. Eur J Orthod, 21(2), 193-198. Pick, R. M., & Pecaro, B. C. (1987a). Use of the CO2 laser in soft tissue dental surgery. Lasers

Pick, R. M., & Pecaro, B. C. (1987b). Use of the CO2 laser in soft tissue dental surgery. Lasers

Pick, R. M., & Powell, G. L. (1993). Laser in dentistry. Soft-tissue procedures. Dent Clin

Pogrel, M. A., Pham, H. D., Guntenhoner, M., & Stern, R. (1993). Profile of hyaluronidase

Seldin, L. W. (2001). Future of Dentistry: Today's Vision: Tomorrow's Reality: American

Stern, R. H., & Sognnaes, R. F. (1964). Laser beam effect on dental hard tissues. J Dent Res,

Dental Association, Health Policy Resources Center.

activity distinguishes carbon dioxide laser from scalpel wound healing. Ann Surg,

dental enamel. Journal of Dental Research, 74(10), 1702-1708.

Gerhardt-de Oliveira, M. (2011). Analysis of the morphology and composition of tooth apices apicectomized using three different ablation techniques. Med Oral

laser on pulpal temperature and surface morphology: an in vitro study. Journal of

electron microscope observations of CO2 laser effects on dental enamel. J Dent Res,

9.3, 9.6, 10.3, and 10.6 microns and at fluences of 1-20 J/cm2. [In Vitro

Research Support, U.S. Gov't, P.H.S.]. Lasers Surg Med, 20(1), 22-31.

Am Dent Assoc, 135(2), 194-201; quiz 228-199.

periodontal therapy. J Can Dent Assoc, 76, a30.

Patol Oral Cir Bucal, 16(2), e225-230.

Dentistry, 29(8), 521-529.

74(10), 1702-1708.

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in Surgery and Medicine, 7(2).

Surg Med, 7(2), 207-213.

North Am, 37(2), 281-296.

217(2), 196-200.

43(5).

10.1089/104454704774076190

changes in the reflectance of CO2 laser-irradiated dental hard tissues at lambda =

P. (2004). Comparative study of dentine permeability after apicectomy and surface treatment with 9.6 microm TEA CO2 and Er:YAG laser irradiation. [Research Support, Non-U.S. Gov't]. J Clin Laser Med Surg, 22(2), 129-139. doi:

achieve dentinal sealing effect with CO2 laser without detrimental effect to pulpal tissue (Dederich, 1999). Furthermore, researchers showed that pulsed CO2 laser produced >1000 celsius temperature increase at the surface, enough to melt and recrystalize enamel and minimal changes deeper than 40um which is critical to avoid collateral damage (J. D. Featherstone & D. G. Nelson, 1987; Wigdor et al., 1995). For dental decay, CO2 lasers have mixed results ranging from thermal damage, dentin/pulp sterilization, and mineralization under treated surface (Melcer, 1986). Surface etching with CO2 laser showed 300% increased in dentin resin bond but no change in enamel bonding (Obata et al., 1999; Wigdor et al., 1995). Superpulse CO2 laser produced fastest debonding of orthodontic brackets (Obata et al., 1999). CO2 laser has also been used in otherwise hopeless prognosis cases of vertical root fracture with radiographical success at one-year follow-up. Teeth bleaching agent activation with CO2 laser causes higher temperature changes and due to lack of controlled clinical trials, ADA does not support its clinical use (Luk, Tam, & Hubert, 2004; N/A, 1998).

#### **4. Future of dental applications of CO2 lasers**

The future looks promising for CO2 laser use in the field of dentistry. We need more clinical research specifically randomized clinical trials to evaluate effectiveness of CO2 laser compared to traditional methods. The evidence will help develop standard clinical guidelines for practicing dentists.

#### **5. Conclusion**

Currently, CO2 lasers have been used widely in dentistry for soft tissue procedures. More research will help provide practice guidelines. Clinical research including randomized trials are needed to provide specifications for parameter settings, delivery mode, and other guidelines for hard tissue procedures. More information regarding shorter wavelength CO2 lasers in recent years makes future of CO2 laser promising in dentistry.

#### **6. Acknowledgment**

We would like to thank Lutheram Medical Center- Department of Dental Medicine for providing support for this project.

#### **7. References**


achieve dentinal sealing effect with CO2 laser without detrimental effect to pulpal tissue (Dederich, 1999). Furthermore, researchers showed that pulsed CO2 laser produced >1000 celsius temperature increase at the surface, enough to melt and recrystalize enamel and minimal changes deeper than 40um which is critical to avoid collateral damage (J. D. Featherstone & D. G. Nelson, 1987; Wigdor et al., 1995). For dental decay, CO2 lasers have mixed results ranging from thermal damage, dentin/pulp sterilization, and mineralization under treated surface (Melcer, 1986). Surface etching with CO2 laser showed 300% increased in dentin resin bond but no change in enamel bonding (Obata et al., 1999; Wigdor et al., 1995). Superpulse CO2 laser produced fastest debonding of orthodontic brackets (Obata et al., 1999). CO2 laser has also been used in otherwise hopeless prognosis cases of vertical root fracture with radiographical success at one-year follow-up. Teeth bleaching agent activation with CO2 laser causes higher temperature changes and due to lack of controlled clinical

trials, ADA does not support its clinical use (Luk, Tam, & Hubert, 2004; N/A, 1998).

The future looks promising for CO2 laser use in the field of dentistry. We need more clinical research specifically randomized clinical trials to evaluate effectiveness of CO2 laser compared to traditional methods. The evidence will help develop standard clinical

Currently, CO2 lasers have been used widely in dentistry for soft tissue procedures. More research will help provide practice guidelines. Clinical research including randomized trials are needed to provide specifications for parameter settings, delivery mode, and other guidelines for hard tissue procedures. More information regarding shorter wavelength CO2

We would like to thank Lutheram Medical Center- Department of Dental Medicine for

Boehm, R., Rich, J., Webster, J., & Janke, S. (1977). Thermal stress effects and surface cracking associated with laser use on human teeth. J Biomech Eng, 99, 189-194. Convissar, R. A., & Goldstein, E. E. (2003). An overview of lasers in dentistry. [Case Reports,

Dederich, D. N. (1999). CO2 laser fusion of a vertical root fracture. J Am Dent Assoc, 130(8),

Featherstone, J. D., & Nelson, D. G. (1987). Laser effects on dental hard tissues. [Research

Featherstone, J. D. B., & Nelson, D. G. A. (1987). Laser effects on dental hard tissues.

lasers in recent years makes future of CO2 laser promising in dentistry.

Support, U.S. Gov't, P.H.S.]. Adv Dent Res, 1(1), 21-26.

**4. Future of dental applications of CO2 lasers** 

guidelines for practicing dentists.

**5. Conclusion** 

**6. Acknowledgment** 

**7. References** 

providing support for this project.

1195-1199.

Review]. Gen Dent, 51(5), 436-440.

Advances in Dental Research, 1(1), 21-26.


**16** 

*Iran* 

Nasrin Zand

**Non-Thermal, Non-Ablative CO2 Laser Therapy** 

CO2 laser has been used as a very useful device in surgery for ablation, coagulation and cutting the tissues for the last four decades. It is interesting to know that this high power laser can also be used as a therapeutic laser for immediate pain reduction in some oral lesions without any visible side effects such as ulceration, erosion formation and even erythema.

Recently few case reports and clinical trials have been published about using CO2 laser in non-ablative manner to reduce pain in oral lesions. In these studies, the oral painful lesions were irradiated through a layer of transparent, non-anesthetic gel with high water content to reduce the beam absorption by the soft tissue. The patients reported immediate and significant pain relief after laser irradiation. The procedure was painless and anesthesia was not required. This technique was called non-thermal, Non-Ablative CO2 Laser Therapy (NACLT). The results of powermetry and thermometry demonstrated the low power nature of NACLT. However there are some differences between analgesic effects of NACLT and

To provide a comprehensive understanding of NACLT, this chapter is organized in several sections. First, due to low level therapeutic nature of NACLT, conventional low power therapeutic lasers, their biological effects and their pain relieving properties are reviewed. Then, a discussion about the interesting analgesic effects of CO2 lasers is presented. In the next section, NACLT as a new low level laser therapy procedure and its pain relieving applications in painful oral lesions is discussed. Finally, the presumed mechanisms of

Low-level laser (or light) therapy (LLLT) has been investigated and used clinically for over 40 years. However, it is only in relatively recent times that LLLT has become scientifically

and clinically accepted by even a fraction of the medical community (Hamblin 2010).

the other classical low power lasers which will be discussed in the next sections.

**1. Introduction** 

analgesic effects of NACLT are covered.

**2.1 History** 

**2. Low level laser therapy (laser phototherapy)** 

**(NACLT): A New Approach to Relieve Pain in** 

*Academic Center for Education, Culture and Research (ACECR), Tehran* 

**Some Painful Oral Diseases** 

*Iranian Center for Medical Lasers (ICML),* 


### **Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT): A New Approach to Relieve Pain in Some Painful Oral Diseases**

Nasrin Zand *Iranian Center for Medical Lasers (ICML), Academic Center for Education, Culture and Research (ACECR), Tehran Iran* 

#### **1. Introduction**

386 CO2 Laser – Optimisation and Application

Stern, R. H., Vahl, J., & Sognnaes, R. F. (1972). Lased enamel: ultrastructural observations of pulsed carbon dioxide laser effects. Journal of Dental Research, 51(2), 455-460. Sulewski JG, (2010). Making the most of 16th Annual conference and exhibition: A practical orientation for attendees. Academy of Laser Dentistry, Miami, Florida. Taylor, R., Shklar, G., & Roeber, F. (1965). THE EFFECTS OF LASER RADIATION ON

Vaderhobli, R. M., White, J. M., Le, C., Ho, S., & Jordan, R. (2010). In vitro study of the soft

White, J. M., Chaudhry, S. I., Kudler, J. J., Sekandari, N., Schoelch, M. L., & Silverman Jr, S.

White, J. M., Gekelman, D., Shin, K. B., Park, J. S., Swenson, T. O., Rouse, B. P., . . . Oto, M. G.

Wigdor, H. A., Walsh, J. T., Jr., Featherstone, J. D., Visuri, S. R., Fried, D., & Waldvogel, J. L.

Wilder-Smith, P., Dang, J., & Kurosaki, T. (1997). Investigating the range of surgical effects

Oral Surg Oral Med Oral Pathol, 19, 786-795.

laser medicine & surgery, 16(6), 299.

Review]. Lasers Surg Med, 16(2), 103-133.

of applied scientific research?

Lasers Surg Med, 42(3), 257-263. doi: 10.1002/lsm.20888

TEETH, DENTAL PULP, AND ORAL MUCOSA OF EXPERIMENTAL ANIMALS.

tissue effects of microsecond-pulsed CO(2) laser parameters during soft tissue incision and sulcular debridement. [In Vitro, Research Support, Non-U.S. Gov't].

(1998). Nd: YAG and CO2 laser therapy of oral mucosal lesions. Journal of clinical

(2002). Laser interaction with dental soft tissues: What do we know from our years

(1995). Lasers in dentistry. [Case Reports, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, Non-P.H.S., Research Support, U.S. Gov't, P.H.S.,

on soft tissue produced by a carbon dioxide laser. J Am Dent Assoc, 128(5), 583-588.

CO2 laser has been used as a very useful device in surgery for ablation, coagulation and cutting the tissues for the last four decades. It is interesting to know that this high power laser can also be used as a therapeutic laser for immediate pain reduction in some oral lesions without any visible side effects such as ulceration, erosion formation and even erythema.

Recently few case reports and clinical trials have been published about using CO2 laser in non-ablative manner to reduce pain in oral lesions. In these studies, the oral painful lesions were irradiated through a layer of transparent, non-anesthetic gel with high water content to reduce the beam absorption by the soft tissue. The patients reported immediate and significant pain relief after laser irradiation. The procedure was painless and anesthesia was not required. This technique was called non-thermal, Non-Ablative CO2 Laser Therapy (NACLT). The results of powermetry and thermometry demonstrated the low power nature of NACLT. However there are some differences between analgesic effects of NACLT and the other classical low power lasers which will be discussed in the next sections.

To provide a comprehensive understanding of NACLT, this chapter is organized in several sections. First, due to low level therapeutic nature of NACLT, conventional low power therapeutic lasers, their biological effects and their pain relieving properties are reviewed. Then, a discussion about the interesting analgesic effects of CO2 lasers is presented. In the next section, NACLT as a new low level laser therapy procedure and its pain relieving applications in painful oral lesions is discussed. Finally, the presumed mechanisms of analgesic effects of NACLT are covered.

#### **2. Low level laser therapy (laser phototherapy)**

#### **2.1 History**

Low-level laser (or light) therapy (LLLT) has been investigated and used clinically for over 40 years. However, it is only in relatively recent times that LLLT has become scientifically and clinically accepted by even a fraction of the medical community (Hamblin 2010).

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

2002; Gigo-Benato, Geuna et al. 2005).

(Hamblin, Waynant et al. 2006).

**2.3 Pain relieving effects of low level therapeutic lasers** 

A New Approach to Relieve Pain in Some Painful Oral Diseases 389

level (therapeutic laser) in NACLT, too). Some of the researchers favour the term "laser

Low level laser (or light) therapy (LLLT) is the application of light (usually a low power laser or LED in the range of 1mW – 500mW) to a pathology to promote wound healing and tissue repair, reduce inflammation and relieve pain. The light is typically of narrow spectral width in the red or near infrared spectrum (600nm – 1000nm); at power densities (between 1mw-5W/cm2) (Huang, Chen et al. 2009), not associated with macroscopic thermal effects, in contrast to thermally mediated surgical applications (Chow, David et al. 2007). In using high power surgical lasers, the collimation of laser light leads to the emission of a narrow, intense beam of light and is used for precise tissue destruction (photothermal effect). However, in LLLT, light radiation intensities are so low that the resulting biological effects are ascribable to physical or chemical changes associated with the interaction of cells and tissues with the laser radiation, and not simply to a result of heating (Snyder, Byrnes et al.

The main areas of medicine where laser phototherapy has a known and major role are as follows: promoting wound healing, tissue repair and prevention of tissue death, relief of inflammation in chronic diseases and injuries with its associated pain and edema, relief of

The first law of photobiology states that for low power visible light to have any effect on a living biological system, the photons must be absorbed by electronic absorption bands belonging to some molecular photoacceptors, or chromophores (Sutherland 2002; Huang, Chen et al. 2009). Red and near infrared light is absorbed by photoreceptors contained in the protein components of the respiratory chain located in mitochondria, in particular cytochrome c oxidase and flavoproteins like NADH-dehydrogenase. This can lead to a short time activation of respiratory chain and oxidation of NADH pool leading to changes in the redox state of both mitochondria and cytoplasm, leading to increased ATP production, and biological responses at the cellular level through cascades of biochemical reactions (Karu 1989; Karu, Pyatibrat et al. 2004; Karu and Kolyakov 2005). These effects in turn lead to increased cell proliferation and migration, modulation in levels of cytokines, growth factors, inflammatory mediators, and increased tissue oxygenation. The results of these biochemical and cellular changes in animals and patients lead to valuable biological effects such as promoting wound healing and tissue repair, relief of inflammation, pain reduction, and amelioration of damage after heart attacks, stroke, nerve injury and even retinal toxicity

Low-level laser therapy (LLLT) is increasingly recognized as an appropriate option for pain relief. In fact, it is for this indication that biostimulative lasers have been approved for marketing by the U.S. Food and Drug Administration through the premarket notification/510(k) (Gigo-Benato, Geuna et al. 2005). Many studies have demonstrated the efficacy of phototherapy in various pain syndromes (Tuner and Hode 2010). Responding to the increasing levels of evidence, the World Health Organization's Committee of the Decay

neurogenic pain and some neurological problems (Hamblin, Waynant et al. 2006).

phototherapy (LPT)" which is an emerging terminology (Tuner and Hode 2010).

**2.2 A brief review on biological effects of low level therapeutic lasers** 

The history of the use of laser phototherapy in medicine goes back to the late 1960s, only eight years after the invention of the first laser (Ruby laser) by Theodore Maiman. In 1967, Endre Mester in Semmelweis University, Budapest, Hungary decided to test if laser radiation might cause cancer in mice. He shaved the dorsal hair of the mice, divided them into two groups and irradiated the shaved areas with a low powered ruby laser (694-nm) in one group. They did not get cancer and to his surprise the hair on the treated group grew back more quickly than the untreated group. This was the first demonstration of "laser biostimulation" (Hamblin, Waynant et al. 2006).

In early 1960's, the first low level laser, Helium-Neon was developed by Professor Ali Javan. It emits visible, red light with a wavelength of 632.8nm. This low power laser has been used extensively in experimental and therapeutic studies. Today, the semiconductor lasers, including InGaAlP lasers (633-700nm), GaAlAs lasers (780-890nm, invisible, near infrared area), GaAs laser (904nm, invisible, near infrared area) are widely used by researchers and clinicians.

LLLT originally thought to be a peculiar property of laser light (soft or cold lasers); the subject has now broadened, using non-coherent light (light-emitting diodes, LEDs). Today, medical treatment with coherent-light sources (lasers) or noncoherent light (LEDs) has passed through its childhood and adolescence (Hamblin, Waynant et al. 2006). Currently, low-level laser (or light) therapy (LLLT) is practiced as part of physical therapy in many parts of the world. Although LLLT was used mainly for wound healing and pain relief, the medical applications of LLLT have broadened to include diseases such as stroke, myocardial infarction, and degenerative or traumatic brain disorders (Hashmi, Huang et al. 2010).

Although many experimental and clinical studies have reported the positive effects of phototherapy to promote wound healing , pain relief and anti-inflammatory effects, some negative reports also have been published, further confounding the issue (Demidova-Rice, Salomatina et al. 2007), for instance regarding the application of laser phototherapy on wound healing (Posten, Wrone et al. 2005). This controversy seems to be due to two main reasons; first of all, the basic biochemical mechanisms underlying these biological effects are not completely understood. Secondly, the complexity of rationally choosing amongst a large number of laser irradiation parameters (such as wavelength, fluence, power density, pulse structure and treatment timing), inappropriate anatomical treatment location and concurrent patient medication (such as steroidal and non-steroidal anti-inflammatories which can inhibit healing) has led to conflicting results and publication of a number of unfavourable, as well as many favourable, studies. In particular a biphasic dose response has been frequently observed where low levels of light have a much better effect than higher levels (Gigo-Benato, Geuna et al. 2005; Aimbire, Albertini et al. 2006; Hamblin, Waynant et al. 2006; Goncalves, Souza et al. 2007; Huang, Chen et al. 2009; Hamblin 2010).

It should be noticed that LLLT has a diversified terminology. It is also called "cold laser", "soft laser", "biostimulation" , "photobiomodulation", "low intensity laser therapy" , "low energy laser therapy", "laser phototherapy (LPT)", "laser therapy", and "non-ablative irradiation". Some investigators state that using frequent terms, such as "low power laser therapy" is misleading, since high power lasers, too, can be used for laser phototherapy (Tuner and Hode 2010) (as we will discuss in the next sections, CO2 laser is applied as a low

The history of the use of laser phototherapy in medicine goes back to the late 1960s, only eight years after the invention of the first laser (Ruby laser) by Theodore Maiman. In 1967, Endre Mester in Semmelweis University, Budapest, Hungary decided to test if laser radiation might cause cancer in mice. He shaved the dorsal hair of the mice, divided them into two groups and irradiated the shaved areas with a low powered ruby laser (694-nm) in one group. They did not get cancer and to his surprise the hair on the treated group grew back more quickly than the untreated group. This was the first demonstration of "laser

In early 1960's, the first low level laser, Helium-Neon was developed by Professor Ali Javan. It emits visible, red light with a wavelength of 632.8nm. This low power laser has been used extensively in experimental and therapeutic studies. Today, the semiconductor lasers, including InGaAlP lasers (633-700nm), GaAlAs lasers (780-890nm, invisible, near infrared area), GaAs laser (904nm, invisible, near infrared area) are widely used by researchers and

LLLT originally thought to be a peculiar property of laser light (soft or cold lasers); the subject has now broadened, using non-coherent light (light-emitting diodes, LEDs). Today, medical treatment with coherent-light sources (lasers) or noncoherent light (LEDs) has passed through its childhood and adolescence (Hamblin, Waynant et al. 2006). Currently, low-level laser (or light) therapy (LLLT) is practiced as part of physical therapy in many parts of the world. Although LLLT was used mainly for wound healing and pain relief, the medical applications of LLLT have broadened to include diseases such as stroke, myocardial infarction, and degenerative or traumatic brain disorders (Hashmi, Huang et

Although many experimental and clinical studies have reported the positive effects of phototherapy to promote wound healing , pain relief and anti-inflammatory effects, some negative reports also have been published, further confounding the issue (Demidova-Rice, Salomatina et al. 2007), for instance regarding the application of laser phototherapy on wound healing (Posten, Wrone et al. 2005). This controversy seems to be due to two main reasons; first of all, the basic biochemical mechanisms underlying these biological effects are not completely understood. Secondly, the complexity of rationally choosing amongst a large number of laser irradiation parameters (such as wavelength, fluence, power density, pulse structure and treatment timing), inappropriate anatomical treatment location and concurrent patient medication (such as steroidal and non-steroidal anti-inflammatories which can inhibit healing) has led to conflicting results and publication of a number of unfavourable, as well as many favourable, studies. In particular a biphasic dose response has been frequently observed where low levels of light have a much better effect than higher levels (Gigo-Benato, Geuna et al. 2005; Aimbire, Albertini et al. 2006; Hamblin, Waynant et

al. 2006; Goncalves, Souza et al. 2007; Huang, Chen et al. 2009; Hamblin 2010).

It should be noticed that LLLT has a diversified terminology. It is also called "cold laser", "soft laser", "biostimulation" , "photobiomodulation", "low intensity laser therapy" , "low energy laser therapy", "laser phototherapy (LPT)", "laser therapy", and "non-ablative irradiation". Some investigators state that using frequent terms, such as "low power laser therapy" is misleading, since high power lasers, too, can be used for laser phototherapy (Tuner and Hode 2010) (as we will discuss in the next sections, CO2 laser is applied as a low

biostimulation" (Hamblin, Waynant et al. 2006).

clinicians.

al. 2010).

level (therapeutic laser) in NACLT, too). Some of the researchers favour the term "laser phototherapy (LPT)" which is an emerging terminology (Tuner and Hode 2010).

#### **2.2 A brief review on biological effects of low level therapeutic lasers**

Low level laser (or light) therapy (LLLT) is the application of light (usually a low power laser or LED in the range of 1mW – 500mW) to a pathology to promote wound healing and tissue repair, reduce inflammation and relieve pain. The light is typically of narrow spectral width in the red or near infrared spectrum (600nm – 1000nm); at power densities (between 1mw-5W/cm2) (Huang, Chen et al. 2009), not associated with macroscopic thermal effects, in contrast to thermally mediated surgical applications (Chow, David et al. 2007). In using high power surgical lasers, the collimation of laser light leads to the emission of a narrow, intense beam of light and is used for precise tissue destruction (photothermal effect). However, in LLLT, light radiation intensities are so low that the resulting biological effects are ascribable to physical or chemical changes associated with the interaction of cells and tissues with the laser radiation, and not simply to a result of heating (Snyder, Byrnes et al. 2002; Gigo-Benato, Geuna et al. 2005).

The main areas of medicine where laser phototherapy has a known and major role are as follows: promoting wound healing, tissue repair and prevention of tissue death, relief of inflammation in chronic diseases and injuries with its associated pain and edema, relief of neurogenic pain and some neurological problems (Hamblin, Waynant et al. 2006).

The first law of photobiology states that for low power visible light to have any effect on a living biological system, the photons must be absorbed by electronic absorption bands belonging to some molecular photoacceptors, or chromophores (Sutherland 2002; Huang, Chen et al. 2009). Red and near infrared light is absorbed by photoreceptors contained in the protein components of the respiratory chain located in mitochondria, in particular cytochrome c oxidase and flavoproteins like NADH-dehydrogenase. This can lead to a short time activation of respiratory chain and oxidation of NADH pool leading to changes in the redox state of both mitochondria and cytoplasm, leading to increased ATP production, and biological responses at the cellular level through cascades of biochemical reactions (Karu 1989; Karu, Pyatibrat et al. 2004; Karu and Kolyakov 2005). These effects in turn lead to increased cell proliferation and migration, modulation in levels of cytokines, growth factors, inflammatory mediators, and increased tissue oxygenation. The results of these biochemical and cellular changes in animals and patients lead to valuable biological effects such as promoting wound healing and tissue repair, relief of inflammation, pain reduction, and amelioration of damage after heart attacks, stroke, nerve injury and even retinal toxicity (Hamblin, Waynant et al. 2006).

#### **2.3 Pain relieving effects of low level therapeutic lasers**

Low-level laser therapy (LLLT) is increasingly recognized as an appropriate option for pain relief. In fact, it is for this indication that biostimulative lasers have been approved for marketing by the U.S. Food and Drug Administration through the premarket notification/510(k) (Gigo-Benato, Geuna et al. 2005). Many studies have demonstrated the efficacy of phototherapy in various pain syndromes (Tuner and Hode 2010). Responding to the increasing levels of evidence, the World Health Organization's Committee of the Decay

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

**3. Pain relieving effects of carbon dioxide lasers** 

Mansouri et al. 2009; Zand, Najafi et al. 2010).

(Tuner and Hode 2010).

two main groups:

1996; Tuner and Hode 2010)*.*

(Duncavage and Ossoff 1986).

A New Approach to Relieve Pain in Some Painful Oral Diseases 391

• Systemic effect, some researchers propose that laser phototherapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites

Laser phototherapy is a relatively new application of carbon dioxide lasers, in spite of the fact that papers on the subject were published as early as the mid-eighties (Tuner and Hode 2010). CO2 laser biostimulative and pain relieving effects can be assessed in the following

• The lower post operative pain following CO2 laser surgery compared to conventional surgery, which is attributed to the simultaneous low level laser therapy (photobiomodulation) effects of high power CO2 laser irradiation (Tuner and Hode 2010). • Application of CO2 laser, as a phototherapeutic laser. "For using CO2 laser as a low power therapeutic laser, one needs to make the beam wide enough not to burn. Another option is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity biostimulative laser therapy range" (Tuner and Hode 2010). NACLT is a new technique, in which CO2 laser is used in non-thermal, non-ablative manner as a biomodulative, low intensity laser for immediate and significant pain relief. (Zand, Ataie-Fashtami et al. 2009; Zand,

This section is organized as follows. At first some clinical studies which demonstrate the pain relieving effects of high power surgical CO2 lasers are reviewed, and then low power biostimulative CO2 laser studies are briefly reviewed. In the next section, NACLT as a new

Some investigators who used the classical thermal effects of CO2 laser (vaporization, cutting and coagulation) reported less post-operative pain following CO2 laser surgery, and a reduced requirement for post-operative analgesics (Duncavage and Ossoff 1986; Colvard and Kuo 1991; Demidov, Rykov et al. 1992; Chia, Darzi et al. 1995; Kaplan, Kott et al. 1996; Andre 2003).

Kaplan, one of the pioneers of CO2 laser surgery, attributed the excellent healing and lower post operative pain experienced with CO2 laser surgery compared to conventional surgery to the simultaneous laser therapy effects of CO2 laser irradiation. Kaplan stated that laser surgery and laser therapy should be regarded *as two sides of the same coin* (Kaplan, Kott et al.

Duncavage reported that the advantages of the CO2 laser surgery included homeostasis, precise visualization, and less edema and pain than the conventional techniques

Colvard and Kuo used high-power, ablative CO2 laser at a power output of 4 W under local anesthesia for painful minor aphthous ulcers of 14 patients. In all, 88.8% of the patients in the study were completely pain free following anesthetic resolution, and none of the

patients required post-operative medication for pain relief. (Colvard and Kuo 1991).

low power, biomodulative laser therapy protocol for pain reduction is introduced.

**3.1 Pain relieving effects of carbon dioxide laser as a high power surgical laser** 

of the Bone and Joint has also recently incorporated LLLT into guidelines for treatment of neck pain (Haldeman, Carroll et al. 2008; Chow, Armati et al. 2011).

*"Efficacy of LLLT in painful clinical conditions has been established by several recent systematic reviews and meta-analyses [level 1 evidence, according to the Australian Government, NHMRC (1999)] (Chow, David et al. 2007). This level of evidence relates to chronic neck pain(Chow and Barnsley 2005), tendinitis (Bjordal, Couppe et al. 2001), chronic joint disorders (Bjordal, Couppe et al. 2003), and chronic pain (Enwemeka, Parker et al. 2004) . Randomized controlled trials (RCTs) provide level II evidence for the efficacy of laser therapy in chronic low back pain (Umegaki, Tanaka et al. 1989; Soriano and Rios 1998; Basford, Sheffield et al. 1999) . In other reviews of laser therapy for painful conditions such as rheumatoid arthritis (Brosseau, Robinson et al. 2005) and musculoskeletal pain (Gam, Thorsen et al. 1993; De Bie, De. Vet et al. 1998), the evidence is equivocal. Such variability in outcomes may be due to the multiplicity of parameters used, including wavelengths, energy, and power densities, with differing frequencies of application(Chow and Barnsley 2005)." (Chow, David et al. 2007)*

#### **2.4 Mechanisms of analgesic effects of low level laser therapy**

The basic biological mechanisms behind the analgesic effects of conventional LLLT are not completely understood. Some of the explanations for these pain relieving effects are as follows:


of the Bone and Joint has also recently incorporated LLLT into guidelines for treatment of

*"Efficacy of LLLT in painful clinical conditions has been established by several recent systematic reviews and meta-analyses [level 1 evidence, according to the Australian Government, NHMRC (1999)] (Chow, David et al. 2007). This level of evidence relates to chronic neck pain(Chow and Barnsley 2005), tendinitis (Bjordal, Couppe et al. 2001), chronic joint disorders (Bjordal, Couppe et al. 2003), and chronic pain (Enwemeka, Parker et al. 2004) . Randomized controlled trials (RCTs) provide level II evidence for the efficacy of laser therapy in chronic low back pain (Umegaki, Tanaka et al. 1989; Soriano and Rios 1998; Basford, Sheffield et al. 1999) . In other reviews of laser therapy for painful conditions such as rheumatoid arthritis (Brosseau, Robinson et al. 2005) and musculoskeletal pain (Gam, Thorsen et al. 1993; De Bie, De. Vet et al. 1998), the evidence is equivocal. Such variability in outcomes may be due to the multiplicity of parameters used, including wavelengths, energy, and power densities, with differing frequencies of* 

The basic biological mechanisms behind the analgesic effects of conventional LLLT are not completely understood. Some of the explanations for these pain relieving effects are as follows: • Reversible blockage of action potential generation of nociceptive signals in primary afferent neurons and specific reversible inhibition and functional impairment of Aδ and C fibers, which transmit nociceptive stimuli (Wakabayashi, Hamba et al. 1993; Kasai, Kono et al. 1996; Orchardson, Peacock et al. 1997; Chow, Armati et al. 2011). • Increase in β-endorphin synthesis and release (Labajos 1988; Montesinos 1988;

• Inhibiting cyclooxygenase, interrupting conversion of arachidonic acid in to prostaglandins, especially prostaglandin E2 (PGE2) (Shimizu, Yamaguchi et al. 1995;

• Suppression of Substance P, a neuropeptide associated with nociception (Ohno 1997). • Suppression of bradykinin activity, a pro-inflammatory neuropeptide that irritates nociceptors and is a key element in clinical pain and the associated inflammation

• Increased production of serotonin, affecting negatively neurotransmission (Tuner and

• Decreased inflammation and subsequently decreased inflammatory sensitization of

• Improvement of local microcirculation, increased tissue oxygenation, shift of metabolism from anaerobic to aerobic pathways, decreased production of acidic

• Increased lymphatic flow and consequently reducing edema, which leads to decreased

• Involvement of nitric oxide in analgesic effects of therapeutic lasers (Mrowiec 1997) • Single oxygen production, which in small amounts, is very important in biochemical

• Increased synaptic activity of acetylcholine esterase (Simunovic 2000)

processes and may be important in biostimulation

metabolites which stimulate the pain receptors

neck pain (Haldeman, Carroll et al. 2008; Chow, Armati et al. 2011).

*application(Chow and Barnsley 2005)." (Chow, David et al. 2007)*

**2.4 Mechanisms of analgesic effects of low level laser therapy** 

Hagiwara, Iwasaka et al. 2007).

Mizutani, Musya et al. 2004).

Hode 2010)

(Maeda 1989; Jimbo, Noda et al. 1998)

small-diameter afferent nerve endings

sensitization of pain receptors

• Systemic effect, some researchers propose that laser phototherapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010).

#### **3. Pain relieving effects of carbon dioxide lasers**

Laser phototherapy is a relatively new application of carbon dioxide lasers, in spite of the fact that papers on the subject were published as early as the mid-eighties (Tuner and Hode 2010). CO2 laser biostimulative and pain relieving effects can be assessed in the following two main groups:


This section is organized as follows. At first some clinical studies which demonstrate the pain relieving effects of high power surgical CO2 lasers are reviewed, and then low power biostimulative CO2 laser studies are briefly reviewed. In the next section, NACLT as a new low power, biomodulative laser therapy protocol for pain reduction is introduced.

#### **3.1 Pain relieving effects of carbon dioxide laser as a high power surgical laser**

Some investigators who used the classical thermal effects of CO2 laser (vaporization, cutting and coagulation) reported less post-operative pain following CO2 laser surgery, and a reduced requirement for post-operative analgesics (Duncavage and Ossoff 1986; Colvard and Kuo 1991; Demidov, Rykov et al. 1992; Chia, Darzi et al. 1995; Kaplan, Kott et al. 1996; Andre 2003).

Kaplan, one of the pioneers of CO2 laser surgery, attributed the excellent healing and lower post operative pain experienced with CO2 laser surgery compared to conventional surgery to the simultaneous laser therapy effects of CO2 laser irradiation. Kaplan stated that laser surgery and laser therapy should be regarded *as two sides of the same coin* (Kaplan, Kott et al. 1996; Tuner and Hode 2010)*.*

Duncavage reported that the advantages of the CO2 laser surgery included homeostasis, precise visualization, and less edema and pain than the conventional techniques (Duncavage and Ossoff 1986).

Colvard and Kuo used high-power, ablative CO2 laser at a power output of 4 W under local anesthesia for painful minor aphthous ulcers of 14 patients. In all, 88.8% of the patients in the study were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. (Colvard and Kuo 1991).

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

for the treatment of chronic pharyngitis (Nicola and Nicola 1994).

of treatment protocols deserves further studies (Longo, Simunovic et al. 1997).

and Hode 2010).

and Hode 2010).

**4. NACLT (Non-Ablative CO2 Laser Therapy)** 

aggravation of the oral lesions (Elad, Or et al. 2003).

A New Approach to Relieve Pain in Some Painful Oral Diseases 393

their incident energy and power density were set within the laser therapy range by spreading out the beam over such a large surface that the laser did not cause burning (Tuner

Nicola used CO2 low power laser treating chronic pharyngitis. 85 patients with non-specific chronic pharyngitis were elected to be treated: Group Ι, 40 patients with predominance of hyperaemic aspect; and group II, 45 patients, predominance of hypertrophied aspect. Both groups were treated for 8 to 10 sessions. They concluded that this method was very suitable

In another study, 846 patients with different types of fibromyositic rheumatisms were submitted to defocalized laser therapy from 1980 to 1995. They employed Diodes and CO2 lasers. Control groups were used to compare results with those of traditional methods. Results were evaluated on the basis of subjective (such as local pain) and objective criteria. On the whole, results were positive in comparison with other methods both as regards recovery time and persistence of results. Approximately 2/3 of the patients benefited from the treatment indicated that there were greater advantages in use of laser over other presently available methods. Longo and his collogues recommended that standardalization

The CO2-laser can also be used as an acupuncture tool. Simulation of acupuncture points has been carried out both with biostimulating power densities (e.g.100mW/cm2) and burning/coagulation/ evaporation power densities. Some clinics state that CO2 lasers give better results on acupuncture points than HeNe lasers. "As the CO2 laser's beam cannot penetrate more than around 0.5 mm into tissue, the effects must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites. It is well known that these kinds of secondary effect also occur at the traditional wavelengths of 633, 830, and 904 nm" (Tuner

Recently, there have been few reports about using CO2 laser in non-ablative manner to reduce pain in painful mucosal lesions (Elad, Or et al. 2003; Sharon-Buller and Sela 2004; Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).

In a report, Elad et al suggested that CO2 laser treatment could be of benefit to control pain in severe oral chronic graft-versus-host-disease (Elad, Or et al. 2003). In this study, the oral lesions of four patients were irradiated by CO2 laser during 17 sessions. The CO2 laser was applied, over mucosal lesions, using 1W power for 2-3s/1mm2. The treated mucosa was kept wet during the process. The treatment was pain free, and anesthesia was not required. The average VAS scores before, during, and immediately after CO2 laser treatment were 8.09, 3.47, and 4.88 respectively. There was no visual damage to the oral mucosa and no

In another case report, aphthous ulcers of two patients were irradiated with CO2 laser at 1.0- 1.5 W power, with a defocused hand piece for 5 seconds. Before laser irradiation, a thin film

Demidov and his colleagues used high-energy CO2 laser as a laser scalpel in 120 cases of breast surgery including 70 operations for cancer. They reported reduced pain sensitivity in the region of the wounds in the postoperative period (Demidov, Rykov et al. 1992).

In another study Chia reported that high-power CO2 laser haemorrhoidectomy was associated with a reduced requirement for post-operative analgesia (Chia, Darzi et al. 1995).

Andre used classical high power CO2 laser at 10–15 W in continuous mode under local anesthesia for treating ingrowing nails of 302 patients. He reported that ingrowing nails were easily operated with the CO2 laser; bleeding was minimal, infection was rare, the wounds healed without exhudative drainage and cosmetic results were good. In addition the immediate post-operative pain was less severe than after classical nail surgery with scalpel (Andre 2003).

In another study, Tada et al. compared the clinical effects and postoperative course of the scanning CO2 laser surgery and conventional surgical method to evaluate the clinical effectiveness of the former for the treatment of ingrowing nail deformities. They demonstrated that statistically, the operating time and the duration of postoperative pain were reduced significantly by the scanning CO2 laser. Furthermore, patients treated with CO2 laser were able to return to daily life significantly sooner (Tada, Hatoko et al. 2004).

Kaviani et al. investigated whether the CO2 laser surgery was superior to conventional surgical techniques for minor breast surgery in a randomized clinical trial. They demonstrated that application of the CO2 laser in breast mass biopsy had some advantages, including a lower requirement for local anesthetic and a lower rate of intraoperative bleeding; however it did not reduce the postoperative pain grade severity (Kaviani, Fateh et al. 2008).

Demetriades used ablative CO2 laser in painful oral aphthous ulcer of a patient with Behçet's Syndrome. His experience showed transient pain relief following ablative CO2 laser irradiation (Demetriades, Hansford et al. 2009).

#### **3.2 Pain relieving effects of carbon dioxide laser as a low level (therapeutic) laser**

It is interesting to know that in addition to classical low level therapeutic lasers, surgical lasers could also be used as therapeutic instruments, for example; defocused CO2 laser 10,600 nm, defocused ruby laser 694 nm and defocused Nd:YAG laser 1064 nm can be used for photobiomodulation. "When high power laser are used for biomodulation, one only needs to make the beam wide enough not to burn. The patient will then feel only a mild heat. An alternative is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity laser therapy range" (Tuner and Hode 2010).

The famous investigation of Mester with a low powered ruby laser (694-nm) on the shaved areas of the mice and quickly growing back of hairs can be an example of using a surgical laser as a therapeutic, biostimulative one (please see 2.1. History).

At Uppsala Academic Hospital, a CO2-laser was tested successfully for biostimulative treatment of epicondylitis. This method was called EDL (Emitted Defocused Laser-light). It should be noted, however, that CO2-lasers were not used as surgical lasers in this study;

Demidov and his colleagues used high-energy CO2 laser as a laser scalpel in 120 cases of breast surgery including 70 operations for cancer. They reported reduced pain sensitivity in

In another study Chia reported that high-power CO2 laser haemorrhoidectomy was associated with a reduced requirement for post-operative analgesia (Chia, Darzi et al. 1995). Andre used classical high power CO2 laser at 10–15 W in continuous mode under local anesthesia for treating ingrowing nails of 302 patients. He reported that ingrowing nails were easily operated with the CO2 laser; bleeding was minimal, infection was rare, the wounds healed without exhudative drainage and cosmetic results were good. In addition the immediate post-operative pain was less severe than after classical nail surgery with

In another study, Tada et al. compared the clinical effects and postoperative course of the scanning CO2 laser surgery and conventional surgical method to evaluate the clinical effectiveness of the former for the treatment of ingrowing nail deformities. They demonstrated that statistically, the operating time and the duration of postoperative pain were reduced significantly by the scanning CO2 laser. Furthermore, patients treated with CO2 laser were able to return to daily life significantly sooner (Tada, Hatoko et al. 2004).

Kaviani et al. investigated whether the CO2 laser surgery was superior to conventional surgical techniques for minor breast surgery in a randomized clinical trial. They demonstrated that application of the CO2 laser in breast mass biopsy had some advantages, including a lower requirement for local anesthetic and a lower rate of intraoperative bleeding; however

Demetriades used ablative CO2 laser in painful oral aphthous ulcer of a patient with Behçet's Syndrome. His experience showed transient pain relief following ablative CO2 laser

It is interesting to know that in addition to classical low level therapeutic lasers, surgical lasers could also be used as therapeutic instruments, for example; defocused CO2 laser 10,600 nm, defocused ruby laser 694 nm and defocused Nd:YAG laser 1064 nm can be used for photobiomodulation. "When high power laser are used for biomodulation, one only needs to make the beam wide enough not to burn. The patient will then feel only a mild heat. An alternative is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity laser therapy range" (Tuner and

The famous investigation of Mester with a low powered ruby laser (694-nm) on the shaved areas of the mice and quickly growing back of hairs can be an example of using a surgical

At Uppsala Academic Hospital, a CO2-laser was tested successfully for biostimulative treatment of epicondylitis. This method was called EDL (Emitted Defocused Laser-light). It should be noted, however, that CO2-lasers were not used as surgical lasers in this study;

laser as a therapeutic, biostimulative one (please see 2.1. History).

**3.2 Pain relieving effects of carbon dioxide laser as a low level (therapeutic) laser** 

it did not reduce the postoperative pain grade severity (Kaviani, Fateh et al. 2008).

irradiation (Demetriades, Hansford et al. 2009).

the region of the wounds in the postoperative period (Demidov, Rykov et al. 1992).

scalpel (Andre 2003).

Hode 2010).

their incident energy and power density were set within the laser therapy range by spreading out the beam over such a large surface that the laser did not cause burning (Tuner and Hode 2010).

Nicola used CO2 low power laser treating chronic pharyngitis. 85 patients with non-specific chronic pharyngitis were elected to be treated: Group Ι, 40 patients with predominance of hyperaemic aspect; and group II, 45 patients, predominance of hypertrophied aspect. Both groups were treated for 8 to 10 sessions. They concluded that this method was very suitable for the treatment of chronic pharyngitis (Nicola and Nicola 1994).

In another study, 846 patients with different types of fibromyositic rheumatisms were submitted to defocalized laser therapy from 1980 to 1995. They employed Diodes and CO2 lasers. Control groups were used to compare results with those of traditional methods. Results were evaluated on the basis of subjective (such as local pain) and objective criteria. On the whole, results were positive in comparison with other methods both as regards recovery time and persistence of results. Approximately 2/3 of the patients benefited from the treatment indicated that there were greater advantages in use of laser over other presently available methods. Longo and his collogues recommended that standardalization of treatment protocols deserves further studies (Longo, Simunovic et al. 1997).

The CO2-laser can also be used as an acupuncture tool. Simulation of acupuncture points has been carried out both with biostimulating power densities (e.g.100mW/cm2) and burning/coagulation/ evaporation power densities. Some clinics state that CO2 lasers give better results on acupuncture points than HeNe lasers. "As the CO2 laser's beam cannot penetrate more than around 0.5 mm into tissue, the effects must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites. It is well known that these kinds of secondary effect also occur at the traditional wavelengths of 633, 830, and 904 nm" (Tuner and Hode 2010).

### **4. NACLT (Non-Ablative CO2 Laser Therapy)**

Recently, there have been few reports about using CO2 laser in non-ablative manner to reduce pain in painful mucosal lesions (Elad, Or et al. 2003; Sharon-Buller and Sela 2004; Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).

In a report, Elad et al suggested that CO2 laser treatment could be of benefit to control pain in severe oral chronic graft-versus-host-disease (Elad, Or et al. 2003). In this study, the oral lesions of four patients were irradiated by CO2 laser during 17 sessions. The CO2 laser was applied, over mucosal lesions, using 1W power for 2-3s/1mm2. The treated mucosa was kept wet during the process. The treatment was pain free, and anesthesia was not required. The average VAS scores before, during, and immediately after CO2 laser treatment were 8.09, 3.47, and 4.88 respectively. There was no visual damage to the oral mucosa and no aggravation of the oral lesions (Elad, Or et al. 2003).

In another case report, aphthous ulcers of two patients were irradiated with CO2 laser at 1.0- 1.5 W power, with a defocused hand piece for 5 seconds. Before laser irradiation, a thin film

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

2009; Zand, Najafi et al. 2010).

**4.1 NACLT protocol** 

burning.

evaluated in further studies.

A New Approach to Relieve Pain in Some Painful Oral Diseases 395

There are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional phototherapeutic lasers, so that immediately after NACLT, the patients of the studies have been able to eat and drink easily (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al.

At first, all standard precautions of laser utilization should be considered. Before laser irradiation, the patient and surgical staff should be given appropriate protective eye shields and eye glasses matched to the laser wavelength (10,600 nm) to protect inadvertent laser impact.1 Before laser irradiation, a layer of a transparent, non-anesthetic gel with high water content is placed on the lesion. In our studies, we use a transparent gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm on the lesion, is used. The CO2 laser is operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with a circular motion. The irradiation time depends on the size of the lesion. For example, in our studies the irradiation time for minor aphthous ulcers is about 5-10 seconds. When using NACLT for larger lesions, such as; pemphigus vulgaris, every one centimeter square of the lesion has been irradiated for 5 seconds in each pass, and repeated immediately if the contact pain of the lesion persists. Between the passes, the prior gel should be wiped gently and replaced by a new layer of gel, otherwise the water content of the gel will decrease which may lead to increasing the beam absorption by the lesion and subsequent tissue injury and even

1 In NACLT studies, we used eye glasses matched to the CO2 laser wavelength (10,600 nm), but we presume that it might be safer to use eye glasses matched to both the 10,600 nm and the guiding beam to protect the eyes from the reflected beam from the surface of the gel, the presumption that should be

Fig. 1. Minor aphthous ulcers before and immediately after NACLT

of Elmex Gel (a transparent gel with high water content) was placed on the lesions to reduce the beam absorption by the soft tissue. Anesthesia was not required since the treatment was not painful. The patients reported immediate pain relief after laser irradiation (Sharon-Buller and Sela 2004).

In these two reports, water (Elad, Or et al. 2003) and transparent gel with high water content (Sharon-Buller and Sela 2004) were used to reduce the beam absorption by the soft tissue.

These interesting results encouraged us to conduct a randomized controlled clinical trial to confirm the pain-relieving effect of CO2 laser in minor aphthous ulcers as a prototype of painful oral lesions (Zand, Ataie-Fashtami et al. 2009). The results of this clinical trial demonstrated that a single session of low-intensity, non-thermal, CO2 laser irradiation could reduce pain in minor aphthous ulcers immediately and significantly, with no visible side effects (Zand, Ataie-Fashtami et al. 2009), the technique was called NACLT (Non-Ablative CO2 Laser Therapy) afterwards.

In order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent gel with high water content to reduce the beam absorption by the tissue. In addition, the CO2 laser is operated with a de-focused hand piece 5–6 mm distant from the mucosal surface, scanning rapidly over the lesion with circular motion (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). The results of the powermetry have shown that the final laser power output, after passing through the gel, is reduced to 2-5 mW, which is in the range of low power lasers. Thermometry has also shown no significant temperature rise on the surface of the ulcers and under the gel layer during the laser irradiation, supporting the low power nature of the applied CO2 laser in NACLT(Zand, Ataie-Fashtami et al. 2009). It appears that due to high water content of the gel, it absorbs CO2 laser irradiation considerably, resulting in significant drops in the power output, by a factor of 200-500. In fact by irradiation of CO2 laser through a transparent gel with high water content, CO2 laser can be used as a nondestructive, non-thermal laser to reduce pain in some oral lesions immediately and significantly. This technique was called non-ablative CO2 laser therapy (NACLT), in order to avoid any confusion with classical high power thermal CO2 laser effects. This technique could also be called non-thermal CO2 laser therapy (NTCLT) to avoid misinterpretation with fractional non-ablative lasers used for skin rejuvenation (Zand, Ataie-Fashtami et al. 2009).

NACLT is a pain free procedure and neither systemic nor topical anesthesia is required. The patients don't feel warmth in their lesions during the procedure, in contrast to conventional defocused CO2 laser therapy in which the patients feel mild warmth. Up to now, in the series of studies about the analgesic effects of NACLT, we have observed no visual effects of thermal damage to the oral mucosa such as tissue ablation, ulceration, erythema or aggravation of the lesions following the careful application of the technique (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). So that the beforeafter NACLT pictures of the lesions cannot be differentiated from each other easily (Figure 1). Since there is no tissue ablation and plume formation during NACLT, in contrast to the classical ablative CO2 laser surgery, it seems rational to conclude that this procedure has no potential for carrying viral particles to the surgeon and other operating room staff (Zand, Ataie-Fashtami et al. 2009).

There are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional phototherapeutic lasers, so that immediately after NACLT, the patients of the studies have been able to eat and drink easily (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).

Fig. 1. Minor aphthous ulcers before and immediately after NACLT

#### **4.1 NACLT protocol**

394 CO2 Laser – Optimisation and Application

of Elmex Gel (a transparent gel with high water content) was placed on the lesions to reduce the beam absorption by the soft tissue. Anesthesia was not required since the treatment was not painful. The patients reported immediate pain relief after laser irradiation (Sharon-

In these two reports, water (Elad, Or et al. 2003) and transparent gel with high water content (Sharon-Buller and Sela 2004) were used to reduce the beam absorption by the soft tissue.

These interesting results encouraged us to conduct a randomized controlled clinical trial to confirm the pain-relieving effect of CO2 laser in minor aphthous ulcers as a prototype of painful oral lesions (Zand, Ataie-Fashtami et al. 2009). The results of this clinical trial demonstrated that a single session of low-intensity, non-thermal, CO2 laser irradiation could reduce pain in minor aphthous ulcers immediately and significantly, with no visible side effects (Zand, Ataie-Fashtami et al. 2009), the technique was called NACLT (Non-Ablative

In order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent gel with high water content to reduce the beam absorption by the tissue. In addition, the CO2 laser is operated with a de-focused hand piece 5–6 mm distant from the mucosal surface, scanning rapidly over the lesion with circular motion (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). The results of the powermetry have shown that the final laser power output, after passing through the gel, is reduced to 2-5 mW, which is in the range of low power lasers. Thermometry has also shown no significant temperature rise on the surface of the ulcers and under the gel layer during the laser irradiation, supporting the low power nature of the applied CO2 laser in NACLT(Zand, Ataie-Fashtami et al. 2009). It appears that due to high water content of the gel, it absorbs CO2 laser irradiation considerably, resulting in significant drops in the power output, by a factor of 200-500. In fact by irradiation of CO2 laser through a transparent gel with high water content, CO2 laser can be used as a nondestructive, non-thermal laser to reduce pain in some oral lesions immediately and significantly. This technique was called non-ablative CO2 laser therapy (NACLT), in order to avoid any confusion with classical high power thermal CO2 laser effects. This technique could also be called non-thermal CO2 laser therapy (NTCLT) to avoid misinterpretation with fractional non-ablative lasers used for skin rejuvenation (Zand, Ataie-Fashtami et al.

NACLT is a pain free procedure and neither systemic nor topical anesthesia is required. The patients don't feel warmth in their lesions during the procedure, in contrast to conventional defocused CO2 laser therapy in which the patients feel mild warmth. Up to now, in the series of studies about the analgesic effects of NACLT, we have observed no visual effects of thermal damage to the oral mucosa such as tissue ablation, ulceration, erythema or aggravation of the lesions following the careful application of the technique (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). So that the beforeafter NACLT pictures of the lesions cannot be differentiated from each other easily (Figure 1). Since there is no tissue ablation and plume formation during NACLT, in contrast to the classical ablative CO2 laser surgery, it seems rational to conclude that this procedure has no potential for carrying viral particles to the surgeon and other operating room staff (Zand,

Buller and Sela 2004).

CO2 Laser Therapy) afterwards.

2009).

Ataie-Fashtami et al. 2009).

At first, all standard precautions of laser utilization should be considered. Before laser irradiation, the patient and surgical staff should be given appropriate protective eye shields and eye glasses matched to the laser wavelength (10,600 nm) to protect inadvertent laser impact.1 Before laser irradiation, a layer of a transparent, non-anesthetic gel with high water content is placed on the lesion. In our studies, we use a transparent gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm on the lesion, is used. The CO2 laser is operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with a circular motion. The irradiation time depends on the size of the lesion. For example, in our studies the irradiation time for minor aphthous ulcers is about 5-10 seconds. When using NACLT for larger lesions, such as; pemphigus vulgaris, every one centimeter square of the lesion has been irradiated for 5 seconds in each pass, and repeated immediately if the contact pain of the lesion persists. Between the passes, the prior gel should be wiped gently and replaced by a new layer of gel, otherwise the water content of the gel will decrease which may lead to increasing the beam absorption by the lesion and subsequent tissue injury and even burning.

 1 In NACLT studies, we used eye glasses matched to the CO2 laser wavelength (10,600 nm), but we presume that it might be safer to use eye glasses matched to both the 10,600 nm and the guiding beam to protect the eyes from the reflected beam from the surface of the gel, the presumption that should be evaluated in further studies.

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

**4.2.1.2 NACLT and minor oral aphthous stomatitis** 

**4.2.1.3 NACLT and major oral aphthous stomatitis** 

Fig. 3. Major aphthous ulcer

Fashtami et al. 2009).

A New Approach to Relieve Pain in Some Painful Oral Diseases 397

A randomized controlled clinical trial was designed to evaluate the pain relieving effects of a single-session of NACLT in minor recurrent aphthous stomatitis as a prototype of painful oral ulcers. Fifteen patients, each with two discrete aphthous ulcers, were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. In each patient, the laser lesion was treated with NACLT, while the placebo lesion was irradiated with the same laser, but with an inactive probe. The patients scored and recorded the pain severity of their lesions on a 10-grade visual analogue scale (VAS) up to 4 days post operatively. In the laser group, the pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). The procedure itself was not painful, so anesthesia was not required. The patients reported no warmth in their lesions during laser treatment. There was no visual effect of thermal damage to the oral mucosa such as ablation, coagulation or erythema. The results showed that a single-session of NACLT reduced pain in minor aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-

A pilot randomized controlled clinical trial was designed to evaluate the analgesic effects of a single-session of NACLT in major recurrent aphthous ulcers. Five patients, each with two discrete major aphthous ulcers were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. The lesions in laser group were irradiated with CO2 laser (λ = 10,600 nm; Lancet-2, Russia) through a thick layer of transparent, non-anesthetic gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm. The CO2 laser was operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with circular motion. The patients' idiopathic (non-contact) and contact pain severity scores were recorded before and immediately after NACLT. These scores were also recorded up to 4 days post- operatively. The results of the study demonstrated that in the laser group, both the non-contact and contact pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no

#### **4.2 NACLT applications in clinical studies**

#### **4.2.1 Recurrent oral aphthous stomatitis**

#### **4.2.1.1 Definition**

Recurrent aphthous stomatitis (RAS) is a common oral disorder of uncertain etiopathogenesis (Scully, Gorsky et al. 2003), characterized by painful, round or ovoid ulcers with circumscribed margins, yellowish fibrinoid base, surrounded with erythematous haloes. The lesions typically first presenting in childhood or adolescence, recur at varying intervals throughout life (Jurge, Kuffer et al. 2006). The frequency and severity of the ulcerations usually decreases with age (Arikan, Birol et al. 2006). RAS occurs worldwide although it appears most common in the developed world (Jurge, Kuffer et al. 2006).

Recurrent aphthous stomatitis (RAS) is classified into three clinical forms, namely minor (miRAS), major (maRAS) and herpetiformis. Minor aphthous ulcers, which comprise over 80–90% of cases (Shashy and Ridley 2000), are less than 1 cm in diameter, last up to 7–14 days, and they heal without scar formation. Major aphthous ulcers are over 1 cm in diameter, and their healing may take 20 to 30 days at a time and often heal with scarring. Herpetiform ulcers (HUs) are multiple, clustered, 1–3 mm lesions that may coalesce into larger ulcers. They typically heal within 15 days (Prolo P 2006).

Although the exact underlying pathophysiology of RAS is not completely known, some evidences propose that aphthous ulcers are related to a focal immune dysfunction in which T lymphocytes have a significant role (Shashy and Ridley 2000; Jurge, Kuffer et al. 2006). Many etiologic, predisposing, and precipitating factors, such as genetic factors, immunologic problems, trauma, hypersensitivity to foods and drugs, hormonal changes, hematological deficiencies, cessation of smoking, and psychological stresses have been propsed (Shashy and Ridley 2000; Arikan, Birol et al. 2006).

Since there is no consensus regarding the cause of recurrent oral aphthous ulcers, it is difficult to have completely effective treatment or prevention (Shashy and Ridley 2000). There are currently few agents that have been found in randomized controlled clinical trials to cure aphthous ulcers (Jurge, Kuffer et al. 2006). As a result, the management of RAS is directed largely toward symptomatic relief. The main problem with aphthous ulcers is their pain which may be so severe. Many different therapeutic agents, including topical corticosteroids, mouth rinses, antibiotics, local anesthetic gels or pastilles, and treatment modalities, such as silver nitrate cautery and cryotherapy, have been tried for pain control in miRAS patients (Alidaee, Taheri et al. 2005; Arikan, Birol et al. 2006).

Fig. 2. Minor aphthous ulcer

Fig. 3. Major aphthous ulcer

396 CO2 Laser – Optimisation and Application

Recurrent aphthous stomatitis (RAS) is a common oral disorder of uncertain etiopathogenesis (Scully, Gorsky et al. 2003), characterized by painful, round or ovoid ulcers with circumscribed margins, yellowish fibrinoid base, surrounded with erythematous haloes. The lesions typically first presenting in childhood or adolescence, recur at varying intervals throughout life (Jurge, Kuffer et al. 2006). The frequency and severity of the ulcerations usually decreases with age (Arikan, Birol et al. 2006). RAS occurs worldwide

Recurrent aphthous stomatitis (RAS) is classified into three clinical forms, namely minor (miRAS), major (maRAS) and herpetiformis. Minor aphthous ulcers, which comprise over 80–90% of cases (Shashy and Ridley 2000), are less than 1 cm in diameter, last up to 7–14 days, and they heal without scar formation. Major aphthous ulcers are over 1 cm in diameter, and their healing may take 20 to 30 days at a time and often heal with scarring. Herpetiform ulcers (HUs) are multiple, clustered, 1–3 mm lesions that may coalesce into

Although the exact underlying pathophysiology of RAS is not completely known, some evidences propose that aphthous ulcers are related to a focal immune dysfunction in which T lymphocytes have a significant role (Shashy and Ridley 2000; Jurge, Kuffer et al. 2006). Many etiologic, predisposing, and precipitating factors, such as genetic factors, immunologic problems, trauma, hypersensitivity to foods and drugs, hormonal changes, hematological deficiencies, cessation of smoking, and psychological stresses have been

Since there is no consensus regarding the cause of recurrent oral aphthous ulcers, it is difficult to have completely effective treatment or prevention (Shashy and Ridley 2000). There are currently few agents that have been found in randomized controlled clinical trials to cure aphthous ulcers (Jurge, Kuffer et al. 2006). As a result, the management of RAS is directed largely toward symptomatic relief. The main problem with aphthous ulcers is their pain which may be so severe. Many different therapeutic agents, including topical corticosteroids, mouth rinses, antibiotics, local anesthetic gels or pastilles, and treatment modalities, such as silver nitrate cautery and cryotherapy, have been tried for pain control in

although it appears most common in the developed world (Jurge, Kuffer et al. 2006).

larger ulcers. They typically heal within 15 days (Prolo P 2006).

propsed (Shashy and Ridley 2000; Arikan, Birol et al. 2006).

miRAS patients (Alidaee, Taheri et al. 2005; Arikan, Birol et al. 2006).

Fig. 2. Minor aphthous ulcer

**4.2 NACLT applications in clinical studies 4.2.1 Recurrent oral aphthous stomatitis** 

**4.2.1.1 Definition** 

#### **4.2.1.2 NACLT and minor oral aphthous stomatitis**

A randomized controlled clinical trial was designed to evaluate the pain relieving effects of a single-session of NACLT in minor recurrent aphthous stomatitis as a prototype of painful oral ulcers. Fifteen patients, each with two discrete aphthous ulcers, were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. In each patient, the laser lesion was treated with NACLT, while the placebo lesion was irradiated with the same laser, but with an inactive probe. The patients scored and recorded the pain severity of their lesions on a 10-grade visual analogue scale (VAS) up to 4 days post operatively. In the laser group, the pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). The procedure itself was not painful, so anesthesia was not required. The patients reported no warmth in their lesions during laser treatment. There was no visual effect of thermal damage to the oral mucosa such as ablation, coagulation or erythema. The results showed that a single-session of NACLT reduced pain in minor aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami et al. 2009).

#### **4.2.1.3 NACLT and major oral aphthous stomatitis**

A pilot randomized controlled clinical trial was designed to evaluate the analgesic effects of a single-session of NACLT in major recurrent aphthous ulcers. Five patients, each with two discrete major aphthous ulcers were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. The lesions in laser group were irradiated with CO2 laser (λ = 10,600 nm; Lancet-2, Russia) through a thick layer of transparent, non-anesthetic gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm. The CO2 laser was operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with circular motion. The patients' idiopathic (non-contact) and contact pain severity scores were recorded before and immediately after NACLT. These scores were also recorded up to 4 days post- operatively. The results of the study demonstrated that in the laser group, both the non-contact and contact pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

Managing aphthous ulcers: Laser Treatment Applied

CO2-laser treatment of ulcerative lesions

Relieving pain in minor aphthous stomatitis by a single session of non-thermal carbon dioxide laser irradiation

Analgesic effects of single session of Non-Ablative CO2 Laser Therapy (NACLT) in major aphthous ulcers: (a preliminary study)

**4.2.2 Behçet's disease** 

**4.2.2.1 Definition** 

A New Approach to Relieve Pain in Some Painful Oral Diseases 399

Title Author Study Type of irradiation

Before-after clinical trial 1991

Case report 2004

Randomized controlled clinical trial (RCT) 2008

Randomized controlled clinical trial (RCT) 2009

Behçet's disease (BD) which is classified among vasculitides is a complex, multisystem inflammatory disease characterized by oral and genital aphthae, cutaneous lesions, arthritis, ocular, gastrointestinal, and neurologic manifestations (Meador, Ehrlich et al. 2002; Suzuki

The most common clinical feature is the presence of recurrent and usually painful mucocutaneous ulcers (Lin and Liang 2006). Oral aphthosis is the most frequent and

Colvard M, Kuo P

> Sharon-Buller A, et al.

Zand N, Ataiefashtami L. et al.

Zand N, Ataiefashtami L. et al.

Table 1. Irradiation of aphthous ulcers with CO2 laser

Kurokawa and Suzuki 2004; Lin and Liang 2006).

Need to

surgery + 18

Ablative CO2 laser

CO2 laser irradiation of the lesions through a thin film of transparent gel with high water content

CO2 laser Irradiation of the lesions through a thick (3-4mm) layer of transparent gel with high water content; (NACLT)

CO2 laser Irradiation of the lesions through a thick (3-4mm) layer of transparent gel with high water content; (NACLT)

Anesthesia

Number of

\_ 2

\_ 15


patients

changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). There were not any visible side effects following NACLT. None of the patients reported warmth feeling in their lesions during laser treatment. The results of the study suggested that a single-session of NACLT could reduce pain in major aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami et al. 2009). This study is in progress.

#### **4.2.1.4 Literature review**

Colvard and Kuo evaluated the potential efficacy of the high-power, surgical CO2 laser for pain relief in 28 painful minor aphthous ulcers of 14 patients. Their anesthetic protocol included pre-operative pain medication (oral administration of ketoprofen) and local anesthesia by infiltration of 1:200,000 2% isocaine with 1:200000 neocobefrin to overcome the painful nature of the procedure. During the procedure, CO2 laser was used as a classical, ablative manner with power output 4 W and as much necrotic tissue as possible was removed. Over all 88.8% of the patients were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. The authors concluded that CO2 laser should be included as an alternative modality for the treatment of miRAS, due to its ability to reduce or eliminate pain (Colvard and Kuo 1991). In this study, CO2 laser was used in classical, high power ablative manner. However the post operative analgesic effects of the procedure demonstrated the simultaneous biomodulative effects of CO2 laser irradiation. The same concept Kaplan stated that laser surgery and low level laser therapy should be regarded as two sides of the same coin.

Fekrazad et al. evaluated the effects of Nd: YAG laser (power: 3 W, energy: 100 mj, pulse repetition rate: 30Hz, irradiation time: 60 sec) in 138 patients with aphtous ulcers. The patients were randomly assigned into three groups, as follows: (1) treatment with a focalized beam; (2) treatment with a non-focalised beam and (3) placebo treatment. In group (1) the laser beam was administered from a distance of 6 mm from the centre of the ulcer without using a clear and defined point of irradiation. In group (2) a well defined point beam of the laser was irradiated from a distance of 2 mm from the center of the ulcer, in a helical fashion. In group (3) the HeNe Laser was used as placebo with inactive probe. In group (1) and (2) a significant reduction of pain was observed compared to group (3). The duration of pain and the duration of recovery period were shortest in group (2) (Fekrazad, Jafari et al. 2006).

De Souza TO et al. assessed the effect of low-level laser therapy on pain control and the repair of recurrent aphthous stomatitis. Twenty patients with recurrent aphthous ulcers were divided into two groups. The patients in Group I (n = 5) treated with topical triamcinolone acetonide and the patients in Group II (n = 15) received laser treatment with an InGaA1P diode laser (670 nm, 50 mW, 3 J/cm2 per point) in daily sessions on consecutive days. All patients were assessed daily, and the following clinical parameters were determined during each session: pain intensity before and after treatment and clinical measurement of lesion size. The results showed that 75% of the patients reported a reduction in pain in the same session after laser treatment, and total regression of the lesion occurred after 4 days. Total regression in the corticosteroid group was from 5 to 7 days. They concluded that LLLT with these laser parameters demonstrated analgesic and healing effects with regard to recurrent aphthous stomatitis (De Souza, Martins et al. 2010).

changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). There were not any visible side effects following NACLT. None of the patients reported warmth feeling in their lesions during laser treatment. The results of the study suggested that a single-session of NACLT could reduce pain in major aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami

Colvard and Kuo evaluated the potential efficacy of the high-power, surgical CO2 laser for pain relief in 28 painful minor aphthous ulcers of 14 patients. Their anesthetic protocol included pre-operative pain medication (oral administration of ketoprofen) and local anesthesia by infiltration of 1:200,000 2% isocaine with 1:200000 neocobefrin to overcome the painful nature of the procedure. During the procedure, CO2 laser was used as a classical, ablative manner with power output 4 W and as much necrotic tissue as possible was removed. Over all 88.8% of the patients were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. The authors concluded that CO2 laser should be included as an alternative modality for the treatment of miRAS, due to its ability to reduce or eliminate pain (Colvard and Kuo 1991). In this study, CO2 laser was used in classical, high power ablative manner. However the post operative analgesic effects of the procedure demonstrated the simultaneous biomodulative effects of CO2 laser irradiation. The same concept Kaplan stated that laser surgery and low

Fekrazad et al. evaluated the effects of Nd: YAG laser (power: 3 W, energy: 100 mj, pulse repetition rate: 30Hz, irradiation time: 60 sec) in 138 patients with aphtous ulcers. The patients were randomly assigned into three groups, as follows: (1) treatment with a focalized beam; (2) treatment with a non-focalised beam and (3) placebo treatment. In group (1) the laser beam was administered from a distance of 6 mm from the centre of the ulcer without using a clear and defined point of irradiation. In group (2) a well defined point beam of the laser was irradiated from a distance of 2 mm from the center of the ulcer, in a helical fashion. In group (3) the HeNe Laser was used as placebo with inactive probe. In group (1) and (2) a significant reduction of pain was observed compared to group (3). The duration of pain and the duration of recovery period were shortest in group (2) (Fekrazad,

De Souza TO et al. assessed the effect of low-level laser therapy on pain control and the repair of recurrent aphthous stomatitis. Twenty patients with recurrent aphthous ulcers were divided into two groups. The patients in Group I (n = 5) treated with topical triamcinolone acetonide and the patients in Group II (n = 15) received laser treatment with an InGaA1P diode laser (670 nm, 50 mW, 3 J/cm2 per point) in daily sessions on consecutive days. All patients were assessed daily, and the following clinical parameters were determined during each session: pain intensity before and after treatment and clinical measurement of lesion size. The results showed that 75% of the patients reported a reduction in pain in the same session after laser treatment, and total regression of the lesion occurred after 4 days. Total regression in the corticosteroid group was from 5 to 7 days. They concluded that LLLT with these laser parameters demonstrated analgesic and healing effects with regard to recurrent aphthous stomatitis (De Souza, Martins et al. 2010).

level laser therapy should be regarded as two sides of the same coin.

et al. 2009). This study is in progress.

**4.2.1.4 Literature review** 

Jafari et al. 2006).


Table 1. Irradiation of aphthous ulcers with CO2 laser

#### **4.2.2 Behçet's disease**

#### **4.2.2.1 Definition**

Behçet's disease (BD) which is classified among vasculitides is a complex, multisystem inflammatory disease characterized by oral and genital aphthae, cutaneous lesions, arthritis, ocular, gastrointestinal, and neurologic manifestations (Meador, Ehrlich et al. 2002; Suzuki Kurokawa and Suzuki 2004; Lin and Liang 2006).

The most common clinical feature is the presence of recurrent and usually painful mucocutaneous ulcers (Lin and Liang 2006). Oral aphthosis is the most frequent and

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

was required. The study is in progress.

**4.2.2.2 NACLT and oral aphthous ulcers of Behçet's disease** 

participate in the study according the inclusion/exclusion criteria.

**4.2.2.3 NACLT and genital aphthous ulcers of Behçet's disease** 

ulcers of Behçet's disease without any complications.

significant pain relieving effect.

A New Approach to Relieve Pain in Some Painful Oral Diseases 401

A pilot before-after clinical trial was designed to evaluate the analgesic effects of a singlesession of NACLT in painful aphthous ulcers of Behçet's disease. Up until the time of this publication,, three patients with known Behcet's disease have been eligible and consented to

Four painful oral aphthous ulcers of the three patients were treated by NACLT. The pain severity of the lesions were dramatically declined immediately after irradiation (p<0.001). This analgesic effect was consistently sustained during the five days follow-up periods. Up until the time of this publication, the results of this pilot study suggest that a single session of NACLT could relieve pain in oral aphthous ulcers of Behçet's disease immediately and significantly without visible side effects of thermal damage or aggravation of the lesions. Similar to the other NACLT studies, the procedure itself was pain free and no anesthesia

In another case report that is being published, the extremely painful genital aphthous ulcers of a 23-year-old female with Behcet's disease were irradiated by NACLT. Before laser irradiation the pain of the lesions was so severe which impeded daily functions, such as sitting, walking, and even sleeping and did not respond to conventional analgesics. The non-contact and contact visual analogue scale (VAS) pain scores of the left genital ulcer were 8 and 10 and the scores of the right sided ulcer were 6 and 10 respectively. Immediately after NACLT of the ulcers and its surrounding erythematous rim, the contact pain of the lesions relieved completely (so that she could even walk downstairs without difficulties). Similar to the other prior NACLT investigations, there were no visual side effects of thermal damage to the lesions, such as tissue ablation or aggravation of the lesions following NACLT. The procedure was painless and neither systemic nor local anesthesia was required. This analgesic effect of NACLT was sustained during the healing period and she experienced no problem in daily functions and did not require topical or systemic analgesics. She just had mild burning sensation during urination for the first three days which relieved after healing of the lesions. It should be noted that concomitantly, treatment with prednisolone 30mg/day and colchicine 2mg/ day was initiated in the hospital. Additionally, the depth of the genital ulcers decreased remarkably two days after NACLT. The ulcers healed completely within 11 days which seemed to be much shorter than what was expected. Interestingly in spite of the large size of the ulcers, they left a very small (6mm) scar. The results of this case report suggest that NACLT could be potentially considered as an alternative method for pain relief in painful genital aphthous

It should be noticed that Behçet's syndrome is a serious multisystem disease, which in some cases it may lead to systemic complications such as; severe ocular problems (even blindness), intestinal, central nervous system,… involvement. Therefore the patients must be warned that NACLT should not substitute the systemic therapy of the disease in spite of its

Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the efficacy and safety of NACLT in reducing the pain of oral and genital aphthous ulcers of

constant manifestation of Behçet's disease (Davatchi, Shahram et al. 2005) and usually the initial presenting symptom in most, if not all, patients (Lin and Liang 2006) . The distinct difference between the clinical features of aphthous ulcers of RAS and Behçet's Syndrome remains unclear. The aphthous ulcers of Behçet's disease are typically painful punched-out ulcers with a white yellowish fibrinoid base, surrounded with erythematous halo. They range in size from a few millimeters to 2 cm. These ulcers typically heal spontaneously within 1 to 3 weeks,usually without scarring (Ghate and Jorizzo 1999; Lin and Liang 2006).

Genital ulceration occurs in approximately 75% of the patients with Behçet's disease (Lin and Liang 2006). The genital aphthous ulcers are morphologically similar to the oral ulcers, except that lesions are usually larger, more painful, heal more slowly, recur less frequently and can have scarring tendency (Davatchi, Shahram et al. 2005; Lin and Liang 2006). In females they are often larger than 10 mm, and deeper than oral lesions. They are localized on the vulva, vagina, and rarely cervix. The giant aphthous lesion of the vulva is frequent, causing dysfunction and leaving sometimes indelible cicatrix. In males, genital aphthosis is often seen on the scrotum, but may be seen also on the shaft of penis or on the meatus. Sometimes they become giant lesions (Davatchi, Shahram et al. 2005) . Genital ulcerations of Behçet's disease may be very painful, exert a negative impact on the patient's quality of life, and these lesions are often refractory to multiple treatments(Kasugai, Watanabe et al. 2010).

Treatment of Behçet's disease is based on the clinical symptoms and severity of systemic involvement, including topical therapies as well as colchicine, dapsone, thalidomide, and immunosuppressants, interferon-alpha/beta, anti-tumor necrosis factor antibody, the latter specially in treatment for the cases with poor prognosis including eye, intestine, vessel and central nervous system involvement (Suzuki Kurokawa and Suzuki 2004).

The mucosal lesions, especially genital lesions can often become refractory to multiple treatments and present challenges to physicians. Topical or intralesional corticosteroids, oral pentoxifylline, sucralfate, dapsone, colchicine, and systemic low-dose corticosteroids, used either alone or in combination, are safe and having varying evidence for effect in mild to moderate mucocutaneous disease. Azathioprine or methotrexate can be used if the lesions are refractory to the previously mentioned therapies. Tumor necrosis factor (TNF) inhibitors such as infliximab or etanercept should be considered as the next step treatments . Tacrolimus, cyclosporine, and interferon-alpha-2a should be used generally only if TNF inhibitors have failed as a result of their toxicitie*s* (Lin and Liang 2006)*.*

Fig. 4. Behçet's Disease

constant manifestation of Behçet's disease (Davatchi, Shahram et al. 2005) and usually the initial presenting symptom in most, if not all, patients (Lin and Liang 2006) . The distinct difference between the clinical features of aphthous ulcers of RAS and Behçet's Syndrome remains unclear. The aphthous ulcers of Behçet's disease are typically painful punched-out ulcers with a white yellowish fibrinoid base, surrounded with erythematous halo. They range in size from a few millimeters to 2 cm. These ulcers typically heal spontaneously within 1 to 3 weeks,usually without scarring (Ghate and Jorizzo 1999; Lin and Liang 2006). Genital ulceration occurs in approximately 75% of the patients with Behçet's disease (Lin and Liang 2006). The genital aphthous ulcers are morphologically similar to the oral ulcers, except that lesions are usually larger, more painful, heal more slowly, recur less frequently and can have scarring tendency (Davatchi, Shahram et al. 2005; Lin and Liang 2006). In females they are often larger than 10 mm, and deeper than oral lesions. They are localized on the vulva, vagina, and rarely cervix. The giant aphthous lesion of the vulva is frequent, causing dysfunction and leaving sometimes indelible cicatrix. In males, genital aphthosis is often seen on the scrotum, but may be seen also on the shaft of penis or on the meatus. Sometimes they become giant lesions (Davatchi, Shahram et al. 2005) . Genital ulcerations of Behçet's disease may be very painful, exert a negative impact on the patient's quality of life, and these lesions are often refractory to multiple treatments(Kasugai, Watanabe et al. 2010). Treatment of Behçet's disease is based on the clinical symptoms and severity of systemic involvement, including topical therapies as well as colchicine, dapsone, thalidomide, and immunosuppressants, interferon-alpha/beta, anti-tumor necrosis factor antibody, the latter specially in treatment for the cases with poor prognosis including eye, intestine, vessel and

central nervous system involvement (Suzuki Kurokawa and Suzuki 2004).

inhibitors have failed as a result of their toxicitie*s* (Lin and Liang 2006)*.*

Fig. 4. Behçet's Disease

The mucosal lesions, especially genital lesions can often become refractory to multiple treatments and present challenges to physicians. Topical or intralesional corticosteroids, oral pentoxifylline, sucralfate, dapsone, colchicine, and systemic low-dose corticosteroids, used either alone or in combination, are safe and having varying evidence for effect in mild to moderate mucocutaneous disease. Azathioprine or methotrexate can be used if the lesions are refractory to the previously mentioned therapies. Tumor necrosis factor (TNF) inhibitors such as infliximab or etanercept should be considered as the next step treatments . Tacrolimus, cyclosporine, and interferon-alpha-2a should be used generally only if TNF

#### **4.2.2.2 NACLT and oral aphthous ulcers of Behçet's disease**

A pilot before-after clinical trial was designed to evaluate the analgesic effects of a singlesession of NACLT in painful aphthous ulcers of Behçet's disease. Up until the time of this publication,, three patients with known Behcet's disease have been eligible and consented to participate in the study according the inclusion/exclusion criteria.

Four painful oral aphthous ulcers of the three patients were treated by NACLT. The pain severity of the lesions were dramatically declined immediately after irradiation (p<0.001). This analgesic effect was consistently sustained during the five days follow-up periods. Up until the time of this publication, the results of this pilot study suggest that a single session of NACLT could relieve pain in oral aphthous ulcers of Behçet's disease immediately and significantly without visible side effects of thermal damage or aggravation of the lesions. Similar to the other NACLT studies, the procedure itself was pain free and no anesthesia was required. The study is in progress.

#### **4.2.2.3 NACLT and genital aphthous ulcers of Behçet's disease**

In another case report that is being published, the extremely painful genital aphthous ulcers of a 23-year-old female with Behcet's disease were irradiated by NACLT. Before laser irradiation the pain of the lesions was so severe which impeded daily functions, such as sitting, walking, and even sleeping and did not respond to conventional analgesics. The non-contact and contact visual analogue scale (VAS) pain scores of the left genital ulcer were 8 and 10 and the scores of the right sided ulcer were 6 and 10 respectively. Immediately after NACLT of the ulcers and its surrounding erythematous rim, the contact pain of the lesions relieved completely (so that she could even walk downstairs without difficulties). Similar to the other prior NACLT investigations, there were no visual side effects of thermal damage to the lesions, such as tissue ablation or aggravation of the lesions following NACLT. The procedure was painless and neither systemic nor local anesthesia was required. This analgesic effect of NACLT was sustained during the healing period and she experienced no problem in daily functions and did not require topical or systemic analgesics. She just had mild burning sensation during urination for the first three days which relieved after healing of the lesions. It should be noted that concomitantly, treatment with prednisolone 30mg/day and colchicine 2mg/ day was initiated in the hospital. Additionally, the depth of the genital ulcers decreased remarkably two days after NACLT. The ulcers healed completely within 11 days which seemed to be much shorter than what was expected. Interestingly in spite of the large size of the ulcers, they left a very small (6mm) scar. The results of this case report suggest that NACLT could be potentially considered as an alternative method for pain relief in painful genital aphthous ulcers of Behçet's disease without any complications.

It should be noticed that Behçet's syndrome is a serious multisystem disease, which in some cases it may lead to systemic complications such as; severe ocular problems (even blindness), intestinal, central nervous system,… involvement. Therefore the patients must be warned that NACLT should not substitute the systemic therapy of the disease in spite of its significant pain relieving effect.

Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the efficacy and safety of NACLT in reducing the pain of oral and genital aphthous ulcers of

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

**4.2.3 Pemphigus vulgaris** 

2005; Bystryn and Rudolph 2005).

during conventional systemic therapy.

Fig. 5. Pemphigus vulgaris

**4.2.3.2 NACLT and oral lesions of pemphigus vulgaris** 

**4.2.3.1 Definition** 

A New Approach to Relieve Pain in Some Painful Oral Diseases 403

Pemphigus Vulgaris (PV) is a rare, potentially life-threatening, autoimmune blistering disease of the skin and mucous membranes. Although the disease can affect anyone, it is most prevalent in people of Mediterranean or Jewish ancestry(Bystryn and Rudolph 2005). The prevalence of the disease is 30/100,000 and annual incidence has been reported between 1 and 5 in 100,000 according to different studies in Iran (Chams-Davatchi, Valikhani et al. 2005; Asilian, Yoosefi et al. 2006). The lesions are characterized by intra-epidermal vesicles with acantholysis and an intact basal layer. In the majority of patients, painful mucous membrane erosions are the presenting sign of pemphigus vulgaris and may be the only sign for weeks to months before any bullous skin lesion develops. The mucous membranes most often affected are those of the oral cavity, in which intact blisters are rare, probably because they are fragile and break easily, leaving scattered and often extensive erosions. The lesions are usually multiple, superficial, and irregular in shape, and arise from mucosa of healthy appearance. Although any surface can be involved, the most common sites are the buccal

Oral lesions in pemphigus vulgaris may be so painful during the active period of the disease that may interfere with their eating, drinking and even speaking (Black, Mignogna et al.

High doses of systemic corticosteroids plus immunosuppressive agents have dramatically declined the mortality rate of the disease. Understandably, owing to the life threatening nature of PV, the main focus of the peer reviewed literature has been on suppression and remission of PV (Rashid and Candido 2008). However, remission is not instantaneous and takes time to achieve. This delay in remission allows ample opportunity for complications to develop, secondary to the pain associated with PV. This can be highlighted by cases of repeated dehydration and malnutrition seen in PV patients (Rashid and Candido 2008). Therefore it seems necessary to obtain new modalities for pain control of these oral lesions

A pilot before-after clinical trial was designed to evaluate the analgesic effects of application of a single session of NACLT in oral lesions of PV. Thirty eight painful oral lesions of ten patients with PV were irradiated with CO2 laser by NACLT protocol. The patients scored and recorded the pain severity of their lesions on a visual analogue scale (VAS) up to 7 days

and labial mucosa, the palate, and the tongue (Bystryn and Rudolph 2005).

Behcet's disease. In addition such studies can demonstrate whether NACLT could accelerate wound healing in these lesions and specially prevent scar formation in genital aphthous ulcers of Behcet's disease or not.

#### **4.2.2.4 Literature review**

Demetriades used ablative CO2 laser in four painful oral aphthous ulcers of a patient with Behçet's Syndrome. Before laser irradiation, the lesions were infiltrated with a minimal amount of lidocaine 2% with 1:100,000 epinephrine. A CO2-laser set at 2W superpulse mode with a 0.4 mm ceramic tip was used, in a defocused way to lightly char the surface of the ulcers. The patient tolerated the procedure well. On subsequent follow-up, one week after the procedure, the patient reported considerable relief of symptoms on most of the treated ulcers. The oropharyngeal ulcer displayed only moderate response, but the patient reported an overall improvement of his quality of life (Demetriades, Hansford et al. 2009).


Table 2. Irradiation of aphthous ulcers of Behcet's disease with CO2 laser

#### **4.2.3 Pemphigus vulgaris**

#### **4.2.3.1 Definition**

402 CO2 Laser – Optimisation and Application

Behcet's disease. In addition such studies can demonstrate whether NACLT could accelerate wound healing in these lesions and specially prevent scar formation in genital aphthous

Demetriades used ablative CO2 laser in four painful oral aphthous ulcers of a patient with Behçet's Syndrome. Before laser irradiation, the lesions were infiltrated with a minimal amount of lidocaine 2% with 1:100,000 epinephrine. A CO2-laser set at 2W superpulse mode with a 0.4 mm ceramic tip was used, in a defocused way to lightly char the surface of the ulcers. The patient tolerated the procedure well. On subsequent follow-up, one week after the procedure, the patient reported considerable relief of symptoms on most of the treated ulcers. The oropharyngeal ulcer displayed only moderate response, but the patient reported an overall improvement of his quality of life (Demetriades, Hansford et

Title Author Study Type of

Demetriades M et al

Zand N, Fateh M. Et al.

Zand N, Fateh M. Et al.

Table 2. Irradiation of aphthous ulcers of Behcet's disease with CO2 laser

irradiation

Ablative defocused CO2-laser irradiation

NACLT \_

NACLT -

Case report 2009

Case report, under publish

Pilot beforeafter clinical trial/ under publish

Need to

Anesthesia

+

ulcers of Behcet's disease or not.

General manifestations of Behçet's syndrome and the success of CO2-laser as treatment for oral lesions: A review of the literature and case presentation

Relieving pain in painful genital ulcers of Behcet's disease by a single session of non thermal, Non-Ablative CO2 Laser Therapy(NACLT): A Case Report

Immediate pain relief of oral aphthous ulcers of Behcet's disease by nonthermal, Non-Ablative CO2 Laser Therapy (NACLT)

**4.2.2.4 Literature review** 

al. 2009).

Pemphigus Vulgaris (PV) is a rare, potentially life-threatening, autoimmune blistering disease of the skin and mucous membranes. Although the disease can affect anyone, it is most prevalent in people of Mediterranean or Jewish ancestry(Bystryn and Rudolph 2005). The prevalence of the disease is 30/100,000 and annual incidence has been reported between 1 and 5 in 100,000 according to different studies in Iran (Chams-Davatchi, Valikhani et al. 2005; Asilian, Yoosefi et al. 2006). The lesions are characterized by intra-epidermal vesicles with acantholysis and an intact basal layer. In the majority of patients, painful mucous membrane erosions are the presenting sign of pemphigus vulgaris and may be the only sign for weeks to months before any bullous skin lesion develops. The mucous membranes most often affected are those of the oral cavity, in which intact blisters are rare, probably because they are fragile and break easily, leaving scattered and often extensive erosions. The lesions are usually multiple, superficial, and irregular in shape, and arise from mucosa of healthy appearance. Although any surface can be involved, the most common sites are the buccal and labial mucosa, the palate, and the tongue (Bystryn and Rudolph 2005).

Oral lesions in pemphigus vulgaris may be so painful during the active period of the disease that may interfere with their eating, drinking and even speaking (Black, Mignogna et al. 2005; Bystryn and Rudolph 2005).

High doses of systemic corticosteroids plus immunosuppressive agents have dramatically declined the mortality rate of the disease. Understandably, owing to the life threatening nature of PV, the main focus of the peer reviewed literature has been on suppression and remission of PV (Rashid and Candido 2008). However, remission is not instantaneous and takes time to achieve. This delay in remission allows ample opportunity for complications to develop, secondary to the pain associated with PV. This can be highlighted by cases of repeated dehydration and malnutrition seen in PV patients (Rashid and Candido 2008). Therefore it seems necessary to obtain new modalities for pain control of these oral lesions during conventional systemic therapy.

#### **4.2.3.2 NACLT and oral lesions of pemphigus vulgaris**

A pilot before-after clinical trial was designed to evaluate the analgesic effects of application of a single session of NACLT in oral lesions of PV. Thirty eight painful oral lesions of ten patients with PV were irradiated with CO2 laser by NACLT protocol. The patients scored and recorded the pain severity of their lesions on a visual analogue scale (VAS) up to 7 days

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

**4.2.4 Post chemotherapy oral mucositis** 

producing a "pseudomembrane" (Sonis 2004).

Fig. 6. Post Docetaxole chemotherapy oral mucositis

**4.2.4.2 NACLT and mild to moderate post chemotherapy oral mucositis** 

A pilot before-after clinical trial was designed to evaluate the effects of single session of nonthermal, Non-ablative CO2 Laser therapy (NACLT) to reduce pain in mild to moderate oral mucositis following breast cancer chemotherapy with Docetaxole. Six patients were included and their oral lesions were irradiated by NACLT. The patients reported the idiopathic (non-contact) and contact pain of their lesions on VAS (visual analogue scale) before and immediately after laser and up to 7 days post operatively. The results of the

**4.2.4.1 Definition** 

(Arora, Pai et al. 2008).

A New Approach to Relieve Pain in Some Painful Oral Diseases 405

Oral mucositis is a common, debilitating, and potentially serious complication of chemoradiotherapy. Studies have shown that mucositis will develop in about 40% of chemotherapy patients, 80% of bone marrow transplant patients, and 100% of patients treated with radiotherapy to the head and neck (Berger & Kilroy 1997; Sonis et al. 1999).

It presents as erythema, edema, ulceration, bleeding along with pain. The pain of the lesions is aggravated by the patient's swallowing and normal oral functioning. Consequently, oral intake difficulties lead to loss of weight. The progression of oral lesions and their impact on the patient's general condition may require nasogastric tube feeding or temporary discontinuation of the treatment or modification of the therapeutic plan (Arora, Pai et al. 2008). Pathologic evaluation of mucositis reveals mucosal thinning leading to a shallow ulcer thought to be caused by inflammation and depletion of the epithelial basal layer with subsequent denudation and bacterial infection. The wound healing response to this injury is characterized by inflammatory cell infiltration, interstitial exudate, fibrin and cell debris

Various preventive measures causing alteration of mucosa (cryotherapy, allopurinol, pilocarpine, leucovorin), modification of mucosal proliferation (beta-carotene, glutamine, cytokines), and antimicrobial or anti-inflammatory action (chlorhexidine, corticosteroids) have been tried. General oral care, diet, topical mucosal coating agents (sucralfate, magnesium hydroxide), topical anesthetics, and systemic analgesics ( opiods and non opioids) (Arora, Pai et al. 2008), recombinant human keratinocyte growth factor (palifermin) and Amifostine have also been suggested (Kuhn, Porto et al. 2009). However, currently no definitive preventive or therapeutic intervention exists that is completely successful at preventing oral mucositis and treatment for this complication has thus been symptomatic

post operatively. Immediately after NACLT, the severity of idiopathic (non-contact) and contact pain were dramatically declined (p<0.001), so that the patients could eat and drink without any difficulties. This analgesic effect was sustained during follow-up periods. There was no visual effect of thermal damage to the oral mucosa or aggravation of the lesions. The results of this pilot study suggested that a single session of NACLT could reduce pain in oral lesions of pemphigus vulgaris immediately and significantly, without visible side effects (Zand, Mansouri et al. 2009). We recommend that in further studies, the pain severity of the lesions should be followed up for longer periods of time.

*It should be noted that due to the life threatening nature of PV without appropriate systemic treatment, the patients must be warned that NACLT should not alter their conventional treatment at all, in spite of its significant analgesic effect, as we instructed our patients to comply with their prescribed medical regimen.* 

#### **4.2.3.3 Literature review**

In a case report, the oral lesions of two patients with recalcitrant oral pemphigus vulgaris (who were under systemic treatment) were irradiated with CO2 laser at 1-1.5 W in a defocused mode for 5-10 seconds. The patients reported no pain after treatment. Recall examination after 1 month, 3 months and 5 month revealed complete healing of the lesions with no recurrence (Bhardwaj, Joshi et al. 2010).

The pictures of the paper demonstrate the themal, ablative nature within the procedure. Its pain relieving effects can be explained by its simultaneous biomodulative effects of CO2 laser irradiation.


Table 3. Irradiation of oral lesions of pemphigus vulgaris with CO2 laser

#### **4.2.4 Post chemotherapy oral mucositis**

#### **4.2.4.1 Definition**

404 CO2 Laser – Optimisation and Application

post operatively. Immediately after NACLT, the severity of idiopathic (non-contact) and contact pain were dramatically declined (p<0.001), so that the patients could eat and drink without any difficulties. This analgesic effect was sustained during follow-up periods. There was no visual effect of thermal damage to the oral mucosa or aggravation of the lesions. The results of this pilot study suggested that a single session of NACLT could reduce pain in oral lesions of pemphigus vulgaris immediately and significantly, without visible side effects (Zand, Mansouri et al. 2009). We recommend that in further studies, the

*It should be noted that due to the life threatening nature of PV without appropriate systemic treatment, the patients must be warned that NACLT should not alter their conventional treatment at all, in spite of its significant analgesic effect, as we instructed our patients to comply with their* 

In a case report, the oral lesions of two patients with recalcitrant oral pemphigus vulgaris (who were under systemic treatment) were irradiated with CO2 laser at 1-1.5 W in a defocused mode for 5-10 seconds. The patients reported no pain after treatment. Recall examination after 1 month, 3 months and 5 month revealed complete healing of the lesions

The pictures of the paper demonstrate the themal, ablative nature within the procedure. Its pain relieving effects can be explained by its simultaneous biomodulative effects of CO2

> Before-after clinical trial 2009

Case report 2010

irradiation

CO2 laser Irradiation of the lesions in a defocused mode (thermal)

NACLT \_

Need to

Anesthesia

?

Number of

patients

Ten patients/38 lesions

Two patients/? lesions

pain severity of the lesions should be followed up for longer periods of time.

Title Author Study Type of

Zand, N., Mansouri, P. et al.

Bhardwaj, A. et al.

Table 3. Irradiation of oral lesions of pemphigus vulgaris with CO2 laser

*prescribed medical regimen.*  **4.2.3.3 Literature review** 

laser irradiation.

Relieving pain in painful oral lesions of pemphigus vulgaris by a single session, Non-ablative 10600 nm CO2 Laser irradiation (pilot study )

Management of recalcitrant oral pemphigus vulgaris with CO2 laser-Report of two cases

with no recurrence (Bhardwaj, Joshi et al. 2010).

Oral mucositis is a common, debilitating, and potentially serious complication of chemoradiotherapy. Studies have shown that mucositis will develop in about 40% of chemotherapy patients, 80% of bone marrow transplant patients, and 100% of patients treated with radiotherapy to the head and neck (Berger & Kilroy 1997; Sonis et al. 1999).

It presents as erythema, edema, ulceration, bleeding along with pain. The pain of the lesions is aggravated by the patient's swallowing and normal oral functioning. Consequently, oral intake difficulties lead to loss of weight. The progression of oral lesions and their impact on the patient's general condition may require nasogastric tube feeding or temporary discontinuation of the treatment or modification of the therapeutic plan (Arora, Pai et al. 2008).

Pathologic evaluation of mucositis reveals mucosal thinning leading to a shallow ulcer thought to be caused by inflammation and depletion of the epithelial basal layer with subsequent denudation and bacterial infection. The wound healing response to this injury is characterized by inflammatory cell infiltration, interstitial exudate, fibrin and cell debris producing a "pseudomembrane" (Sonis 2004).

Various preventive measures causing alteration of mucosa (cryotherapy, allopurinol, pilocarpine, leucovorin), modification of mucosal proliferation (beta-carotene, glutamine, cytokines), and antimicrobial or anti-inflammatory action (chlorhexidine, corticosteroids) have been tried. General oral care, diet, topical mucosal coating agents (sucralfate, magnesium hydroxide), topical anesthetics, and systemic analgesics ( opiods and non opioids) (Arora, Pai et al. 2008), recombinant human keratinocyte growth factor (palifermin) and Amifostine have also been suggested (Kuhn, Porto et al. 2009). However, currently no definitive preventive or therapeutic intervention exists that is completely successful at preventing oral mucositis and treatment for this complication has thus been symptomatic (Arora, Pai et al. 2008).

Fig. 6. Post Docetaxole chemotherapy oral mucositis

#### **4.2.4.2 NACLT and mild to moderate post chemotherapy oral mucositis**

A pilot before-after clinical trial was designed to evaluate the effects of single session of nonthermal, Non-ablative CO2 Laser therapy (NACLT) to reduce pain in mild to moderate oral mucositis following breast cancer chemotherapy with Docetaxole. Six patients were included and their oral lesions were irradiated by NACLT. The patients reported the idiopathic (non-contact) and contact pain of their lesions on VAS (visual analogue scale) before and immediately after laser and up to 7 days post operatively. The results of the

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

the promoting wound healing effects of NACLT.

2009). (Please see also 4. NACLT)

effect.

performed frequently.

A New Approach to Relieve Pain in Some Painful Oral Diseases 407

results of which are being published. In addition we have used NACLT in few ulcerated and non-ulcerated skin diseases with variable degrees of success. In some lesions, such as post herpetic neuralgia, NACLT acts like other conventional therapeutic lasers, in which the pain relieving effect is completely short standing and needs the several sessions of NACLT to be

In addition, we have evaluated the effects of NACLT in promoting wound healing in few studies with variable degrees of success. Although in few studies, NACLT has shown some valuable results in this field we are not yet ready to express our view in this regard. Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate

*It should be mentioned that since the biological effects of NACLT and their mechanisms are not fully known, it seems ethically questionable to use NACLT in diseases with malignant potential, such as* 

In order to develop an understanding of the mechanisms of analgesic effect of NACLT, powermetry and thermometry were performed in prior studies, the results of which demonstrated the low power nature of the applied CO2 laser (Zand, Ataie-Fashtami et al.

Since the analgesic effect of NACLT is immediate, we assume that at least in part, physiological neural changes such as blockage of action potential generation and conduction of nociceptive signals in primary afferent neurons might take part in this analgesic effect (Zand, Ataie-Fashtami et al. 2009). Destruction of nerve endings is less probable to be induced by NACLT, because, even in the studies in which CO2 laser has been used as a surgical scalpel, there have been no statistically significant differences in the number of intact peripheral nerve structures in laser-treated sites in comparison with sites treated with electrocautery and scalpel (Rocha, Pinheiro et al. 2001). It is not known, whether the other mechanisms such as increase in β-endorphin synthesis and release, changes in bradykinin, prostaglandins, substance P, serotonin, acetylcholine, nitric oxide, singlet oxygen production, and the other biochemical events- which have been proposed to play a part in pain relieving effect of conventional low power lasers - are responsible for analgesic effect of NACLT or not. (Please see also 2.4.Mechanisms of analgesic effects of low power laser therapy) Further basic studies are necessary to elucidate the mechanisms of this analgesic

On the other hand, there are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional low level therapeutic lasers (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). Therefore one could presume that the mechanisms of analgesic effect of NACLT might have some differences from that for conventional low power lasers, an assumption that should be assessed in further basic studies.

*oral lichen planus, consequently we have not assessed the effects of NACLT in such illnesses.* 

**4.3 Presumed mechanisms of pain relieving effects of NACLT** 

study showed that dramatically after NACLT, the severity of pain declined immediately and it was sustained during follow-up periods (P<0.001). Similar to the other NACLT clinical trials, the procedure itself was painless and anesthesia was not required. There was no visible side effect such as ulceration, erosion and even erythema following NACLT. The results of the study suggested that single session of NACLT could reduce pain in lesions of mild/moderate post Docetaxole oral mucositis immediately and dramatically without visible side effects (Zand, Najafi et al. 2010).

#### **4.2.4.3 A brief literature review**

Some studies have shown that laser phototherapy (LPT) can be useful in prevention or treatment of oral mucositis. The principle behind using laser phototherapy (LPT) is that it accelerates wound healing and has anti-inflammatory effects (Arora, Pai et al. 2008). In addition, pain relieving properties of low power lasers seem to be of value in management of painful lesions of oral mucositis (Arora, Pai et al. 2008).

Some researchers state that prophylactic laser application seems more successful than curative laser application, although the reason is not entirely clear (Arora, Pai et al. 2008). Most studies of LLLT in cancer patients have focused on oral mucositis prevention (Bensadoun, Franquin et al. 1994; Bensadoun, Franquin et al. 1999; Rubenstein, Peterson et al. 2004; Bensadoun, Le Page et al. 2006; Kuhn, Porto et al. 2009; Clarkson, Worthington et al. 2010; Bjordal, Bensadoun et al. 2011) . Since the subjects of this section refers to analgesic effects of low power lasers in established chemotherapy-induced oral mucositis (COM), we briefly review some articles in which LLLT has been used for relieving pain in patients with chemotherapy-induced oral mucositis and not the prophylactic laser protocols.

Cauwels and Martens evaluated the capacity of analgesic effect and wound healing of low level laser therapy in 16 children suffering from chemotherapy-induced oral mucositis. All children were treated using a GaAlAs diode laser (wavelength: 830 nm, potency: 150 mW). The energy released was adapted according to the severity of the oral lesions. The same protocol was repeated every 48 hours until healing of each lesion occurred. The results of the study demonstrated that immediately after irradiation of the oral mucositis, pain relief was noticed. Depending on the severity of oral mucositis, on average, 2.5 treatments per lesion in a period of 1 week were sufficient to heal a mucositis lesion. They concluded that LLLT could reduce the severity and duration of mucositis and to relieve pain significantly (Cauwels and Martens 2011).

Nes and Posso investigated the pain relieving effect of LLLT among 13 patients who have developed moderate chemotherapy-induced oral mucositis. The laser used was GaAlAs (830 nm, power: 250 mW, energy density: 35 J cm-2). The patients were treated during a 5-day period, and the pain was measured before and after each laser application. There was a significant ( *P*= 0.007) 67% decrease in the daily average experience of pain felt before and after each treatment, confirming that LLLT can relieve pain among patients who have developed chemotherapy-induced oral mucositis (Nes and Posso 2005).

#### **4.2.5 Other NACLT studies**

We have evaluated the pain relieving effects of NACLT in some other painful mucosal lesions such as painful oral lesions of Stevens Johnson Syndrome, etc. as case reports, the performed frequently.

406 CO2 Laser – Optimisation and Application

study showed that dramatically after NACLT, the severity of pain declined immediately and it was sustained during follow-up periods (P<0.001). Similar to the other NACLT clinical trials, the procedure itself was painless and anesthesia was not required. There was no visible side effect such as ulceration, erosion and even erythema following NACLT. The results of the study suggested that single session of NACLT could reduce pain in lesions of mild/moderate post Docetaxole oral mucositis immediately and dramatically without

Some studies have shown that laser phototherapy (LPT) can be useful in prevention or treatment of oral mucositis. The principle behind using laser phototherapy (LPT) is that it accelerates wound healing and has anti-inflammatory effects (Arora, Pai et al. 2008). In addition, pain relieving properties of low power lasers seem to be of value in management

Some researchers state that prophylactic laser application seems more successful than curative laser application, although the reason is not entirely clear (Arora, Pai et al. 2008). Most studies of LLLT in cancer patients have focused on oral mucositis prevention (Bensadoun, Franquin et al. 1994; Bensadoun, Franquin et al. 1999; Rubenstein, Peterson et al. 2004; Bensadoun, Le Page et al. 2006; Kuhn, Porto et al. 2009; Clarkson, Worthington et al. 2010; Bjordal, Bensadoun et al. 2011) . Since the subjects of this section refers to analgesic effects of low power lasers in established chemotherapy-induced oral mucositis (COM), we briefly review some articles in which LLLT has been used for relieving pain in patients with

Cauwels and Martens evaluated the capacity of analgesic effect and wound healing of low level laser therapy in 16 children suffering from chemotherapy-induced oral mucositis. All children were treated using a GaAlAs diode laser (wavelength: 830 nm, potency: 150 mW). The energy released was adapted according to the severity of the oral lesions. The same protocol was repeated every 48 hours until healing of each lesion occurred. The results of the study demonstrated that immediately after irradiation of the oral mucositis, pain relief was noticed. Depending on the severity of oral mucositis, on average, 2.5 treatments per lesion in a period of 1 week were sufficient to heal a mucositis lesion. They concluded that LLLT could reduce the severity and duration of mucositis and to relieve pain significantly

Nes and Posso investigated the pain relieving effect of LLLT among 13 patients who have developed moderate chemotherapy-induced oral mucositis. The laser used was GaAlAs (830 nm, power: 250 mW, energy density: 35 J cm-2). The patients were treated during a 5-day period, and the pain was measured before and after each laser application. There was a significant ( *P*= 0.007) 67% decrease in the daily average experience of pain felt before and after each treatment, confirming that LLLT can relieve pain among patients who have

We have evaluated the pain relieving effects of NACLT in some other painful mucosal lesions such as painful oral lesions of Stevens Johnson Syndrome, etc. as case reports, the

developed chemotherapy-induced oral mucositis (Nes and Posso 2005).

chemotherapy-induced oral mucositis and not the prophylactic laser protocols.

visible side effects (Zand, Najafi et al. 2010).

of painful lesions of oral mucositis (Arora, Pai et al. 2008).

**4.2.4.3 A brief literature review** 

(Cauwels and Martens 2011).

**4.2.5 Other NACLT studies** 

In addition, we have evaluated the effects of NACLT in promoting wound healing in few studies with variable degrees of success. Although in few studies, NACLT has shown some valuable results in this field we are not yet ready to express our view in this regard. Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the promoting wound healing effects of NACLT.

*It should be mentioned that since the biological effects of NACLT and their mechanisms are not fully known, it seems ethically questionable to use NACLT in diseases with malignant potential, such as oral lichen planus, consequently we have not assessed the effects of NACLT in such illnesses.* 

#### **4.3 Presumed mechanisms of pain relieving effects of NACLT**

In order to develop an understanding of the mechanisms of analgesic effect of NACLT, powermetry and thermometry were performed in prior studies, the results of which demonstrated the low power nature of the applied CO2 laser (Zand, Ataie-Fashtami et al. 2009). (Please see also 4. NACLT)

Since the analgesic effect of NACLT is immediate, we assume that at least in part, physiological neural changes such as blockage of action potential generation and conduction of nociceptive signals in primary afferent neurons might take part in this analgesic effect (Zand, Ataie-Fashtami et al. 2009). Destruction of nerve endings is less probable to be induced by NACLT, because, even in the studies in which CO2 laser has been used as a surgical scalpel, there have been no statistically significant differences in the number of intact peripheral nerve structures in laser-treated sites in comparison with sites treated with electrocautery and scalpel (Rocha, Pinheiro et al. 2001). It is not known, whether the other mechanisms such as increase in β-endorphin synthesis and release, changes in bradykinin, prostaglandins, substance P, serotonin, acetylcholine, nitric oxide, singlet oxygen production, and the other biochemical events- which have been proposed to play a part in pain relieving effect of conventional low power lasers - are responsible for analgesic effect of NACLT or not. (Please see also 2.4.Mechanisms of analgesic effects of low power laser therapy) Further basic studies are necessary to elucidate the mechanisms of this analgesic effect.

On the other hand, there are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional low level therapeutic lasers (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). Therefore one could presume that the mechanisms of analgesic effect of NACLT might have some differences from that for conventional low power lasers, an assumption that should be assessed in further basic studies.

Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

*Venereol* 17(3): 288-290.

profile." *Skinmed* 5(2): 69-71.

**6. References** 

37.

132-135.

49(2): 107-116.

119-130.

A New Approach to Relieve Pain in Some Painful Oral Diseases 409

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In some ulcerated oral lesions such as aphthous ulcers, the pain of the lesions derives from inflammatory sensitization of small-diameter afferent nerve endings that form a plexus at the junction of the epithelial and subepithelial layers. Branches of this plexus extend upward, into the epithelial layer; producing a superficial, focal, inflammatory lesion that is directly associated with exposed sensory nerve endings. Therefore, in such ulcers, CO2 laser irradiation can reach the exposed nerve endings easily and as we assume, for example, block the action potential generation and conduction of nociceptive signals in primary afferent neurons. On the other hand, in other under-publish studies, we have used NACLT for reducing pain in few non-ulcerated lesions, such as pre-aphthous lesions with moderate-good results. As the CO2 laser's beam has a very limited depth of penetration in to the tissue, explaining this analgesic effect of NACLT seems more complex.

Tuner and Hode state that the therapeutic effects of defocused CO2 laser must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010). We don't know whether such mechanisms may at least in part, take part in the analgesic effect of NACLT or not.

Further fundamental studies are necessary to elucidate the mechanisms of analgesic effect of NACLT.

#### **5. Conclusion**

CO2 laser has been used as a very useful high power, thermal laser in surgery for cutting, ablation and coagulation of the tissues for many years. In contrast, in non- thermal, Non-Ablative CO2 Laser Therapy (NACLT), the CO2 laser is used as a low level (phototherapeutic) laser to reduce pain in some oral mucosal lesions without any visual effects of thermal damage to the oral mucosa such as ablation, ulceration or aggravation of the lesions. The results of powermetry and thermometry have demonstrated the low power nature of the applied CO2 laser in NACLT.

As discussed above, in order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent, non-anesthetic gel with high water content. In addition, the CO2 laser is operated with a de-focused hand piece, scanning rapidly over the lesion with circular motion. With these considerations, CO2 laser can be used as a non-destructive, non-thermal, phototherapeutic laser (NACLT) to reduce pain in some oral mucosal lesions immediately and dramatically, so that after NACLT, the patients of the studies have been able to eat and drink easily at once. So far, in the series of NACLT studies, we have not observed any visible side effects following careful performance of the technique.

Certainly, controlled clinical trials with larger sample sizes will be able to prove the analgesic effects of NACLT more definitely. We recommend that in further studies, the pain severity of the lesions would be followed up for longer periods of time.

In addition, it should be emphasized that in serious diseases such as pemphigus vulgaris, Behcet's disease, etc., the patients must be warned that NACLT should not alter their conventional systemic treatment in spite of its significant analgesic effect.

#### **6. References**

408 CO2 Laser – Optimisation and Application

In some ulcerated oral lesions such as aphthous ulcers, the pain of the lesions derives from inflammatory sensitization of small-diameter afferent nerve endings that form a plexus at the junction of the epithelial and subepithelial layers. Branches of this plexus extend upward, into the epithelial layer; producing a superficial, focal, inflammatory lesion that is directly associated with exposed sensory nerve endings. Therefore, in such ulcers, CO2 laser irradiation can reach the exposed nerve endings easily and as we assume, for example, block the action potential generation and conduction of nociceptive signals in primary afferent neurons. On the other hand, in other under-publish studies, we have used NACLT for reducing pain in few non-ulcerated lesions, such as pre-aphthous lesions with moderate-good results. As the CO2 laser's beam has a very limited depth of penetration

Tuner and Hode state that the therapeutic effects of defocused CO2 laser must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010). We don't know whether such

Further fundamental studies are necessary to elucidate the mechanisms of analgesic effect of

CO2 laser has been used as a very useful high power, thermal laser in surgery for cutting, ablation and coagulation of the tissues for many years. In contrast, in non- thermal, Non-Ablative CO2 Laser Therapy (NACLT), the CO2 laser is used as a low level (phototherapeutic) laser to reduce pain in some oral mucosal lesions without any visual effects of thermal damage to the oral mucosa such as ablation, ulceration or aggravation of the lesions. The results of powermetry and thermometry have demonstrated the low power

As discussed above, in order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent, non-anesthetic gel with high water content. In addition, the CO2 laser is operated with a de-focused hand piece, scanning rapidly over the lesion with circular motion. With these considerations, CO2 laser can be used as a non-destructive, non-thermal, phototherapeutic laser (NACLT) to reduce pain in some oral mucosal lesions immediately and dramatically, so that after NACLT, the patients of the studies have been able to eat and drink easily at once. So far, in the series of NACLT studies, we have not observed any visible side effects following careful

Certainly, controlled clinical trials with larger sample sizes will be able to prove the analgesic effects of NACLT more definitely. We recommend that in further studies, the pain

In addition, it should be emphasized that in serious diseases such as pemphigus vulgaris, Behcet's disease, etc., the patients must be warned that NACLT should not alter their

severity of the lesions would be followed up for longer periods of time.

conventional systemic treatment in spite of its significant analgesic effect.

in to the tissue, explaining this analgesic effect of NACLT seems more complex.

mechanisms may at least in part, take part in the analgesic effect of NACLT or not.

NACLT.

**5. Conclusion** 

nature of the applied CO2 laser in NACLT.

performance of the technique.


Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):

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**0**

**17**

*Italy*

**Protons Acceleration by CO**2 **Laser Pulses and**

Pasquale Londrillo, Graziano Servizi, Andrea Sgattoni, Stefano Sinigardi,

The acceleration of electrons with the high electric fields generated in a plasma by a very intense laser pulse was proposed over forty years ago [Tajima & Dawson (1979)], but only the advent of the chirped pulse amplification CPA [Mourou et al. (2006)] allowed to increase the laser power and intensity up to the required values. The continuous progress, since a decade, of compact Ti:Sa lasers allowed 1 GeV good quality electron beams to be generated [Leemans et al. (2006)]. The optical acceleration of protons and ions has been also actively investigated. The highest energy of protons, 60 MeV, has been reached with short high energy pulses of Nd:Yag lasers, developed for inertial fusion [Snavely (2000)]. With compact ultrashort Ti:Sa laser pulses intensities of 1021 W/cm2 are reached and proton beams with energy up to a few

The targets are typically thin metal foils and the acceleration is achieved in the TNSA regime (Target Normal Sheath Acceleration). The laser, interacting with an overcritical plasma, cannot propagate through it and heats the electrons on the surface of the target. A large number of "hot" electrons is hence produced and they are are accelerated in the forward direction and can cross the target. When reaching the rear surface they create an intense electrostatic field which accelerates the protons present on the surface [Passoni & Lontano (2004)]. The energy spectrum is exponential and the angular spread is significant so that the beam is not suitable for free propagation. Energy selection and collimation may reduce the intensity below the threshold required for any application. However other acceleration mechanisms have been considered such as the radiation pressure dominated regime RPA (Radiation Pressure Acceleration), where two distinct mechanism act depending on the thickness *h* of the target. If *h* ∼ *λ* the hole boring regime with break-up of the electron density wave is active [Macchi et al. (2005)], whereas for ultrathin targets the acceleration mechanism is the same as for the relativistic mirror [Londrillo et al. (2010); Macchi et al. (2009)] and high efficiencies can be reached. This regime was recently experimentally observed using a few nanometers carbon targets, a circularly polarized laser pulse with *λ* ∼ 1*μ*m and very high contrast [Henig (2009)]. The efficiency of the RPA should be higher than TNSA and the proton bunches should have a small energy and angular spread; however the requirements of circular polarization and high contrast of the laser pulse render this regime not easily achievable. The subcritical or

**1. Introduction**

tens of MeV are obtained [Zeil et al. (2010)].

**Perspectives for Medical Applications**

Marco Sumini and Giorgio Turchetti *Università di Bologna, INFN Sezione di Bologna*


### **Protons Acceleration by CO**2 **Laser Pulses and Perspectives for Medical Applications**

Pasquale Londrillo, Graziano Servizi, Andrea Sgattoni, Stefano Sinigardi, Marco Sumini and Giorgio Turchetti *Università di Bologna, INFN Sezione di Bologna Italy*

#### **1. Introduction**

414 CO2 Laser – Optimisation and Application

Zand, N., P. Mansouri, et al. (2009). Relieving pain in painful oral lesions of pemphigus

Zand, N., S. Najafi, et al. (2010). "NACLT ( Non-ablative CO2 laser therapy ): a new

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vulgaris by a single session, Non-ablative 10600 nm CO2 Laser irradiation ( pilot study ). *The 29th Annual conference of the American Society for Lasers in surgery and* 

approach to relieve pain in mild to moderate oral mucositis following breast cancer

The acceleration of electrons with the high electric fields generated in a plasma by a very intense laser pulse was proposed over forty years ago [Tajima & Dawson (1979)], but only the advent of the chirped pulse amplification CPA [Mourou et al. (2006)] allowed to increase the laser power and intensity up to the required values. The continuous progress, since a decade, of compact Ti:Sa lasers allowed 1 GeV good quality electron beams to be generated [Leemans et al. (2006)]. The optical acceleration of protons and ions has been also actively investigated. The highest energy of protons, 60 MeV, has been reached with short high energy pulses of Nd:Yag lasers, developed for inertial fusion [Snavely (2000)]. With compact ultrashort Ti:Sa laser pulses intensities of 1021 W/cm2 are reached and proton beams with energy up to a few tens of MeV are obtained [Zeil et al. (2010)].

The targets are typically thin metal foils and the acceleration is achieved in the TNSA regime (Target Normal Sheath Acceleration). The laser, interacting with an overcritical plasma, cannot propagate through it and heats the electrons on the surface of the target. A large number of "hot" electrons is hence produced and they are are accelerated in the forward direction and can cross the target. When reaching the rear surface they create an intense electrostatic field which accelerates the protons present on the surface [Passoni & Lontano (2004)]. The energy spectrum is exponential and the angular spread is significant so that the beam is not suitable for free propagation. Energy selection and collimation may reduce the intensity below the threshold required for any application. However other acceleration mechanisms have been considered such as the radiation pressure dominated regime RPA (Radiation Pressure Acceleration), where two distinct mechanism act depending on the thickness *h* of the target. If *h* ∼ *λ* the hole boring regime with break-up of the electron density wave is active [Macchi et al. (2005)], whereas for ultrathin targets the acceleration mechanism is the same as for the relativistic mirror [Londrillo et al. (2010); Macchi et al. (2009)] and high efficiencies can be reached. This regime was recently experimentally observed using a few nanometers carbon targets, a circularly polarized laser pulse with *λ* ∼ 1*μ*m and very high contrast [Henig (2009)].

The efficiency of the RPA should be higher than TNSA and the proton bunches should have a small energy and angular spread; however the requirements of circular polarization and high contrast of the laser pulse render this regime not easily achievable. The subcritical or

number, were accelerated with a very small energy spread [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. This result inserts the CO2 lasers among the possible candidates for the production of proton beams for medical therapy, possibly combined with a post acceleration device. The CO2 lasers have high efficiency and produce almost circularly polarized light which is suited for acceleration mechanisms like RPA. In particular the possibility of using gas jets at under-critical or slightly overcritical density opens very interesting perspectives because of the high repetition rate allowed, the absence of debris (opposed to the case of solid thin targets) jointly with the

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 417

We can now study the very same physical problem using a CO2 laser instead of a Ti:Sa laser keeping the dynamics of the interaction unchanged. This can be done keeping unchanged the adimensional parameters which characterize the laser-plasma: the ratio of the plasma density over critical density (*n*/*nc*) and the normalized vector potential (*a*). This correspond to consider a CO2 laser pulse with the same peak power but ten times longer (same number of wave cycle) and a laser waist proportional to the wavelength. The plasma density is hundred times lower and the total volume interested by the acceleration is three order of magnitude larger than the case of a Ti:Sa laser. If we assume a definite fraction of the protons in the corresponding volume are accelerated and the density is kept at the critical value, then the number of accelerated protons is proportional to *λ*. If coupled with a high repetition rate, long wavelength pulses may offer a further advantage to reach the doses required by therapy. In the present note we shall review the basic mechanisms for laser acceleration to present the related scaling laws and compare the results one expects from small (1 *μ*) and large (10 *μ*) wavelength pulses. Systematic 2D and 3D simulations were performed with the high order PIC code ALaDyn [Benedetti et al. (2008)] developed by the university of Bologna to provide quantitative results in addition to the qualitative results of scaling laws. We shall also discuss

The paper consists of six sections: after this introduction, in section 2 we recall the basic features and parameters of the laser beam, in section 3 the TNSA regime is reviewed, in section 4 the RPA regime is presented, in section 5 the acceleration on under-critical target is discussed, in section 6 we discuss the transport of the optically accelerated proton bunch, in

A laser pulse is described by an electromagnetic wave packet, which is a solution of Maxwell's

in the vacuum where source charges and currents are absent *ρ* = 0, **j** = 0. The sources arise when the pulse propagates in material medium creating a plasma. The scalar and vector

**B** div **B** = 0

**E** div **E** = 4*πρ* (1)

advantages of circular polarization.

the transport of a protons beam through an optical system.

section 7 we analyze the perspectives for therapy.

rot **<sup>E</sup>** <sup>=</sup> <sup>−</sup><sup>1</sup>

rot **<sup>B</sup>** <sup>=</sup> <sup>4</sup>*<sup>π</sup>*

*c ∂ ∂t*

*<sup>c</sup>* **<sup>j</sup>** <sup>+</sup> 1 *c ∂ ∂t*

**2. Laser beam interaction with matter**

equations

quasi-critical targets are also a possible alternative because the laser-plasma energy coupling can be very high with a much higher energy transfer from the laser pulse to the plasma: a different regime may be achieved where the laser drills a channel and a strong electron current is created on its trail. At the exit from the plasma it creates a magnetic azimuthal field and a longitudinal electric field. As a consequence, the protons are efficiently accelerated and nicely collimated [Bulanov (2010); Nakamura, Bulanov, Esirkepov & Kando (2010); Naumova & Bulanov (2002); Yogo (2008)]. Experimental results with near critical targets confirmed the theoretical and simulation results on the enhancement of maximum energy and reduction of angular spread with respect to the TNSA acceleration mechanism [Fukuda & Bulanov (2009)].

The possibility of reaching energies close to the threshold of 60 MeV for cancer therapy with compact Ti:Sa laser system has stimulated several projects dedicated to medical applications. Indeed the protons or ions deposit most of their energy at the end of their range and are biologically more effective with respect to electrons or X rays since they allow to spare nearby healthy tissues. However the cyclotrons and synchrotrons currently used require large and expensive infrastructures. The use of laser acceleration opens a perspective for more compact and cheaper devices suitable to be installed on a regional scale. Two possible strategies are being considered:


The hybrid acceleration scheme does not require to develop new laser systems but only the improvement of targets and the design of a transport system capable of shaping the beam in such a way to render it suitable for injection. Simulations and experiments are presently being developed to explore the feasibility of transport of an optically accelerated protons bunch reaching the beam quality required for irradiation and for injection into a post-acceleration device [Melone (2011); Nishiuchi (2010); Schollmeier (2008)]. The reduction of the energy spread with a longitudinal phase space rotation provided by a synchronized RF was also proved [Noda (2008)].

Recently, new protons acceleration experiments have been performed taking advantage of short pulses of long wavelength *λ* = 10*μ*m produced by CO2 lasers. This approach provides a parallel research pathway which offers some advantages. Being the wavelength one order of magnitude larger than the optical values typical of Ti:Sa or Nd laser, the plasma critical density for *λ* = 10*μ*m is about 10<sup>19</sup> cm−<sup>3</sup> which can be reached ionizing a supersonic gas-jet. The CO2 lasers deliver a pulse with a native quasi circular polarization which is interesting for triggering the RPA regime at lower intensities.

Recent experiments showed that using a 1 TW pulse of 1 J interacting with a solid target, protons can be accelerated up to 1 MeV with exponential energy spectrum and wide angular spread typical of TNSA regime [Pogorelsky (2010b)]. On a gas jet at the critical density a quasi monochromatic beam of protons at 2 MeV was obtained suggesting that a RPA mechanism is dominating [Pogorelsky (2010a)]. With a train of short pulses and 100 J total energy a different acceleration mechanism was achieved and protons of 25 MeV, even though a low 2 Will-be-set-by-IN-TECH

quasi-critical targets are also a possible alternative because the laser-plasma energy coupling can be very high with a much higher energy transfer from the laser pulse to the plasma: a different regime may be achieved where the laser drills a channel and a strong electron current is created on its trail. At the exit from the plasma it creates a magnetic azimuthal field and a longitudinal electric field. As a consequence, the protons are efficiently accelerated and nicely collimated [Bulanov (2010); Nakamura, Bulanov, Esirkepov & Kando (2010); Naumova & Bulanov (2002); Yogo (2008)]. Experimental results with near critical targets confirmed the theoretical and simulation results on the enhancement of maximum energy and reduction of angular spread with respect to the TNSA acceleration mechanism [Fukuda & Bulanov (2009)]. The possibility of reaching energies close to the threshold of 60 MeV for cancer therapy with compact Ti:Sa laser system has stimulated several projects dedicated to medical applications. Indeed the protons or ions deposit most of their energy at the end of their range and are biologically more effective with respect to electrons or X rays since they allow to spare nearby healthy tissues. However the cyclotrons and synchrotrons currently used require large and expensive infrastructures. The use of laser acceleration opens a perspective for more compact and cheaper devices suitable to be installed on a regional scale. Two possible strategies are

A) increase the power of the laser system in order to reach energies in the 100-200 MeV energy

B) use the laser system as injector into a DTL linac to increase the energy starting from 10 MeV [Antici (2011)] or to inject a 30 MeV laser accelerated protons bunch, into a high field compact linac in order to raise the energy up to 100 or 200 MeV [Londrillo et al. (2011)]. The hybrid acceleration scheme does not require to develop new laser systems but only the improvement of targets and the design of a transport system capable of shaping the beam in such a way to render it suitable for injection. Simulations and experiments are presently being developed to explore the feasibility of transport of an optically accelerated protons bunch reaching the beam quality required for irradiation and for injection into a post-acceleration device [Melone (2011); Nishiuchi (2010); Schollmeier (2008)]. The reduction of the energy spread with a longitudinal phase space rotation provided by a synchronized RF was also

Recently, new protons acceleration experiments have been performed taking advantage of short pulses of long wavelength *λ* = 10*μ*m produced by CO2 lasers. This approach provides a parallel research pathway which offers some advantages. Being the wavelength one order of magnitude larger than the optical values typical of Ti:Sa or Nd laser, the plasma critical density for *λ* = 10*μ*m is about 10<sup>19</sup> cm−<sup>3</sup> which can be reached ionizing a supersonic gas-jet. The CO2 lasers deliver a pulse with a native quasi circular polarization which is interesting

Recent experiments showed that using a 1 TW pulse of 1 J interacting with a solid target, protons can be accelerated up to 1 MeV with exponential energy spectrum and wide angular spread typical of TNSA regime [Pogorelsky (2010b)]. On a gas jet at the critical density a quasi monochromatic beam of protons at 2 MeV was obtained suggesting that a RPA mechanism is dominating [Pogorelsky (2010a)]. With a train of short pulses and 100 J total energy a different acceleration mechanism was achieved and protons of 25 MeV, even though a low

range [Bulanov (2008); Hofmann (2011); Murakami (2008)]

being considered:

proved [Noda (2008)].

for triggering the RPA regime at lower intensities.

number, were accelerated with a very small energy spread [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. This result inserts the CO2 lasers among the possible candidates for the production of proton beams for medical therapy, possibly combined with a post acceleration device. The CO2 lasers have high efficiency and produce almost circularly polarized light which is suited for acceleration mechanisms like RPA. In particular the possibility of using gas jets at under-critical or slightly overcritical density opens very interesting perspectives because of the high repetition rate allowed, the absence of debris (opposed to the case of solid thin targets) jointly with the advantages of circular polarization.

We can now study the very same physical problem using a CO2 laser instead of a Ti:Sa laser keeping the dynamics of the interaction unchanged. This can be done keeping unchanged the adimensional parameters which characterize the laser-plasma: the ratio of the plasma density over critical density (*n*/*nc*) and the normalized vector potential (*a*). This correspond to consider a CO2 laser pulse with the same peak power but ten times longer (same number of wave cycle) and a laser waist proportional to the wavelength. The plasma density is hundred times lower and the total volume interested by the acceleration is three order of magnitude larger than the case of a Ti:Sa laser. If we assume a definite fraction of the protons in the corresponding volume are accelerated and the density is kept at the critical value, then the number of accelerated protons is proportional to *λ*. If coupled with a high repetition rate, long wavelength pulses may offer a further advantage to reach the doses required by therapy.

In the present note we shall review the basic mechanisms for laser acceleration to present the related scaling laws and compare the results one expects from small (1 *μ*) and large (10 *μ*) wavelength pulses. Systematic 2D and 3D simulations were performed with the high order PIC code ALaDyn [Benedetti et al. (2008)] developed by the university of Bologna to provide quantitative results in addition to the qualitative results of scaling laws. We shall also discuss the transport of a protons beam through an optical system.

The paper consists of six sections: after this introduction, in section 2 we recall the basic features and parameters of the laser beam, in section 3 the TNSA regime is reviewed, in section 4 the RPA regime is presented, in section 5 the acceleration on under-critical target is discussed, in section 6 we discuss the transport of the optically accelerated proton bunch, in section 7 we analyze the perspectives for therapy.

#### **2. Laser beam interaction with matter**

A laser pulse is described by an electromagnetic wave packet, which is a solution of Maxwell's equations

$$\begin{aligned} \text{rot}\,\mathbf{E} &= -\frac{1}{c}\frac{\partial}{\partial t}\mathbf{B} & \text{div}\,\mathbf{B} = 0\\ \text{rot}\,\mathbf{B} &= \frac{4\pi}{c}\mathbf{j} + \frac{1}{c}\frac{\partial}{\partial t}\mathbf{E} & \text{div}\,\mathbf{E} = 4\pi\rho \end{aligned} \tag{1}$$

in the vacuum where source charges and currents are absent *ρ* = 0, **j** = 0. The sources arise when the pulse propagates in material medium creating a plasma. The scalar and vector



x

x

t=1



x

*γ* = 

*∂ f ∂***r** + *e***E** +

x



x

*∂ f*

**p**

x

B\_y t=1



x

*<sup>∂</sup>***<sup>p</sup>** <sup>=</sup> <sup>0</sup> (11)

*<sup>m</sup><sup>γ</sup> <sup>f</sup>*(**r**, **<sup>p</sup>**, *<sup>t</sup>*) *<sup>d</sup>***<sup>p</sup>** (12)

*mpc*<sup>2</sup> (13)

x

Energy t=1

E\_x t=1


2

z


2

z

A t=1


2

z


1/2

Fig. 1. Gaussian wave packet for the function *A* defined by eq. (7, 9), for *σ<sup>x</sup>* = 0.25, *σ<sup>z</sup>* = 1, *k*<sup>0</sup> = 4, *c* = 1, at different times (left figures). Electric field *Ex*, magnetic field *By* and energy at

> <sup>1</sup> <sup>+</sup> **<sup>p</sup>**<sup>2</sup> *m*2*c*<sup>2</sup>

Given *N* particles we introduce the phase space density *f*(**r**, **p**, *t*), which evolves according to

where the fields are solution of the Maxwell's equations, the sources being defined by

*f*(**r**, **p**, *t*) *d***p j** = *e*

The set (1), (11), (12) forms the Maxwell-Vlasov equations and provides the dynamical setting to investigate the laser plasma interaction. In actual computations the number *N* of numerical particles, used to sample the phase space density, is considerably lower than the number *N*ph of physical particles and the masses and charges are *N*ph/*N* times larger with respect to the masses and charges of the physical particles.The equations of motion are not affected since they depend only on the ratio *e*/*m*. The computation of charge densities and currents requires

The basic parameter of the electromagnetic wave is a dimensionless quantity which gives the ratio between the electromagnetic energy and the electron (or proton) rest mass energy

*mc*<sup>2</sup> *ap* <sup>=</sup> *eA*

*e mc<sup>γ</sup>* **<sup>p</sup>** <sup>×</sup> **<sup>B</sup>**

2

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 419

z

t=2

t=0.5


2

z


where *e* is the electric charge and *γ* is the relativistic factor

*∂ f <sup>∂</sup><sup>t</sup>* <sup>+</sup> **<sup>p</sup>** *mγ*

an interpolation procedure with smooth functions like splines,

*<sup>a</sup>* <sup>=</sup> *eA*

*ρ*(**r**, *t*) = *e*

2

z

t=0


2

z


time *t* = 1 (right figures).

the Liouville equation

**2.1 Basic parameters**

2

z

potential defined by

$$\mathbf{B} = \text{rot}\,\mathbf{A} \qquad \qquad \mathbf{E} = -\text{grad}\,\Phi - \frac{1}{c}\frac{\partial}{\partial t}\mathbf{A} \tag{2}$$

identically satisfy he first two equations. Choosing a gauge such that

$$
\mathbf{div}\,\mathbf{A} + \frac{1}{c}\Phi = 0\tag{3}
$$

the last two equations become the wave equations

$$\begin{aligned} \Delta \mathbf{A} - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \mathbf{A} &= -\frac{4\pi}{c} \mathbf{j} \\ \Delta \Phi - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \Phi &= -4\pi \rho \end{aligned} \tag{4}$$

In the vacuum *ρ* = 0 and we may choose Φ = 0. The gauge equation in this case reads div **A** = 0 and the electric field is given by **<sup>E</sup>** <sup>=</sup> <sup>−</sup>*c*−1*∂***A**/*∂t*. If we consider a two dimensional wave propagating in the *x*, *z* plane we may set *Ax* = *∂A*/*∂z* and *Az* = −*∂A*/*∂x* where *A* = *A*(*x*, *z*) so that div **A** = 0 is identically satisfied. In this case the electric field is given by

$$E\_X = -\frac{1}{c} \frac{\partial}{\partial t} \frac{\partial}{\partial z} A \qquad E\_y = 0 \qquad E\_z = \frac{1}{c} \frac{\partial}{\partial t} \frac{\partial}{\partial x} A \tag{5}$$

and the magnetic field

$$B\_{\mathbf{X}} = 0 \qquad B\_{\mathbf{Y}} = \left(\frac{\partial^2}{\partial \mathbf{x}^2} + \frac{\partial^2}{\partial z^2}\right) A \qquad B\_{\mathbf{Z}} = \mathbf{0} \tag{6}$$

Given an initial Gaussian wave field specified by

$$A(\mathbf{x}, z, \mathbf{0}) = A\_0(\mathbf{x}, z) \equiv \frac{e^{-\mathbf{x}^2 / 2\sigma\_\mathbf{x}^2}}{\sqrt{2\pi\sigma\_\mathbf{x}^2}} \frac{e^{-z^2 / 2\sigma\_\mathbf{z}^2}}{\sqrt{2\pi\sigma\_\mathbf{z}^2}} \cos(k\_0 z) \tag{7}$$

The evolution at time *t* is obtained by computing its Fourier transform and by taking into account that *A* satisfies the wave equation

$$
\Delta A - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} A = 0 \tag{8}
$$

The result for the propagating wave packet is given by

$$A(\mathbf{x}, z, t) = \frac{1}{(2\pi)^2} \int\_{-\infty}^{+\infty} dk\_{\mathbf{x}} \int\_{-\infty}^{+\infty} dk\_{\mathbf{z}} \, e^{-\sigma\_{\mathbf{z}}^2 k\_{\mathbf{x}}^2/2} e^{-\sigma\_{\mathbf{z}}^2 (k\_{\mathbf{z}} - k\_0)^2/2} \cos(\mathbf{x} k\_{\mathbf{x}} + z k\_{\mathbf{z}} - \omega t) \tag{9}$$

When the wave interacts with a medium it ionizes it if the intensity is sufficiently high and the charged particles move according to the equations of motion

$$\frac{d\mathbf{r}}{dt} = \frac{\mathbf{p}}{m\gamma} \qquad\qquad\qquad \frac{d\mathbf{p}}{dt} = e\mathbf{E} + \frac{e}{mc\gamma} \mathbf{p} \times \mathbf{B} \tag{10}$$

Fig. 1. Gaussian wave packet for the function *A* defined by eq. (7, 9), for *σ<sup>x</sup>* = 0.25, *σ<sup>z</sup>* = 1, *k*<sup>0</sup> = 4, *c* = 1, at different times (left figures). Electric field *Ex*, magnetic field *By* and energy at time *t* = 1 (right figures).

where *e* is the electric charge and *γ* is the relativistic factor

$$\gamma = \left(1 + \frac{\mathbf{p}^2}{m^2 c^2}\right)^{1/2}$$

Given *N* particles we introduce the phase space density *f*(**r**, **p**, *t*), which evolves according to the Liouville equation

$$\left(\frac{\partial f}{\partial t} + \frac{\mathbf{p}}{m\gamma} \frac{\partial f}{\partial \mathbf{r}} + \left(e\mathbf{E} + \frac{e}{mc\gamma} \mathbf{p} \times \mathbf{B}\right) \frac{\partial f}{\partial \mathbf{p}} = 0\tag{11}$$

where the fields are solution of the Maxwell's equations, the sources being defined by

$$
\rho(\mathbf{r},t) = e \int f(\mathbf{r}, \mathbf{p}, t) \, d\mathbf{p} \qquad \qquad \mathbf{j} = e \int \frac{\mathbf{p}}{m\gamma} f(\mathbf{r}, \mathbf{p}, t) \, d\mathbf{p} \tag{12}
$$

The set (1), (11), (12) forms the Maxwell-Vlasov equations and provides the dynamical setting to investigate the laser plasma interaction. In actual computations the number *N* of numerical particles, used to sample the phase space density, is considerably lower than the number *N*ph of physical particles and the masses and charges are *N*ph/*N* times larger with respect to the masses and charges of the physical particles.The equations of motion are not affected since they depend only on the ratio *e*/*m*. The computation of charge densities and currents requires an interpolation procedure with smooth functions like splines,

#### **2.1 Basic parameters**

4 Will-be-set-by-IN-TECH

1 *c*

*<sup>∂</sup>t*<sup>2</sup> **<sup>A</sup>** <sup>=</sup> <sup>−</sup>4*<sup>π</sup>*

*<sup>∂</sup>t*<sup>2</sup> <sup>Φ</sup> <sup>=</sup> <sup>−</sup>4*πρ*

*A Ey* <sup>=</sup> <sup>0</sup> *Ez* <sup>=</sup> <sup>1</sup>

*∂*2 *∂z*<sup>2</sup> 

*x*

*e*−*z*2/2*σ*<sup>2</sup> *z* <sup>2</sup>*πσ*<sup>2</sup> *z*

<sup>2</sup>*πσ*<sup>2</sup> *x*

The evolution at time *t* is obtained by computing its Fourier transform and by taking into

<sup>Δ</sup>*<sup>A</sup>* <sup>−</sup> <sup>1</sup> *c*2 *∂*2

> *dkz e* <sup>−</sup>*σ*<sup>2</sup> *<sup>x</sup> k*<sup>2</sup> *<sup>x</sup>*/2 *e* <sup>−</sup>*σ*<sup>2</sup>

When the wave interacts with a medium it ionizes it if the intensity is sufficiently high and

*d***p**

*dt* <sup>=</sup> *<sup>e</sup>***<sup>E</sup>** <sup>+</sup>

*e*

In the vacuum *ρ* = 0 and we may choose Φ = 0. The gauge equation in this case reads div **A** = 0 and the electric field is given by **<sup>E</sup>** <sup>=</sup> <sup>−</sup>*c*−1*∂***A**/*∂t*. If we consider a two dimensional wave propagating in the *x*, *z* plane we may set *Ax* = *∂A*/*∂z* and *Az* = −*∂A*/*∂x* where *A* = *A*(*x*, *z*)

*c* **j**

*c ∂ ∂t ∂ ∂x*

*c ∂ ∂t*

Φ = 0 (3)

**A** (2)

*A* (5)

*A Bz* = 0 (6)

*<sup>∂</sup>t*<sup>2</sup> *<sup>A</sup>* <sup>=</sup> <sup>0</sup> (8)

*<sup>z</sup>* (*kz*−*k*0)2/2 cos(*xkx* <sup>+</sup> *zkz* <sup>−</sup> *<sup>ω</sup>t*) (9)

*mc<sup>γ</sup>* **<sup>p</sup>** <sup>×</sup> **<sup>B</sup>** (10)

cos(*k*<sup>0</sup> *z*) (7)

(4)

**<sup>B</sup>** <sup>=</sup> rot **A E** <sup>=</sup> <sup>−</sup>grad <sup>Φ</sup> <sup>−</sup> <sup>1</sup>

div **A** +

<sup>Δ</sup>**<sup>A</sup>** <sup>−</sup> <sup>1</sup> *c*2 *∂*2

ΔΦ <sup>−</sup> <sup>1</sup> *c*2 *∂*2

so that div **A** = 0 is identically satisfied. In this case the electric field is given by

 *∂*<sup>2</sup> *<sup>∂</sup>x*<sup>2</sup> <sup>+</sup>

identically satisfy he first two equations. Choosing a gauge such that

the last two equations become the wave equations

*Ex* <sup>=</sup> <sup>−</sup><sup>1</sup> *c ∂ ∂t ∂ ∂z*

Given an initial Gaussian wave field specified by

account that *A* satisfies the wave equation

(2*π*)<sup>2</sup>

*<sup>A</sup>*(*x*, *<sup>z</sup>*, *<sup>t</sup>*) = <sup>1</sup>

The result for the propagating wave packet is given by

 +∞ −∞

*d***r** *dt* <sup>=</sup> **<sup>p</sup>** *mγ*

*dkx*

the charged particles move according to the equations of motion

 +∞ −∞

*Bx* = 0 *By* =

*<sup>A</sup>*(*x*, *<sup>z</sup>*, 0) = *<sup>A</sup>*0(*x*, *<sup>z</sup>*) <sup>≡</sup> *<sup>e</sup>*−*x*2/2*σ*<sup>2</sup>

potential defined by

and the magnetic field

The basic parameter of the electromagnetic wave is a dimensionless quantity which gives the ratio between the electromagnetic energy and the electron (or proton) rest mass energy

$$a = \frac{eA}{mc^2} \qquad \qquad a\_p = \frac{eA}{m\_p c^2} \tag{13}$$

**3. The TNSA regime**

acceleration is observed.

given by

This regime is observed when the laser beam interacts with a metallic foil whose electron density is largely overcritical *n* � *nc*, the thickness *h* of the foil is large with respect to the skin depth *h* � *<sup>s</sup>* and the polarization is linear. When the laser pulse interacts with the overcritical plasma it is reflected by the target. A sizable fraction of its energy can be absorbed by the electrons of the plasma by linear or nonlinear mechanisms. These electrons can travel through the target and expand around both the front and the rear side. The thickness of

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 421

and Θ refers to the temperature of the electrons whose density is *n*. The protons which are present on both surfaces as part of the contaminants deposited on the target (hydrocarbons, water vapors) are accelerated by the electrostatic field built up by the expanding electron cloud preferentially along the normal to the surfaces. If the laser contrast is high enough the front side of the target may be preserved intact until the main part of the pulse interacts with the plasma and a substantially symmetric acceleration takes place in both forward direction (from the rear side) and backward direction (protons from the front side). On the other hand, if the front surface is destroyed by the prepulses or by the laser pedestal, only the forward

We can consider the motion of a single electron in a plane e.m. wave. Assuming that the propagation is along the *z* axis and the vector potential has only the *Ay* component, defining *<sup>a</sup>* <sup>=</sup> *eAy*/(*mc*2) the generalized momenta are *Px* <sup>=</sup> 0, *Py* <sup>=</sup> *py* <sup>−</sup> *mca*, *Pz* <sup>=</sup> *<sup>p</sup>*. If we have a particle in an external field then *Py* = 0 because it is so initially and it is conserved. When collective effects are present we consider a fluid approximation assuming that �*Py*� = 0 which implies �*py*� = *mca*. The longitudinal motion is a coherent one given by the ponderomotive force, whereas the transverse one is random and we may assume the temperature Θ to be

so that for *<sup>a</sup>* � 1 we have *kb*<sup>Θ</sup> � *a mc*2. As a consequence the Debye length *<sup>λ</sup><sup>D</sup>* is given by

1 <sup>4</sup>*<sup>π</sup> rc <sup>n</sup>* <sup>=</sup> *<sup>a</sup>*

where *n*<sup>0</sup> denotes the electron density. An estimate of the electrostatic field is obtained by supposing the electrons charge distribution obeys Boltzmann statistics so that *n* = *n*<sup>0</sup> *eeV*/(*kB*Θ)

1 + tan2

 *<sup>h</sup>* <sup>−</sup> *<sup>z</sup> λ<sup>D</sup>* √2

*λ*2

*d*2*V*

*<sup>V</sup>* <sup>=</sup> *<sup>T</sup>*

simply the potential energy at the origin and can be expressed as

*<sup>e</sup>* log

and *V* satisfies the Poisson-Boltzmann equation

which must be solved with the conditions *V*(*h*) = *V*�

The longitudinal electric field is given by *Ez* = −*V*�

*<sup>D</sup>* <sup>=</sup> *kB*<sup>Θ</sup> *mc*<sup>2</sup>

*kB*<sup>Θ</sup> <sup>=</sup> *<sup>T</sup>* <sup>=</sup> *mc*2[(<sup>1</sup> <sup>+</sup> *<sup>a</sup>*2)1/2 <sup>−</sup> <sup>1</sup>] (21)

*dz*<sup>2</sup> <sup>=</sup> <sup>4</sup>*<sup>π</sup> <sup>n</sup>*<sup>0</sup> *<sup>e</sup>* exp (*eV*/(*kB*Θ)) (23)

(*h*) = 0. The solution is given by

(*z*); the maximum protons energy *E*max is

(24)

<sup>4</sup>*<sup>π</sup> rc <sup>n</sup>* (22)

*<sup>D</sup>* <sup>=</sup> *kB*Θ/(4*<sup>π</sup> <sup>e</sup>*2*n*)

the electron cloud is estimated equal to some Debye lengths *λ<sup>D</sup>* where *λ*<sup>2</sup>

When *a* ∼ 1 the electron becomes relativistic since the energy acquired from the wave is comparable with its rest energy. The intensity is related to the electromagnetic energy by

$$\frac{I}{c} = \frac{B^2 + E^2}{8\pi} = \pi \frac{A^2}{\lambda^2} \tag{14}$$

and consequently letting *rc* = *e*2/(*mc*2) = 3 10−<sup>13</sup> cm be the classical electron radius we have

$$a^2 = \frac{r\_c}{\pi \, mc^3} \, \lambda^2 I = \frac{I}{mc^3 n\_c} \tag{15}$$

where *nc* is the critical density defined below (19). A frequently used formula follows

$$a = 0.85 \, 10^{-9} I^{1/2} (\text{W/cm}^2) \, \lambda (\mu \text{m}) \tag{16}$$

The most relevant plasma parameter is the electron density which determines the plasma oscillations frequency *ω<sup>p</sup>*

$$
\omega\_p^2 = \frac{4\pi e^2 n}{m} = 4\pi r\_c c^2 n \tag{17}
$$

where *n* = *ρ*/*e* is the electron density, and *ρ* is the charge density. The plasma is an active optical medium and its refraction index is

$$m\_{\text{refr}} = \left(1 - \frac{\omega\_p^2}{\omega^2}\right)^{1/2} \tag{18}$$

For *ω<sup>p</sup>* < *ω* the medium is transparent. The density *nc* at which the medium becomes opaque is called critical density and is given by *ω<sup>p</sup>* = *ω* namely

$$n\_{\mathcal{L}} = \frac{\omega^2}{4\pi c^2 r\_{\mathcal{L}}} \simeq \frac{\pi}{\lambda^2 r\_{\mathcal{L}}} = \frac{10^{21}}{\lambda^2 (\mu \text{m})} \text{ cm}^{-3} \tag{19}$$

When the plasma is overcritical the wave becomes evanescent and decays exponentially. The decay length, called skin depth, is given by

$$\ell\_s = \frac{\lambda}{2\pi} \left(\frac{\omega\_p^2}{\omega^2} - 1\right)^{-1/2} = \frac{\lambda}{2\pi} \left(\frac{n}{n\_c} - 1\right)^{-1/2} \tag{20}$$

If we assume the waist to be a fixed multiple of the wavelength, *w* = *κλ* then *a*<sup>2</sup> is proportional to the pulse power and does not depend on the wavelength. On the contrary the critical density in proportional to *λ*<sup>−</sup>2, and is *nc* = 10−<sup>19</sup> cm−<sup>3</sup> for a CO2 laser pulse. This means that a gas jet, from which a plasma density in the range of 1018 <sup>÷</sup> 1020 cm−<sup>3</sup> can be obtained, provides a medium with quasi critical electron density. Moreover since the pulse length is in the range of hundreds of microns, the millimetric thickness of a gas jet is adequate for protons acceleration. Since the pulse durations for a Ti:Sa and CO2 laser are 30 fs and 1 ps respectively, in order to have the same power the ratio of the energies must be 1/30. For the same power and the same value of *a*, the proton energy should be the same but their number would be higher for a CO2 pulse.

#### **3. The TNSA regime**

6 Will-be-set-by-IN-TECH

When *a* ∼ 1 the electron becomes relativistic since the energy acquired from the wave is comparable with its rest energy. The intensity is related to the electromagnetic energy by

<sup>8</sup>*<sup>π</sup>* <sup>=</sup> *<sup>π</sup>*

and consequently letting *rc* = *e*2/(*mc*2) = 3 10−<sup>13</sup> cm be the classical electron radius we have

*<sup>π</sup> mc*<sup>3</sup> *<sup>λ</sup>*<sup>2</sup> *<sup>I</sup>* <sup>=</sup> *<sup>I</sup>*

The most relevant plasma parameter is the electron density which determines the plasma

where *n* = *ρ*/*e* is the electron density, and *ρ* is the charge density. The plasma is an active

For *ω<sup>p</sup>* < *ω* the medium is transparent. The density *nc* at which the medium becomes opaque

When the plasma is overcritical the wave becomes evanescent and decays exponentially. The

If we assume the waist to be a fixed multiple of the wavelength, *w* = *κλ* then *a*<sup>2</sup> is proportional to the pulse power and does not depend on the wavelength. On the contrary the critical density in proportional to *λ*<sup>−</sup>2, and is *nc* = 10−<sup>19</sup> cm−<sup>3</sup> for a CO2 laser pulse. This means that a gas jet, from which a plasma density in the range of 1018 <sup>÷</sup> 1020 cm−<sup>3</sup> can be obtained, provides a medium with quasi critical electron density. Moreover since the pulse length is in the range of hundreds of microns, the millimetric thickness of a gas jet is adequate for protons acceleration. Since the pulse durations for a Ti:Sa and CO2 laser are 30 fs and 1 ps respectively, in order to have the same power the ratio of the energies must be 1/30. For the same power and the same value of *a*, the proton energy should be the same but their number would be

−1/2

 <sup>1</sup> <sup>−</sup> *<sup>ω</sup>*<sup>2</sup> *p ω*2

� *<sup>π</sup> λ*2*rc* *A*2

*mc*3*nc*

1/2

<sup>=</sup> <sup>1021</sup>

<sup>=</sup> *<sup>λ</sup>* 2*π*  *n nc* − 1

*<sup>λ</sup>*<sup>2</sup> (14)

1/2(W/cm2) *λ*(*μ*m) (16)

*<sup>m</sup>* <sup>=</sup> <sup>4</sup>*πrcc*2*<sup>n</sup>* (17)

*<sup>λ</sup>*2(*μ*m) cm−<sup>3</sup> (19)

−1/2

(15)

(18)

(20)

*I*

*<sup>a</sup>*<sup>2</sup> <sup>=</sup> *rc*

*a* = 0.85 10−<sup>9</sup> *I*

*ω*2

oscillations frequency *ω<sup>p</sup>*

optical medium and its refraction index is

decay length, called skin depth, is given by

higher for a CO2 pulse.

is called critical density and is given by *ω<sup>p</sup>* = *ω* namely

�*<sup>s</sup>* <sup>=</sup> *<sup>λ</sup>* 2*π*

*nc* <sup>=</sup> *<sup>ω</sup>*<sup>2</sup> 4*πc*2*rc*

> *ω*<sup>2</sup> *p <sup>ω</sup>*<sup>2</sup> <sup>−</sup> <sup>1</sup>

*<sup>c</sup>* <sup>=</sup> *<sup>B</sup>*<sup>2</sup> <sup>+</sup> *<sup>E</sup>*<sup>2</sup>

where *nc* is the critical density defined below (19). A frequently used formula follows

*<sup>p</sup>* <sup>=</sup> <sup>4</sup>*πe*2*<sup>n</sup>*

*n*refr =

This regime is observed when the laser beam interacts with a metallic foil whose electron density is largely overcritical *n* � *nc*, the thickness *h* of the foil is large with respect to the skin depth *h* � *<sup>s</sup>* and the polarization is linear. When the laser pulse interacts with the overcritical plasma it is reflected by the target. A sizable fraction of its energy can be absorbed by the electrons of the plasma by linear or nonlinear mechanisms. These electrons can travel through the target and expand around both the front and the rear side. The thickness of the electron cloud is estimated equal to some Debye lengths *λ<sup>D</sup>* where *λ*<sup>2</sup> *<sup>D</sup>* <sup>=</sup> *kB*Θ/(4*<sup>π</sup> <sup>e</sup>*2*n*) and Θ refers to the temperature of the electrons whose density is *n*. The protons which are present on both surfaces as part of the contaminants deposited on the target (hydrocarbons, water vapors) are accelerated by the electrostatic field built up by the expanding electron cloud preferentially along the normal to the surfaces. If the laser contrast is high enough the front side of the target may be preserved intact until the main part of the pulse interacts with the plasma and a substantially symmetric acceleration takes place in both forward direction (from the rear side) and backward direction (protons from the front side). On the other hand, if the front surface is destroyed by the prepulses or by the laser pedestal, only the forward acceleration is observed.

We can consider the motion of a single electron in a plane e.m. wave. Assuming that the propagation is along the *z* axis and the vector potential has only the *Ay* component, defining *<sup>a</sup>* <sup>=</sup> *eAy*/(*mc*2) the generalized momenta are *Px* <sup>=</sup> 0, *Py* <sup>=</sup> *py* <sup>−</sup> *mca*, *Pz* <sup>=</sup> *<sup>p</sup>*. If we have a particle in an external field then *Py* = 0 because it is so initially and it is conserved. When collective effects are present we consider a fluid approximation assuming that �*Py*� = 0 which implies �*py*� = *mca*. The longitudinal motion is a coherent one given by the ponderomotive force, whereas the transverse one is random and we may assume the temperature Θ to be given by

$$k\_B \Theta = T = mc^2[(1+a^2)^{1/2} - 1] \tag{21}$$

so that for *<sup>a</sup>* � 1 we have *kb*<sup>Θ</sup> � *a mc*2. As a consequence the Debye length *<sup>λ</sup><sup>D</sup>* is given by

$$
\lambda\_D^2 = \frac{k\_B \Theta}{mc^2} \frac{1}{4\pi r\_c n} = \frac{a}{4\pi r\_c n} \tag{22}
$$

where *n*<sup>0</sup> denotes the electron density. An estimate of the electrostatic field is obtained by supposing the electrons charge distribution obeys Boltzmann statistics so that *n* = *n*<sup>0</sup> *eeV*/(*kB*Θ) and *V* satisfies the Poisson-Boltzmann equation

$$\frac{d^2V}{dz^2} = 4\pi \, n\_0 \, e \, \exp\left(eV/(k\_B\Theta)\right) \tag{23}$$

which must be solved with the conditions *V*(*h*) = *V*� (*h*) = 0. The solution is given by

$$V = \frac{T}{e} \log\left(1 + \tan^2\left(\frac{h-z}{\lambda\_D \sqrt{2}}\right)\right) \tag{24}$$

The longitudinal electric field is given by *Ez* = −*V*� (*z*); the maximum protons energy *E*max is simply the potential energy at the origin and can be expressed as

The main feature of TNSA is that the energy spectrum has an exponential decay with a clear cut-off. Letting *N*(*E*) be the number of protons having energy in the range [0, *E*] the spectrum

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 423

where *E*max is the maximum energy where the exponential distribution is cut off. If *E*<sup>0</sup> � *E*max, so that the error in normalization while replacing the integration upper bound *E*max with ∞ is negligible, then *E*<sup>0</sup> is precisely the average energy and *N*<sup>0</sup> is the total protons number. For instance suppose we have *E*max = 20 MeV and and *E*<sup>0</sup> = 2 MeV with *N*<sup>0</sup> = 1012. Such a number would be obtained with a 6 J pulse if 0.3 J are transferred to the protons. We should notice that the number of protons with a narrow energy in the range [*E*, *E* + Δ*E*] would

and consequently choosing *E* = 10 MeV, Δ*E* = 0.1 MeV we would have *N* = 3 108 protons. The angular spread of the protons is important, so that after collimation with an iris in order to allow focusing with a quadrupole or a solenoid the number would be further reduced, possibly below 107, which is rather low (but might be acceptable) in view of possible

In figure 3 we show the energy spectrum for a metal foil and a foam layer whose parameters are the same as in figure 2. The maximum energy is *E*max � 14 MeV and the average energy

Fig. 3. Energy spectrum for metal foil with foam and the same parameters and the same laser

In figure 4 we show the scaling of the maximum energy and average energy at different *a*. In this case, the average *E* is not computed on all the particles, but instead only on a subset that linearly fits the logarithmic plot, where the beginning and the end of the spectrum are excluded. It turns out that the average energy, in 3D simulations, is ∼ 1/7 of the maximum

energy, both for the bare target and a target with a foam.

Δ*E E*0 *e*

*dE* <sup>=</sup> <sup>0</sup> *<sup>E</sup>* <sup>&</sup>gt; *<sup>E</sup>*max (26)

<sup>−</sup>*E*/*E*<sup>0</sup> (27)

<sup>−</sup>*E*/*E*<sup>0</sup> <sup>0</sup> <sup>≤</sup> *<sup>E</sup>* <sup>&</sup>lt; *<sup>E</sup>*max *dN*

*N*([*E*, *E* + Δ*E*]) � *N*<sup>0</sup>

is given by

be

applications.

is 1.8 MeV.

pulse as figure 2.

*dN dE* <sup>=</sup> *<sup>N</sup>*<sup>0</sup> *E*0 *e*

$$E\_{\text{max}}(\text{MeV}) = \frac{E\_{\text{max}}}{2mc^2} \simeq \frac{a}{2} \log\left(1 + \tan^2\sqrt{2}\right) \simeq 2a \tag{25}$$

This result is compatible with experiments which show a linear dependence of *E*max with *I*1/2. However for very short and very collimated laser pulses the experimentally observed scaling law is *E*max ∝ *I*0.8. More refined theoretical models agree with this scaling law [Zani et al. (2011)].

The efficiency of TNSA acceleration can be enhanced if the efficiency of the energy transfer from laser to the target can be increased. If the laser interacts with a near critical density plasma the energy coupling of the laser with the target is considerably increased, comparing with the case of highly overcritical plasma, and a higher number of "hot" electrons can be obtained. The pre-pulse induced ionization creates a pre-plasma and improves the energy transfer from the laser to the electrons letting the laser interact with a plasma at lower density. The characteristics of the preplasma are not easily controlled in a metallic target being the control of the laser-pedestal and pre-pulses very difficult. However a different design of the target may be considered where a foam layer is deposited on the thin metal foil [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. Such a target leads to a considerably greater laser energy absorption and to possibly an improved control of the laser target interaction. Systematic 2D and 3D PIC simulations have shown that the presence of a foam increases the maximum protons energy..

In figure 2 we show a comparison of the results obtained from a fully 3D simulation of the interaction of a laser beam with *a* = 10 with a thin metal foil with and without the coating of a slightly overcritical foam layer. The saturation of the maximum energy in the considered time interval is quite evident and the gain with the foam layer is almost a factor 3.

Fig. 2. Comparison of the maximum proton energy rise with time for a metal foil (red curve) with a foil on which a foam is superimposed (blue). The laser pulse has *λ* = 0.8 *μ*m, its duration is 25 fs, the waist is 3 *μ*m and the power is *W* = 30 TW so that *a*<sup>0</sup> = 10. The foil is 0, 5 *μ*m thick with a *n* = 40 *nc* whereas the foam layer is 2*μ* thick and *n* = 2*nc*. The polarization is linear and the incidence is normal. The accelerated protons come form the contaminants layer which is modeled as an ultra-thin H layer of 50 nm at density *n* = 9*nc*.

8 Will-be-set-by-IN-TECH

This result is compatible with experiments which show a linear dependence of *E*max with *I*1/2. However for very short and very collimated laser pulses the experimentally observed scaling law is *E*max ∝ *I*0.8. More refined theoretical models agree with this scaling law [Zani

The efficiency of TNSA acceleration can be enhanced if the efficiency of the energy transfer from laser to the target can be increased. If the laser interacts with a near critical density plasma the energy coupling of the laser with the target is considerably increased, comparing with the case of highly overcritical plasma, and a higher number of "hot" electrons can be obtained. The pre-pulse induced ionization creates a pre-plasma and improves the energy transfer from the laser to the electrons letting the laser interact with a plasma at lower density. The characteristics of the preplasma are not easily controlled in a metallic target being the control of the laser-pedestal and pre-pulses very difficult. However a different design of the target may be considered where a foam layer is deposited on the thin metal foil [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. Such a target leads to a considerably greater laser energy absorption and to possibly an improved control of the laser target interaction. Systematic 2D and 3D PIC simulations have shown that the presence of a foam increases the

In figure 2 we show a comparison of the results obtained from a fully 3D simulation of the interaction of a laser beam with *a* = 10 with a thin metal foil with and without the coating of a slightly overcritical foam layer. The saturation of the maximum energy in the considered time

Fig. 2. Comparison of the maximum proton energy rise with time for a metal foil (red curve) with a foil on which a foam is superimposed (blue). The laser pulse has *λ* = 0.8 *μ*m, its duration is 25 fs, the waist is 3 *μ*m and the power is *W* = 30 TW so that *a*<sup>0</sup> = 10. The foil is

0, 5 *μ*m thick with a *n* = 40 *nc* whereas the foam layer is 2*μ* thick and *n* = 2*nc*. The polarization is linear and the incidence is normal. The accelerated protons come form the contaminants layer which is modeled as an ultra-thin H layer of 50 nm at density *n* = 9*nc*.

interval is quite evident and the gain with the foam layer is almost a factor 3.

<sup>2</sup> log 

1 + tan<sup>2</sup>

√ 2 

� 2*a* (25)

<sup>2</sup>*mc*<sup>2</sup> � *<sup>a</sup>*

*<sup>E</sup>*max(MeV) = *<sup>E</sup>*max

et al. (2011)].

maximum protons energy..

The main feature of TNSA is that the energy spectrum has an exponential decay with a clear cut-off. Letting *N*(*E*) be the number of protons having energy in the range [0, *E*] the spectrum is given by

$$\frac{dN}{dE} = \frac{N\_0}{E\_0} e^{-E/E\_0} \quad 0 \le E < E\_{\text{max}} \qquad \frac{dN}{dE} = 0 \quad E > E\_{\text{max}} \tag{26}$$

where *E*max is the maximum energy where the exponential distribution is cut off. If *E*<sup>0</sup> � *E*max, so that the error in normalization while replacing the integration upper bound *E*max with ∞ is negligible, then *E*<sup>0</sup> is precisely the average energy and *N*<sup>0</sup> is the total protons number. For instance suppose we have *E*max = 20 MeV and and *E*<sup>0</sup> = 2 MeV with *N*<sup>0</sup> = 1012. Such a number would be obtained with a 6 J pulse if 0.3 J are transferred to the protons. We should notice that the number of protons with a narrow energy in the range [*E*, *E* + Δ*E*] would be

$$N([E\_\prime E + \Delta E]) \simeq N\_0 \frac{\Delta E}{E\_0} \ e^{-E/E\_0} \tag{27}$$

and consequently choosing *E* = 10 MeV, Δ*E* = 0.1 MeV we would have *N* = 3 108 protons. The angular spread of the protons is important, so that after collimation with an iris in order to allow focusing with a quadrupole or a solenoid the number would be further reduced, possibly below 107, which is rather low (but might be acceptable) in view of possible applications.

In figure 3 we show the energy spectrum for a metal foil and a foam layer whose parameters are the same as in figure 2. The maximum energy is *E*max � 14 MeV and the average energy is 1.8 MeV.

Fig. 3. Energy spectrum for metal foil with foam and the same parameters and the same laser pulse as figure 2.

In figure 4 we show the scaling of the maximum energy and average energy at different *a*. In this case, the average *E* is not computed on all the particles, but instead only on a subset that linearly fits the logarithmic plot, where the beginning and the end of the spectrum are excluded. It turns out that the average energy, in 3D simulations, is ∼ 1/7 of the maximum energy, both for the bare target and a target with a foam.

where **p** and **P** are the ordinary and generalized momenta. For a wave propagating on the *z* direction **A** = **A**(*z* − *ct*) an averaging procedure with respect to the explicit time dependence of the Hamiltonian can be carried out. For a plane wave the time average vanishes �**A**� = 0.

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 425

This is the case in the non relativistic limit since the Hamiltonian becomes quadratic or if the

*<sup>z</sup>* ) � 1 averaging after a first order expansion in this variable shows that (29) is still valid.

The averages of generalized and ordinary momentum are the same and the gradient of the

This force dominates when the laser pulse is circularly polarized, because the electrons heating is strongly suppressed. The features of the acceleration mechanism depend on the target

For targets of thickness comparable with the pulse wavelength, we have the *hole boring* regime. The electrons density wave brakes on a distance comparable with the skin depth and the fluid approximation is no longer applicable. A piecewise constant approximation leads to a linear electric fields and the protons maximum energy can be easily evaluated. The

and for an ion of charge *Z* the energy is multiplied by *Z*. The angular spread of the beam is significantly lower with respect to the TNSA acceleration but the factor *nc*/*n* strongly reduces the energy value. Even though the scaling with *a* is quadratic rather than linear, only at very high values of the intensity the energy overcomes the value reached with TNSA. The only solution is to avoid the use of solid targets and work with a near critical density. This condition is naturally met for a wavelength of 10 *μ*m since the critical density *nc* = 1019 cm−<sup>3</sup> is met on gas jets. An experiment recently performed confirms that such a regime can be met and protons with a narrow energy spectrum can be accelerated [Palmer et al. (2010); Pogorelsky

For ultra-thin targets of thickness comparable with the skin depth the radiation pressure is able to push all the electrons of the foil which create a huge charge separation and all the ions are promptly accelerated. The result is that the target is practically accelerated as a whole as a

**P**<sup>2</sup> + *<sup>e</sup>*<sup>2</sup>

*<sup>c</sup>*<sup>2</sup> �**A**2� *m*2*c*<sup>2</sup>

<sup>2</sup>*mc*2*<sup>γ</sup>* grad �**A**2� <sup>=</sup> <sup>−</sup> *mc*<sup>2</sup>

<sup>2</sup>*mc*<sup>2</sup> � *<sup>a</sup>*<sup>2</sup> *nc*

1/2

(29)

*y*/(*p*<sup>2</sup> *<sup>x</sup>* +

<sup>2</sup>*<sup>γ</sup>* grad �**a**2� (30)

*<sup>n</sup>* (31)

As a consequence under suitable conditions it can be easily shown that

 1 +

vector potential has a single component **<sup>A</sup>** <sup>=</sup> *Ay*(*<sup>z</sup>* <sup>−</sup> *ct*) **<sup>e</sup>***y*. In this case supposing *<sup>p</sup>*<sup>2</sup>

*dt* <sup>=</sup> <sup>−</sup> *<sup>e</sup>*<sup>2</sup>

squared electromagnetic potential is the ponderomotive force in this approximation.

*<sup>E</sup>*max(MeV) = *<sup>E</sup>*max

�*H*� � *mc*<sup>2</sup>

*d***P**

As a consequence the equations of motion read

*d***r** *dt* <sup>=</sup> **<sup>P</sup>** *γ m*

*p*2

geometry.

(2010a)].

**4.2 Relativistic mirror**

**4.1 Hole Boring**

expression one obtains is

We noticed that a good linear fit holds in both cases and that for 2D simulations the maximum value of energy *E*max is about twice the energy obtained for 3D simulations.

Fig. 4. Scalings for the maximum and average energies computed from linear fit on a logarithmic plot for 2D and 3D simulations for a target with a metal foil 0.5 *μ*m thick and density *n* = 80*nc*, a foam layer of 2 *μ*m and density *n* = 2*nc*, a layer of contaminants of 50 nm and density of *n* = 9*nc*.

#### **3.1 CO**2 **results**

Experiments in Brookhaven with 1 TW CO2 laser pulses have shown that protons with maximum energy of 1 MeV can be obtained [Pogorelsky (2010b)]. The scaling laws obtained from both experiments and theoretical/numerical work does not offer significant perspectives to reach proton energies interesting for hadron therapy purposes unless powers in the PW range can be reached. As a consequence TNSA accelerated protons might be proposed for medical applications only coupled with a post acceleration device. Even in this case energies above 10 MeV should be reached. The use of targets with a coating of of 5-10 microns of a silicon foam with near critical density on the illuminated surface may increase the maximum energy by a significant factor (2-4) as simulations and experiments have shown [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. As a consequence the injection energy may be reached with compact lasers. This improved type of targets may be used for CO2 laser pulses as well. The most promising scenarios however are met when quasi critical are used, typically provided by a gas jet. In this case however we face a different acceleration regime which is dominated by the radiation pressure if the electron heating is modest as in the case of a circular polarization.

#### **4. The RPA regime**

The radiation pressure becomes the dominant mechanism in the acceleration of protons when *I* > 1023 W/cm2. However the effect of radiation pressure prevails over electrostatic acceleration even at lower energies when the electrons heating is decreased using a circularly polarized light. Given a vector potential **A** the relativistic Hamiltonian of a charged particle is

$$H = mc^2(\gamma - 1) \qquad \qquad \gamma = \left(1 + \frac{\mathbf{p}^2}{m^2 c^2}\right)^{1/2} \qquad \mathbf{p} = \mathbf{P} - \frac{e}{c}\mathbf{A} \tag{28}$$

where **p** and **P** are the ordinary and generalized momenta. For a wave propagating on the *z* direction **A** = **A**(*z* − *ct*) an averaging procedure with respect to the explicit time dependence of the Hamiltonian can be carried out. For a plane wave the time average vanishes �**A**� = 0. As a consequence under suitable conditions it can be easily shown that

$$
\langle H \rangle \simeq mc^2 \left( 1 + \frac{\mathbf{P}^2 + \frac{\varepsilon^2}{c^2} \langle \mathbf{A}^2 \rangle}{m^2 c^2} \right)^{1/2} \tag{29}
$$

This is the case in the non relativistic limit since the Hamiltonian becomes quadratic or if the vector potential has a single component **<sup>A</sup>** <sup>=</sup> *Ay*(*<sup>z</sup>* <sup>−</sup> *ct*) **<sup>e</sup>***y*. In this case supposing *<sup>p</sup>*<sup>2</sup> *y*/(*p*<sup>2</sup> *<sup>x</sup>* + *p*2 *<sup>z</sup>* ) � 1 averaging after a first order expansion in this variable shows that (29) is still valid. As a consequence the equations of motion read

$$\frac{d\mathbf{r}}{dt} = \frac{\mathbf{P}}{\gamma \, m} \qquad \qquad \frac{d\mathbf{P}}{dt} = -\frac{e^2}{2mc^2\gamma} \operatorname{grad} \left< \mathbf{A}^2 \right> = -\frac{mc^2}{2\gamma} \operatorname{grad} \left< \mathbf{a}^2 \right> \tag{30}$$

The averages of generalized and ordinary momentum are the same and the gradient of the squared electromagnetic potential is the ponderomotive force in this approximation.

This force dominates when the laser pulse is circularly polarized, because the electrons heating is strongly suppressed. The features of the acceleration mechanism depend on the target geometry.

#### **4.1 Hole Boring**

10 Will-be-set-by-IN-TECH

We noticed that a good linear fit holds in both cases and that for 2D simulations the maximum

E (MeV)

Fig. 4. Scalings for the maximum and average energies computed from linear fit on a logarithmic plot for 2D and 3D simulations for a target with a metal foil 0.5 *μ*m thick and density *n* = 80*nc*, a foam layer of 2 *μ*m and density *n* = 2*nc*, a layer of contaminants of 50 nm

Experiments in Brookhaven with 1 TW CO2 laser pulses have shown that protons with maximum energy of 1 MeV can be obtained [Pogorelsky (2010b)]. The scaling laws obtained from both experiments and theoretical/numerical work does not offer significant perspectives to reach proton energies interesting for hadron therapy purposes unless powers in the PW range can be reached. As a consequence TNSA accelerated protons might be proposed for medical applications only coupled with a post acceleration device. Even in this case energies above 10 MeV should be reached. The use of targets with a coating of of 5-10 microns of a silicon foam with near critical density on the illuminated surface may increase the maximum energy by a significant factor (2-4) as simulations and experiments have shown [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. As a consequence the injection energy may be reached with compact lasers. This improved type of targets may be used for CO2 laser pulses as well. The most promising scenarios however are met when quasi critical are used, typically provided by a gas jet. In this case however we face a different acceleration regime which is dominated by the radiation pressure if the electron heating is modest as in the case of a circular

The radiation pressure becomes the dominant mechanism in the acceleration of protons when *I* > 1023 W/cm2. However the effect of radiation pressure prevails over electrostatic acceleration even at lower energies when the electrons heating is decreased using a circularly polarized light. Given a vector potential **A** the relativistic Hamiltonian of a charged particle is

<sup>1</sup> <sup>+</sup> **<sup>p</sup>**<sup>2</sup> *m*2*c*<sup>2</sup> 1/2

**<sup>p</sup>** <sup>=</sup> **<sup>P</sup>** <sup>−</sup> *<sup>e</sup>*

*c*

**A** (28)

*<sup>H</sup>* <sup>=</sup> *mc*2(*<sup>γ</sup>* <sup>−</sup> <sup>1</sup>) *<sup>γ</sup>* <sup>=</sup>

Emax 7 x Eaverage

10 15 20 25 30

3D

a

value of energy *E*max is about twice the energy obtained for 3D simulations.

10 15 20 25 30

2D

a

**3.1 CO**2 **results**

polarization.

**4. The RPA regime**

and density of *n* = 9*nc*.

Emax 5 x Eaverage

E (MeV)

For targets of thickness comparable with the pulse wavelength, we have the *hole boring* regime. The electrons density wave brakes on a distance comparable with the skin depth and the fluid approximation is no longer applicable. A piecewise constant approximation leads to a linear electric fields and the protons maximum energy can be easily evaluated. The expression one obtains is

$$E\_{\text{max}}(\text{MeV}) = \frac{E\_{\text{max}}}{2mc^2} \simeq a^2 \, \frac{n\_c}{n} \tag{31}$$

and for an ion of charge *Z* the energy is multiplied by *Z*. The angular spread of the beam is significantly lower with respect to the TNSA acceleration but the factor *nc*/*n* strongly reduces the energy value. Even though the scaling with *a* is quadratic rather than linear, only at very high values of the intensity the energy overcomes the value reached with TNSA. The only solution is to avoid the use of solid targets and work with a near critical density. This condition is naturally met for a wavelength of 10 *μ*m since the critical density *nc* = 1019 cm−<sup>3</sup> is met on gas jets. An experiment recently performed confirms that such a regime can be met and protons with a narrow energy spectrum can be accelerated [Palmer et al. (2010); Pogorelsky (2010a)].

#### **4.2 Relativistic mirror**

For ultra-thin targets of thickness comparable with the skin depth the radiation pressure is able to push all the electrons of the foil which create a huge charge separation and all the ions are promptly accelerated. The result is that the target is practically accelerated as a whole as a

Fig. 5. Protons energy in MeV versus intensity for a pulse with *λ* = 10*μ*m. various foil thickness are considered: = 5*μ*m (red), = 10*μ*m (green), = 15*μ*m (Blue), = 30*μ*m (purple). The 1D PIC results are compared with the analytical results for the RPA hole boring

where *n* denotes the electrons density. The number *N* of ions in the target and the minimum number *N*<sup>∗</sup> below which transparency is induced and the corresponding minimum thickness

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 427

In this regime, keeping the ratio *h*/*λ* fixed, the volume of the accelerated ions increases as *λ*3, their number as *λ* if *n*/*nc* is also kept fixed. As a consequence, at the transparency limit the parameter *a* and the laser power have a fixed value. The parameter *α* is constant if the energy is kept proportional to *λ*. In this case the total energy and the proton number increase with *λ*, whereas the energy of each proton does not vary. If the laser energy and the pulse duration both increase with *λ* the power remains constant. In Figure 5 we compare the analytic scaling provided by equations (31) and (37) with 1D PIC simulations for a circularly polarized 10*μ*m pulse. The transition between these regimes is evident and the agreement between the analytical and numerical result is quite good. The RPA regime is well suited for CO2 lasers because the native polarization is circular. Slightly overcritical targets are provided by gas jets and in the hole boring regime the choice *n* � *nc* is necessary in order to achieve high energies, according to (31). In addition a quasi monochromatic spectrum is obtained at moderate intensities typical of *CO*<sup>2</sup> lasers. In this case the hole boring scenario is the most promising as recent experiments have shown [Palmer et al. (2010); Pogorelsky (2010a)].

The relativistic mirror model is very attractive on the basis on the analytical results obtained from the 1D model. However 2D and 3D simulations show that a deterioration occurs when the dimensionality is increased, due to the onset of Rayleigh-Taylor like instabilities. Considering also the difficulties met in the preparation of ultrathin targets in order to be close to the transparency limit and the requirements on the contrast, this regime is not likely to be

interesting for application, even using long wavelength pulses, in a near future.

*a λrc* *<sup>h</sup>*<sup>∗</sup> <sup>=</sup> *<sup>N</sup>*<sup>∗</sup>

*n S* <sup>=</sup> *<sup>a</sup>*

*Z λ rc ne*

(40)

*<sup>N</sup>*<sup>∗</sup> <sup>=</sup> *<sup>S</sup> Z*

(solid black line) and the relativistic mirror (dotted line).

*ζ λrc*

*h*<sup>∗</sup> of the target are given by

*<sup>N</sup>* <sup>=</sup> *<sup>n</sup>*

*<sup>Z</sup> h S* <sup>=</sup> *<sup>S</sup> Z*

rigid object behaving like a mirror whose equations of motion are

$$\frac{d\mathbf{x}}{dt} = c\boldsymbol{\mathfrak{E}} \qquad \qquad \frac{d\boldsymbol{\mathfrak{E}}}{dt} = \frac{2I}{\mu c^2} \frac{1-\boldsymbol{\mathfrak{E}}}{1+\boldsymbol{\mathfrak{E}}} (1-\boldsymbol{\mathfrak{E}}^2)^{3/2} \tag{32}$$

where *μ* is the surface density of the mirror and *I* = *I*<sup>0</sup> *f* ((*t* − *x*/*c*)/*τ*laser) is the laser pulse. The function *f*(*s*) vanishes except for |*s*| < 1 where it is positive. The equations of motion 30 have a first integral of motion. Setting

$$\mathbf{t}' = \frac{\mathbf{t}}{\tau\_{\text{laser}}}, \qquad \mathbf{x}' = \frac{\mathbf{x}}{c\tau\_{\text{laser}}}, \qquad w = t' - \mathbf{x}' \qquad \chi = \frac{2I\_0 \,\tau\_{\text{laser}}}{\mu c^2} \tag{33}$$

the equations of motion become

$$\frac{d\beta}{dt'} = \chi \ f(w) \ \frac{1-\beta}{1+\beta} \ (1-\beta^2)^{3/2} \qquad\qquad \frac{dw}{dt'} = 1-\beta \tag{34}$$

and introducing the integrating factor *<sup>C</sup>* = (<sup>1</sup> <sup>+</sup> *<sup>β</sup>*)(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)−1(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)−3/2 the differential form *dH* = *C* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)*d<sup>β</sup>* <sup>−</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)/(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*) *dw* becomes exact and the first integral is

$$H = \chi \int\_{-1}^{w} f(w') dw' - \left(\frac{1+\beta}{1-\beta}\right)^{1/2} \tag{35}$$

The initial condition corresponds to *w* = −1, *β* = 0 so that *H* = −1. At the end of the pulse we have *<sup>w</sup>* <sup>=</sup> 1 and *<sup>β</sup>* <sup>=</sup> *<sup>β</sup>*<sup>∗</sup> which is the highest speed value. Denoting by *<sup>F</sup>* <sup>=</sup> <sup>1</sup> <sup>−</sup><sup>1</sup> *<sup>f</sup>*(*w*)*dw* the fluence we have *<sup>χ</sup><sup>F</sup>* <sup>−</sup> [(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*∗)/(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*∗)]1/2 <sup>=</sup> <sup>−</sup>1. We obtain *<sup>β</sup>*<sup>∗</sup> and *<sup>γ</sup>*<sup>∗</sup> as a function of *α* = *χ F* which is given by

$$
\alpha = \chi F = \frac{2I\_0 \, F \, \tau\_{\text{laser S}} \, \text{S}}{\mu \, c^2 \, \text{S}} = \frac{2E\_{\text{laser}}}{E\_{\text{rest mirr}}} \tag{36}
$$

Expressing *<sup>γ</sup>*<sup>∗</sup> = (1<sup>−</sup> *<sup>β</sup>*<sup>2</sup> <sup>∗</sup>)−1/2 as a function of *<sup>α</sup>* we obtain the expression for the kinetic energy of the ion which is given by

$$E\_{\text{max}} = Am\_p c^2 (\gamma\_\* - 1) = Am\_p c^2 \frac{\alpha^2}{2 + 2\alpha} = \frac{E\_{\text{laser}}}{N} \frac{\alpha}{1 + \alpha} \tag{37}$$

where (36) has been used taking into account *<sup>E</sup>*rest mirr = *NAmpc*<sup>2</sup> where *<sup>N</sup>* is the number of ions in the mirror. As a consequence the efficiency of the acceleration process is given by

$$\eta = \frac{E\_{\text{mirr}}}{E\_{\text{laser}}} = \frac{\alpha}{1+\alpha} \tag{38}$$

From equation (36) it appears that the thinner is the mirror the higher is the efficiency and the protons energy because *μ* = *AZ*−<sup>1</sup> *mp h n*. However a limit is imposed by the transparency limit Macchi et al. (2009). The target remains opaque and is accelerated as a mirror provided that

$$a \le \zeta = \pi \frac{n}{n\_c} \left. \frac{h}{\lambda} \right| \tag{39}$$

12 Will-be-set-by-IN-TECH

*dt* <sup>=</sup> <sup>2</sup>*<sup>I</sup> μc*<sup>2</sup>

where *μ* is the surface density of the mirror and *I* = *I*<sup>0</sup> *f* ((*t* − *x*/*c*)/*τ*laser) is the laser pulse. The function *f*(*s*) vanishes except for |*s*| < 1 where it is positive. The equations of motion 30

, *w* = *t*

<sup>1</sup> <sup>+</sup> *<sup>β</sup>* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2 *dw*

and introducing the integrating factor *<sup>C</sup>* = (<sup>1</sup> <sup>+</sup> *<sup>β</sup>*)(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)−1(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)−3/2 the differential

)*dw*� −

The initial condition corresponds to *w* = −1, *β* = 0 so that *H* = −1. At the end of the pulse

the fluence we have *<sup>χ</sup><sup>F</sup>* <sup>−</sup> [(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*∗)/(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*∗)]1/2 <sup>=</sup> <sup>−</sup>1. We obtain *<sup>β</sup>*<sup>∗</sup> and *<sup>γ</sup>*<sup>∗</sup> as a function of

where (36) has been used taking into account *<sup>E</sup>*rest mirr = *NAmpc*<sup>2</sup> where *<sup>N</sup>* is the number of ions in the mirror. As a consequence the efficiency of the acceleration process is given by

From equation (36) it appears that the thinner is the mirror the higher is the efficiency and the protons energy because *μ* = *AZ*−<sup>1</sup> *mp h n*. However a limit is imposed by the transparency limit Macchi et al. (2009). The target remains opaque and is accelerated as a mirror provided

*a* ≤ *ζ* = *π*

<sup>=</sup> *<sup>α</sup>*

*n nc h*

*<sup>η</sup>* <sup>=</sup> *<sup>E</sup>*mirr *E*laser 1 + *β* 1 − *β*

*<sup>μ</sup> <sup>c</sup>*<sup>2</sup> *<sup>S</sup>* <sup>=</sup> <sup>2</sup>*E*laser

1/2

*E*rest mirr

<sup>2</sup> <sup>+</sup> <sup>2</sup>*<sup>α</sup>* <sup>=</sup> *<sup>E</sup>*laser

*N*

*α*

<sup>1</sup> <sup>+</sup> *<sup>α</sup>* (38)

*<sup>λ</sup>* (39)

<sup>∗</sup>)−1/2 as a function of *<sup>α</sup>* we obtain the expression for the kinetic energy

(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)*d<sup>β</sup>* <sup>−</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)/(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*) *dw*

we have *<sup>w</sup>* <sup>=</sup> 1 and *<sup>β</sup>* <sup>=</sup> *<sup>β</sup>*<sup>∗</sup> which is the highest speed value. Denoting by *<sup>F</sup>* <sup>=</sup> <sup>1</sup>

*<sup>α</sup>* <sup>=</sup> *<sup>χ</sup><sup>F</sup>* <sup>=</sup> <sup>2</sup>*I*<sup>0</sup> *<sup>F</sup> <sup>τ</sup>*laser *<sup>S</sup>*

*<sup>E</sup>*max <sup>=</sup> *Ampc*2(*γ*<sup>∗</sup> <sup>−</sup> <sup>1</sup>) = *Ampc*<sup>2</sup> *<sup>α</sup>*<sup>2</sup>

1 − *β*

<sup>1</sup> <sup>+</sup> *<sup>β</sup>* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2 (32)

*<sup>μ</sup> <sup>c</sup>*<sup>2</sup> (33)

becomes exact and the

<sup>1</sup> <sup>+</sup> *<sup>α</sup>* (37)

(35)

(36)

<sup>−</sup><sup>1</sup> *<sup>f</sup>*(*w*)*dw*

*dt*� <sup>=</sup> <sup>1</sup> <sup>−</sup> *<sup>β</sup>* (34)

� <sup>−</sup> *<sup>x</sup>*� *<sup>χ</sup>* <sup>=</sup> <sup>2</sup>*I*<sup>0</sup> *<sup>τ</sup>*laser

rigid object behaving like a mirror whose equations of motion are

, *<sup>x</sup>*� <sup>=</sup> *<sup>x</sup>*

*dt*� <sup>=</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) <sup>1</sup> <sup>−</sup> *<sup>β</sup>*

*H* = *χ*

 *w* −1 *f*(*w*�

*cτ*laser

*dt* <sup>=</sup> *<sup>c</sup><sup>β</sup> <sup>d</sup><sup>β</sup>*

*dx*

have a first integral of motion. Setting

*dβ*

*t* � <sup>=</sup> *<sup>t</sup> τ*laser

the equations of motion become

*α* = *χ F* which is given by

Expressing *<sup>γ</sup>*<sup>∗</sup> = (1<sup>−</sup> *<sup>β</sup>*<sup>2</sup>

that

of the ion which is given by

form *dH* = *C*

first integral is

Fig. 5. Protons energy in MeV versus intensity for a pulse with *λ* = 10*μ*m. various foil thickness are considered: = 5*μ*m (red), = 10*μ*m (green), = 15*μ*m (Blue), = 30*μ*m (purple). The 1D PIC results are compared with the analytical results for the RPA hole boring (solid black line) and the relativistic mirror (dotted line).

where *n* denotes the electrons density. The number *N* of ions in the target and the minimum number *N*<sup>∗</sup> below which transparency is induced and the corresponding minimum thickness *h*<sup>∗</sup> of the target are given by

$$N = \frac{n}{Z}h \\ S = \frac{S}{Z} \begin{array}{c} \frac{S}{\lambda r\_{\odot}} \\ \end{array} \qquad \qquad N\_{\*} = \frac{S}{Z} \begin{array}{c} a \\ \frac{\lambda r\_{\odot}}{\lambda r\_{\odot}} \\ \end{array} \qquad \qquad h\_{\*} = \frac{N\_{\*}}{n \cdot S} = \frac{a}{Z \cdot \lambda \, r\_{\odot} \, n\_{\odot}} \tag{40}$$

In this regime, keeping the ratio *h*/*λ* fixed, the volume of the accelerated ions increases as *λ*3, their number as *λ* if *n*/*nc* is also kept fixed. As a consequence, at the transparency limit the parameter *a* and the laser power have a fixed value. The parameter *α* is constant if the energy is kept proportional to *λ*. In this case the total energy and the proton number increase with *λ*, whereas the energy of each proton does not vary. If the laser energy and the pulse duration both increase with *λ* the power remains constant. In Figure 5 we compare the analytic scaling provided by equations (31) and (37) with 1D PIC simulations for a circularly polarized 10*μ*m pulse. The transition between these regimes is evident and the agreement between the analytical and numerical result is quite good. The RPA regime is well suited for CO2 lasers because the native polarization is circular. Slightly overcritical targets are provided by gas jets and in the hole boring regime the choice *n* � *nc* is necessary in order to achieve high energies, according to (31). In addition a quasi monochromatic spectrum is obtained at moderate intensities typical of *CO*<sup>2</sup> lasers. In this case the hole boring scenario is the most promising as recent experiments have shown [Palmer et al. (2010); Pogorelsky (2010a)].

The relativistic mirror model is very attractive on the basis on the analytical results obtained from the 1D model. However 2D and 3D simulations show that a deterioration occurs when the dimensionality is increased, due to the onset of Rayleigh-Taylor like instabilities. Considering also the difficulties met in the preparation of ultrathin targets in order to be close to the transparency limit and the requirements on the contrast, this regime is not likely to be interesting for application, even using long wavelength pulses, in a near future.

and protons density at two different times exhibiting the formation of the channel are shown in figures 6 and 7 for a 3D simulation. The spectrum of the protons is still exponential and

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 429

Fig. 6. Electrons (left frame) and protons (right frame) density in the case of a laser pulse of 200 TW, with wavelength *λ* = 0.8 *μ*m, pulse duration *τ* = 25 fs and *a* = 32, incident on a target 30 *μ*m thick of density *n* = *nc* at time *t* = 40*μ*/*c* from the the beginning of the laser

the angular dispersion is better than for TNSA. In figure 8 we show the energy spectrum for a 200 TW laser pulse, the same as for figures 6 and 7. When a target is slightly overcritical it is transparent if the intensity is high enough. Indeed one has to take into account the self

As a consequence by increasing the intensity the medium becomes transparent and the laser drills a channel. If the intensities are moderate this effect is small and a medium at a few times the critical density remains opaque and the hole boring acceleration prevails. Using a *CO*<sup>2</sup> laser with a gas jet one can have both regimes. By increasing the density at a fixed intensity the medium looses its transparency and the highest protons energy decreases reaches a minimum

*<sup>γ</sup> <sup>π</sup> λ*2*rc*

*a* (45)

*nc* <sup>=</sup> *<sup>π</sup> λ*2*rc*

and then increases because the RPA hole boring regime sets in [Willingale (2009)].

target interaction. The simulation is 3D.

Fig. 7. The same as figure 6 at time *t* = 60*μ*/*c*

induced transparency since the critical density becomes

#### **5. Near critical targets**

The acceleration of protons on targets with a nearly critical electron density has been recently investigated on several experiments and PIC simulations. Positive features of this type of targets are a better efficiency in the energy transfer from the laser to the electrons and the absence of debris since the medium is transparent. The optimal length of the target is a few times the length of the laser pulse which drills a channel and accelerates longitudinally the electrons which create a magnetic field circulating around the beam axis. The magnetic field moves behind the laser pulse until it exits in the vacuum where it expands; the electrons, whose energy is dissipated, are displaced by the magnetic field and create a quasi static electric field. The ions coming from the filament around the axis are accelerated and collimated. More specifically the mechanism controlling the proton acceleration is provided by the formation of a slowly evolving magnetic dipole (a toroidal configuration in 3D geometry) behind the leading laser pulse. This structure is generated by the coherent return axial current due to the accelerated electron beam, which contains a large fraction of the laser pulse energy. The magnetic vortex, when exiting on a low density (or a vacuum) region, expands symmetrically thus creating a strong induction axial electric field. At higher electron density *nc* < *n* < 3*nc* this mechanism is the most effective in the acceleration process. At lower density *n* < *nc*, a significant contribution comes also from the electrostatic field due to charge separation at the channel rear side, much alike the TNSA regime.

The maximum energy of protons depends on the target thickness and density and a scaling law is obtained by equating the laser energy to the electrons energy, following the waveguide model, provided that the length of the plasma channel *h* is much larger than the length of the laser pulse *h* � *Lp* to insure that the depletion of the laser energy is complete. Using equation (15), *I* = *a*<sup>2</sup> *mc*3*nc*, the laser energy in a channel reads

$$E\_{\rm laser} = \pi \mathbf{R}^2 \,\mathrm{\tau}I = \pi \mathbf{R}^2 L\_{\rm laser} \, a^2 \, m c^2 \, n\_\odot \tag{41}$$

The electrons energy is given by

$$E\_{\rm el} = \pi R^2 \hbar \, n \, a \, m c^2 \tag{42}$$

and equating the energies we obtain

$$a \sim \frac{nh}{n\_{\text{c}} L\_{\text{laser}}} \tag{43}$$

Another scaling provides the optimal channel length which is given by

$$m \sim h^{3/2} \tag{44}$$

If the transition to the vacuum is not abrupt but the target with electron density *n* and length *h* continues with a decreasing density before reaching the vacuum, some improvements on the top energy and the collimation can be obtained. The main advantages with respect to the TNSA regime are that the energies reached are two or three times higher, the collimation is improved and the efficiency is higher. For wavelength in the micron range the problem is to find the right targets. This is solved naturally for pulses with wavelength in the 10 *μm* range, since gas jets can be used. Indeed promising results have been obtained in recent experiments. We have performed several 2D and 3D PIC simulations of quasi critical targets. The electrons 14 Will-be-set-by-IN-TECH

The acceleration of protons on targets with a nearly critical electron density has been recently investigated on several experiments and PIC simulations. Positive features of this type of targets are a better efficiency in the energy transfer from the laser to the electrons and the absence of debris since the medium is transparent. The optimal length of the target is a few times the length of the laser pulse which drills a channel and accelerates longitudinally the electrons which create a magnetic field circulating around the beam axis. The magnetic field moves behind the laser pulse until it exits in the vacuum where it expands; the electrons, whose energy is dissipated, are displaced by the magnetic field and create a quasi static electric field. The ions coming from the filament around the axis are accelerated and collimated. More specifically the mechanism controlling the proton acceleration is provided by the formation of a slowly evolving magnetic dipole (a toroidal configuration in 3D geometry) behind the leading laser pulse. This structure is generated by the coherent return axial current due to the accelerated electron beam, which contains a large fraction of the laser pulse energy. The magnetic vortex, when exiting on a low density (or a vacuum) region, expands symmetrically thus creating a strong induction axial electric field. At higher electron density *nc* < *n* < 3*nc* this mechanism is the most effective in the acceleration process. At lower density *n* < *nc*, a significant contribution comes also from the electrostatic field due to charge separation at the

The maximum energy of protons depends on the target thickness and density and a scaling law is obtained by equating the laser energy to the electrons energy, following the waveguide model, provided that the length of the plasma channel *h* is much larger than the length of the laser pulse *h* � *Lp* to insure that the depletion of the laser energy is complete. Using equation

> *<sup>a</sup>* <sup>∼</sup> *nh ncL*laser

If the transition to the vacuum is not abrupt but the target with electron density *n* and length *h* continues with a decreasing density before reaching the vacuum, some improvements on the top energy and the collimation can be obtained. The main advantages with respect to the TNSA regime are that the energies reached are two or three times higher, the collimation is improved and the efficiency is higher. For wavelength in the micron range the problem is to find the right targets. This is solved naturally for pulses with wavelength in the 10 *μm* range, since gas jets can be used. Indeed promising results have been obtained in recent experiments. We have performed several 2D and 3D PIC simulations of quasi critical targets. The electrons

Another scaling provides the optimal channel length which is given by

*<sup>E</sup>*laser <sup>=</sup> *<sup>π</sup>R*<sup>2</sup> *<sup>τ</sup><sup>I</sup>* <sup>=</sup> *<sup>π</sup>R*2*L*laser *<sup>a</sup>*<sup>2</sup> *mc*<sup>2</sup> *nc* (41)

*<sup>E</sup>*el <sup>=</sup> *<sup>π</sup>R*<sup>2</sup> *h n a mc*<sup>2</sup> (42)

*<sup>n</sup>* <sup>∼</sup> *<sup>h</sup>*3/2 (44)

(43)

**5. Near critical targets**

channel rear side, much alike the TNSA regime.

(15), *I* = *a*<sup>2</sup> *mc*3*nc*, the laser energy in a channel reads

The electrons energy is given by

and equating the energies we obtain

and protons density at two different times exhibiting the formation of the channel are shown in figures 6 and 7 for a 3D simulation. The spectrum of the protons is still exponential and

Fig. 6. Electrons (left frame) and protons (right frame) density in the case of a laser pulse of 200 TW, with wavelength *λ* = 0.8 *μ*m, pulse duration *τ* = 25 fs and *a* = 32, incident on a target 30 *μ*m thick of density *n* = *nc* at time *t* = 40*μ*/*c* from the the beginning of the laser target interaction. The simulation is 3D.

Fig. 7. The same as figure 6 at time *t* = 60*μ*/*c*

the angular dispersion is better than for TNSA. In figure 8 we show the energy spectrum for a 200 TW laser pulse, the same as for figures 6 and 7. When a target is slightly overcritical it is transparent if the intensity is high enough. Indeed one has to take into account the self induced transparency since the critical density becomes

$$m\_{\mathcal{C}} = \frac{\pi}{\lambda^2 r\_{\mathcal{C}}} \gamma \simeq \frac{\pi}{\lambda^2 r\_{\mathcal{C}}} a \tag{45}$$

As a consequence by increasing the intensity the medium becomes transparent and the laser drills a channel. If the intensities are moderate this effect is small and a medium at a few times the critical density remains opaque and the hole boring acceleration prevails. Using a *CO*<sup>2</sup> laser with a gas jet one can have both regimes. By increasing the density at a fixed intensity the medium looses its transparency and the highest protons energy decreases reaches a minimum and then increases because the RPA hole boring regime sets in [Willingale (2009)].

of transport was performed with a proton bunch accelerated by the laser PHELIX with an energy spectrum up to 30 MeV using quadrupoles or a solenoid, which proved to keep the emittance to a lower value [Hofmann (2009; 2011)]. A proposal was made for a 100 MeV device capable of delivering 10<sup>5</sup> protons per shot on the tissues starting from a 300 TW laser beam and *a* = 60 so that a dose of 40 Gy could be delivered on a target tissue of 0.03 g in 2 minutes at 10 Hz [Sakaki et al. (2009)] using a gantry with quadrupoles and bending magnets. Radiobiology experiments were carried out on cancer cells irradiating them with proton bunches of 0.8-2.4 MeV, obtained from laser accelerations, and the break-up of DNA double strands was observed [Yogo (2009)]. A design study for post-acceleration of a 10 MeV beam into a DTL was carried out showing that with a moderate power laser *W* ≤ 100 TW and the use of microlenses right after the interaction region an injectable beam with parameters

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 431

Fig. 9. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm, where the *B* field is described by a function like *B*(*z*) = 1/(1 + *e*−*z*/*λ*). An energy cut is set at 5 < *E* < 5.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy

Fig. 10. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 15.6 cm, *B* = 10 Tesla and *λ* = 2 cm. An energy cut is set at 9.5 < *E* < 10.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue)

We have simulated the transport of a proton beam produced with a 30 TW laser pulse having a waist of 3 *<sup>μ</sup>*m so that *<sup>I</sup>* <sup>=</sup> 2 1020 W/cm<sup>2</sup> and *<sup>a</sup>* � 10. The simulated target was 0.5 *<sup>μ</sup>*m thick, orthogonal to the *z* propagation axis, with *n* = 40*nc*, with a 2 *μ*m coating with *n* = 2*nc* and a 50 nm layer of contaminants with *n* = 9*nc* on the opposite side. The maximum energy was ∼ 14 MeV. We have placed a collimator formed by a screen with a hole of 0.5 mm radius at 1

suitable for therapy could be obtained [Antici (2011)]

spectra. The laser has the same parameters as figure 2.

cm from the interaction region followed by a solenoid.

and final (red) energy spectra

Fig. 8. Energy spectrum for a target at critical density for a laser pulse with *W* = 200 TW for the same parameters as figures 6

#### **6. The transport**

The use of protons or ions beams for medical therapy has to face severe constraints, which are not yet met by present laser produced ion bunches. Indeed the energy range is 60 ≤ *E* ≤ 250 MeV, the average overall dose is 60 Gray (1 Gy corresponds to 1 mJ per gram) and the full dose delivery is reached in several treatments. Assuming that the dose for a single proton session is 10 Gy and that it is delivered in 2 minutes with 10 Hz pulses, the number of protons per shot to reach this dose on a 1 g tissue would be 106. For a TNSA beam with a maximum energy slightly above 60 MeV, such an intensity is not easily achievable because the energy spectrum is exponential and the average energy is much lower (1/7 in 3D simulations) than the maximum energy. The situation for beams obtained from the interaction with a quasi critical target is more favorable, since the collimation is better and the maximum energy is higher. The energy currently reached with compact Ti:Sa lasers are around 20 MeV. This value has been recently overcome with a CO2 laser working with a gas target, where a monochromatic protons bunch was produced. As a consequence, presently the conditions to use for therapy the proton beams accelerated by compact high repetition lasers are not yet met. The increase of power from 100-200 TW to 1 PW is likely to allow to reach the threshold of 60 MeV of maximum energy for therapeutic use even though the intensity might be lower than required.

A possible alternative consists in maintaining the energy in the 10-30 MeV range and post-accelerating the beam. Suitable devices have already been developed to accelerate a proton beam coming from a cyclotron in this energy range. The injection energy varies from 10 MeV for a rather large DTL device [Antici (2011)] to 30 MeV for compact high field linacs like ACLIP [Amaldi et al. (2009)]. Preliminary experiments and some simulations have been already carried out. The injection of a beam in a RF cavity at 1 Hz for monochromatizing it has been experimentally proved even though at low energy (2 MeV) [Nishiuchi (2010)]. Several experiments on beam transport have been performed using quadrupoles or solenoids to focus it. A beam with *E* ≤ 14 MeV has been transported through a line formed by two collimators and two permanent magnetic quadrupoles [Schollmeier (2008)]. An experiment 16 Will-be-set-by-IN-TECH

Fig. 8. Energy spectrum for a target at critical density for a laser pulse with *W* = 200 TW for

The use of protons or ions beams for medical therapy has to face severe constraints, which are not yet met by present laser produced ion bunches. Indeed the energy range is 60 ≤ *E* ≤ 250 MeV, the average overall dose is 60 Gray (1 Gy corresponds to 1 mJ per gram) and the full dose delivery is reached in several treatments. Assuming that the dose for a single proton session is 10 Gy and that it is delivered in 2 minutes with 10 Hz pulses, the number of protons per shot to reach this dose on a 1 g tissue would be 106. For a TNSA beam with a maximum energy slightly above 60 MeV, such an intensity is not easily achievable because the energy spectrum is exponential and the average energy is much lower (1/7 in 3D simulations) than the maximum energy. The situation for beams obtained from the interaction with a quasi critical target is more favorable, since the collimation is better and the maximum energy is higher. The energy currently reached with compact Ti:Sa lasers are around 20 MeV. This value has been recently overcome with a CO2 laser working with a gas target, where a monochromatic protons bunch was produced. As a consequence, presently the conditions to use for therapy the proton beams accelerated by compact high repetition lasers are not yet met. The increase of power from 100-200 TW to 1 PW is likely to allow to reach the threshold of 60 MeV of maximum energy for therapeutic use even though the intensity might be lower

A possible alternative consists in maintaining the energy in the 10-30 MeV range and post-accelerating the beam. Suitable devices have already been developed to accelerate a proton beam coming from a cyclotron in this energy range. The injection energy varies from 10 MeV for a rather large DTL device [Antici (2011)] to 30 MeV for compact high field linacs like ACLIP [Amaldi et al. (2009)]. Preliminary experiments and some simulations have been already carried out. The injection of a beam in a RF cavity at 1 Hz for monochromatizing it has been experimentally proved even though at low energy (2 MeV) [Nishiuchi (2010)]. Several experiments on beam transport have been performed using quadrupoles or solenoids to focus it. A beam with *E* ≤ 14 MeV has been transported through a line formed by two collimators and two permanent magnetic quadrupoles [Schollmeier (2008)]. An experiment

the same parameters as figures 6

**6. The transport**

than required.

of transport was performed with a proton bunch accelerated by the laser PHELIX with an energy spectrum up to 30 MeV using quadrupoles or a solenoid, which proved to keep the emittance to a lower value [Hofmann (2009; 2011)]. A proposal was made for a 100 MeV device capable of delivering 10<sup>5</sup> protons per shot on the tissues starting from a 300 TW laser beam and *a* = 60 so that a dose of 40 Gy could be delivered on a target tissue of 0.03 g in 2 minutes at 10 Hz [Sakaki et al. (2009)] using a gantry with quadrupoles and bending magnets. Radiobiology experiments were carried out on cancer cells irradiating them with proton bunches of 0.8-2.4 MeV, obtained from laser accelerations, and the break-up of DNA double strands was observed [Yogo (2009)]. A design study for post-acceleration of a 10 MeV beam into a DTL was carried out showing that with a moderate power laser *W* ≤ 100 TW and the use of microlenses right after the interaction region an injectable beam with parameters suitable for therapy could be obtained [Antici (2011)]

Fig. 9. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm, where the *B* field is described by a function like *B*(*z*) = 1/(1 + *e*−*z*/*λ*). An energy cut is set at 5 < *E* < 5.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy spectra. The laser has the same parameters as figure 2.

Fig. 10. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 15.6 cm, *B* = 10 Tesla and *λ* = 2 cm. An energy cut is set at 9.5 < *E* < 10.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy spectra

We have simulated the transport of a proton beam produced with a 30 TW laser pulse having a waist of 3 *<sup>μ</sup>*m so that *<sup>I</sup>* <sup>=</sup> 2 1020 W/cm<sup>2</sup> and *<sup>a</sup>* � 10. The simulated target was 0.5 *<sup>μ</sup>*m thick, orthogonal to the *z* propagation axis, with *n* = 40*nc*, with a 2 *μ*m coating with *n* = 2*nc* and a 50 nm layer of contaminants with *n* = 9*nc* on the opposite side. The maximum energy was ∼ 14 MeV. We have placed a collimator formed by a screen with a hole of 0.5 mm radius at 1 cm from the interaction region followed by a solenoid.

of depth exhibits a sharp peak, known as Bragg peak. Healthy tissues are spared and this therapy is applicable to the most severe cases which are not suitable for surgery. The protons are usually accelerated by cyclotrons and a large gantry is needed to rotate the beam around the patient. Carbon ions are accelerated by synchrotrons, which have a larger size, require heavy transport lines and a very large gantry. Even though the number of centers for protons and ions therapy is increasing they are up to now limited to national facilities. A reduction of size and cost of a protons accelerator for therapy would allow the creation of regional centers extending this treatment to a larger fraction of patients. Devices based on innovative techniques such as the superconducting cyclotron or the dielectric wall accelerator have been proposed but conclusive results have not yet been achieved. Laser acceleration of protons has entered this competition even though several years are needed before the feasibility is actually demonstrated. The typical dose for therapy is 60 Gy which means 60 mJ on a mass of 1 g. Split over 6 session this amounts to 10 mJ and corresponds to 10<sup>9</sup> protons of 60 MeV which is the threshold for very superficial tumors. Supposing each bunch contains 106 protons at 10 Hz repetition rate this dose is delivered in a couple of minutes. The major problem is that the energy and intensity required can hardly be reached with compact existing Ti:Sa lasers. Suppose that a bunch of 10<sup>12</sup> protons is accelerated by TNSA having an average energy of 10 MeV and maximum energy of 70 MeV, according to previous scaling, then the total proton energy is 1.6 J and supposing a 10% efficiency the laser energy would be 16 J. Since the spectrum is exponential the fraction of protons at 60 MeV with Δ*E*/*E* = 1% would be 1.5 108. This is a very demanding requirement on the laser. If we choose instead an average energy of 5 MeV we would have the same number of protons at an energy of 30 MeV, for the same

Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 433

After post-acceleration one could reach not only the 60 MeV threshold but also higher

The real problem with TNSA or improved TNSA, achieved with a foam deposition at quasi critical density on the illuminated surface, is that the protons having the desired energy are a very small fraction of the total and carry out a very small fraction of the total energy. Increasing the repetition rate form 10 to 100 Hz would help but would not solve the problem. The way out is to produce a quasi monochromatic spectrum. This has been achieved with a CO2 laser on a gas jet and this result is of extreme interest. The use of ultrathin nanometric targets allows to obtain quasi monochromatic beams via RPA, but in spite of the scientific interest this regime is still very far out from possible applications due to the the extremely high contrast required on the laser beam. The use of gas targets and a suitably shaped beam pulse seem to be the corner stones in the production of a quasi monochromatic beam. In this respect the CO2 laser beams have an advantage with respect to pulses of shorter wavelength. For a quasi monochromatic beam the intensity is still rather low because only a small fraction of the laser energy is transferred to the beam. Once the energy and intensity requirements are satisfied other conditions have to be satisfied to render the proton beam suitable for therapy: the shot to shot stability must be kept within a narrow range and suitable dose control systems have to be developed [Linz & Alonso (2007)]. As a consequence it will take several years before laser accelerators can meet the requirements for clinical use. During this period the laser performance will be improved, new targets will be developed, transport and post acceleration systems will be tested and beam quality and stability will be pursued. Even though most of the research activity will be devoted to short wavelength lasers, the

Δ*E*/*E*.

energies, suitable for deep tumors.

Fig. 11. Left frame: emittances *�<sup>x</sup>* (red) and *�<sup>y</sup>* (blue) for a beam produced with a 30 TW laser pulse transported through a chicane (selecting around 5 MeV), an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm. Middle frame: envelopes *σ<sup>x</sup>* (red) and *σ<sup>y</sup>* (blue) Right frame: initial (blue) and final (red) energy spectra

In figure 9 we show the emittance and the envelope for a small fraction of the bunch, with a cut in the energy spectrum between 5 and 5.5 MeV. We have considered also the transport of the fraction of the beam with an energy between 9.5 MeV and 10.5 MeV. The results are comparable and the emittance and envelopes are shown in Figure 10. An energy selection can be made with a physical device such as a chicane or an RF cavity to achieve the rotation in the longitudinal phase space. The chicane applied to the full bunch leads to an emittance growth along the axis where the dipoles bend the beam. By applying suitable collimators the emittance and envelopes can be reduced to reasonable values but only a small fraction of the beam reaches the end of the transport line. In Figure 11, we show the full bunch propagating along a chicane and then focusing in a solenoid with field of *B* = 10 Tesla.

Other more effective methods based on selection of particles at the desired energy by putting collimator at the corresponding focus point of a solenoid are under investigation.

No transport experiments have been performed with CO2 laser accelerated proton beams, but the situation is certainly much more favorable if a quasi-monochromatic peak can be obtained with adequate intensity [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. In this case no energy selection is required and the transport on the beam becomes quite easy because tight focusing can be achieved preventing emittance increase and keeping the beam size to small values suitable for injection in a linac.

#### **7. Conclusions: Protons and ions therapy**

Cancer treatment, a priority for health care in advanced countries, is based on surgery, chemo-therapy and radiation therapy. Early detection of tumors increases the survival period but there are limits to massive and frequent screening. Unlike chemotherapy radiation therapy killing of malignant cells is quite homogeneous and can be effective even for massive tumors. The most common treatment is based on X rays, driven by electron accelerators. The reason is compactness of the accelerating device and moderate cost, affordable by medium size hospitals. However most of ionizing radiations, including X rays, exhibit a peak in dose deposition close to the entry point and a subsequent exponential decay. As a consequence the dose deposition in healthy tissues is important, even when this undesirable effect is minimized by irradiating from different directions and modulating the intensity (IMRT). The dose deposition mechanism for protons and ions is different and the dose curve as function 18 Will-be-set-by-IN-TECH

Fig. 11. Left frame: emittances *�<sup>x</sup>* (red) and *�<sup>y</sup>* (blue) for a beam produced with a 30 TW laser pulse transported through a chicane (selecting around 5 MeV), an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm. Middle frame: envelopes *σ<sup>x</sup>* (red) and *σ<sup>y</sup>* (blue) Right frame: initial (blue) and final (red) energy spectra

In figure 9 we show the emittance and the envelope for a small fraction of the bunch, with a cut in the energy spectrum between 5 and 5.5 MeV. We have considered also the transport of the fraction of the beam with an energy between 9.5 MeV and 10.5 MeV. The results are comparable and the emittance and envelopes are shown in Figure 10. An energy selection can be made with a physical device such as a chicane or an RF cavity to achieve the rotation in the longitudinal phase space. The chicane applied to the full bunch leads to an emittance growth along the axis where the dipoles bend the beam. By applying suitable collimators the emittance and envelopes can be reduced to reasonable values but only a small fraction of the beam reaches the end of the transport line. In Figure 11, we show the full bunch propagating

Other more effective methods based on selection of particles at the desired energy by putting

No transport experiments have been performed with CO2 laser accelerated proton beams, but the situation is certainly much more favorable if a quasi-monochromatic peak can be obtained with adequate intensity [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. In this case no energy selection is required and the transport on the beam becomes quite easy because tight focusing can be achieved preventing emittance increase and keeping the beam size to small values suitable for

Cancer treatment, a priority for health care in advanced countries, is based on surgery, chemo-therapy and radiation therapy. Early detection of tumors increases the survival period but there are limits to massive and frequent screening. Unlike chemotherapy radiation therapy killing of malignant cells is quite homogeneous and can be effective even for massive tumors. The most common treatment is based on X rays, driven by electron accelerators. The reason is compactness of the accelerating device and moderate cost, affordable by medium size hospitals. However most of ionizing radiations, including X rays, exhibit a peak in dose deposition close to the entry point and a subsequent exponential decay. As a consequence the dose deposition in healthy tissues is important, even when this undesirable effect is minimized by irradiating from different directions and modulating the intensity (IMRT). The dose deposition mechanism for protons and ions is different and the dose curve as function

along a chicane and then focusing in a solenoid with field of *B* = 10 Tesla.

injection in a linac.

**7. Conclusions: Protons and ions therapy**

collimator at the corresponding focus point of a solenoid are under investigation.

of depth exhibits a sharp peak, known as Bragg peak. Healthy tissues are spared and this therapy is applicable to the most severe cases which are not suitable for surgery. The protons are usually accelerated by cyclotrons and a large gantry is needed to rotate the beam around the patient. Carbon ions are accelerated by synchrotrons, which have a larger size, require heavy transport lines and a very large gantry. Even though the number of centers for protons and ions therapy is increasing they are up to now limited to national facilities. A reduction of size and cost of a protons accelerator for therapy would allow the creation of regional centers extending this treatment to a larger fraction of patients. Devices based on innovative techniques such as the superconducting cyclotron or the dielectric wall accelerator have been proposed but conclusive results have not yet been achieved. Laser acceleration of protons has entered this competition even though several years are needed before the feasibility is actually demonstrated. The typical dose for therapy is 60 Gy which means 60 mJ on a mass of 1 g. Split over 6 session this amounts to 10 mJ and corresponds to 10<sup>9</sup> protons of 60 MeV which is the threshold for very superficial tumors. Supposing each bunch contains 106 protons at 10 Hz repetition rate this dose is delivered in a couple of minutes. The major problem is that the energy and intensity required can hardly be reached with compact existing Ti:Sa lasers. Suppose that a bunch of 10<sup>12</sup> protons is accelerated by TNSA having an average energy of 10 MeV and maximum energy of 70 MeV, according to previous scaling, then the total proton energy is 1.6 J and supposing a 10% efficiency the laser energy would be 16 J. Since the spectrum is exponential the fraction of protons at 60 MeV with Δ*E*/*E* = 1% would be 1.5 108. This is a very demanding requirement on the laser. If we choose instead an average energy of 5 MeV we would have the same number of protons at an energy of 30 MeV, for the same Δ*E*/*E*.

After post-acceleration one could reach not only the 60 MeV threshold but also higher energies, suitable for deep tumors.

The real problem with TNSA or improved TNSA, achieved with a foam deposition at quasi critical density on the illuminated surface, is that the protons having the desired energy are a very small fraction of the total and carry out a very small fraction of the total energy. Increasing the repetition rate form 10 to 100 Hz would help but would not solve the problem. The way out is to produce a quasi monochromatic spectrum. This has been achieved with a CO2 laser on a gas jet and this result is of extreme interest. The use of ultrathin nanometric targets allows to obtain quasi monochromatic beams via RPA, but in spite of the scientific interest this regime is still very far out from possible applications due to the the extremely high contrast required on the laser beam. The use of gas targets and a suitably shaped beam pulse seem to be the corner stones in the production of a quasi monochromatic beam. In this respect the CO2 laser beams have an advantage with respect to pulses of shorter wavelength. For a quasi monochromatic beam the intensity is still rather low because only a small fraction of the laser energy is transferred to the beam. Once the energy and intensity requirements are satisfied other conditions have to be satisfied to render the proton beam suitable for therapy: the shot to shot stability must be kept within a narrow range and suitable dose control systems have to be developed [Linz & Alonso (2007)]. As a consequence it will take several years before laser accelerators can meet the requirements for clinical use. During this period the laser performance will be improved, new targets will be developed, transport and post acceleration systems will be tested and beam quality and stability will be pursued. Even though most of the research activity will be devoted to short wavelength lasers, the

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#### **8. Acknowledgments**

We would like to thank the Italian Ministry of Foreign Affairs (MAE) for a grant we received for the scientific cooperation with Japan to develop the research project PROMETHEUS, devoted to a research infrastructure on laser driven proton sources for biomedical applications. We thank the Fondazione del Monte di Bologna e Ravenna for a grant devoted to the feasibility study of a hybrid accelerator devoted to biomedical Research within the framework of PROMETHEUS. We acknowledge the Alma Mater Foundation for the governance of the PROMETHEUS project.

#### **9. References**


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20 Will-be-set-by-IN-TECH

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### *Edited by Dan C. Dumitras*

The present book includes several contributions aiming a deeper understanding of the basic processes in the operation of CO2 lasers (lasing on non-traditional bands, frequency stabilization, photoacoustic spectroscopy) and achievement of new systems (CO2 lasers generating ultrashort pulses or high average power, lasers based on diffusion cooled V-fold geometry, transmission of IR radiation through hollow core microstructured fibers). The second part of the book is dedicated to applications in material processing (heat treatment, welding, synthesis of new materials, micro fluidics) and in medicine (clinical applications, dentistry, non-ablative therapy, acceleration of protons for cancer treatment).

CO2 Laser - Optimisation and Application

CO2 Laser

Optimisation and Application

*Edited by Dan C. Dumitras*

Photo by IdealPhoto30 / iStock