**Laser-Produced Heavy Ion Plasmas as Efficient Soft X-Ray Sources**

Takeshi Higashiguchi, Padraig Dunne and Gerry O'Sullivan

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63455

#### **Abstract**

We demonstrate extreme ultraviolet (EUV) and soft x-ray sources in the 2- to 7 -nm spec‐ tral region related to the beyond extreme ultraviolet (BEUV) question at 6.*x* nm and a wa‐ ter window source based on laser-produced high-Z plasmas. Strong emissions from multiply charged ions merge to produce intense unresolved transition array (UTA) to‐ ward extending below the carbon K-edge (4.37 nm). An outline of a microscope design for single-shot live- cell imaging is proposed based on a high-*Z* UTA plasma source, cou‐ pled to x-ray optics. We will discuss the progress and Z-scaling of UTA emission spectra to achieve lab-scale table-top, efficient, high-brightness high-Z plasma EUV-soft x-ray sources for *in vivo* bio-imaging applications.

**Keywords:** High-*Z*, unresolved transition array (UTA), extreme ultraviolet (EUV), soft xray, water window

### **1. Introduction**

Laboratory- scale source development of shorter- wavelength spectral regions in the extreme ultraviolet (EUV) and soft x-ray has been motivated by their applications in a number of highprofile areas of science and technology. One such topic is the challenge of three-dimensional imaging and single-shot flash photography of microscopic biological structures, such as macromolecules and cells, *in vivo*. For x-ray microscopy, the x-ray source should emit a sufficient photon flux to expose the image of the biosample on the detector. Recently, the most practical source of high-power, high-brightness x-rays has been radiation from synchrotrons and x-ray– free electron lasers (XFEL) [1]. Compact sources using liquid nitrogen droplets are being developed for the use of the zone plates for the transmission microscopy. Recently, the wavelength at 2.48- nm narrowband emission from a liquid-nitrogen-jet laser-plasma [2] was

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successfully combined with the latest normal-incidence multilayer condenser optics and 20 nm zone-plate optics to work laboratory water-window x-ray microscopy [3] with resolution less than 25 nm and synchrotron-like image quality on biological and environment science samples. The development of a high-brightness source based on a focused electron beam impacting a liquid water jet resulting in 2.36-nm emission has also been studied [4]. The total collected energy, however, is low, when one combines the narrowband line emission with the low reflection coefficient of the collector mirror. As a result, long exposures are needed to take a picture, and there is not yet published evidence of single-shot, flash exposures by the use of a laboratory-scale source. In order to overcome the low efficiency imposed by line emission sources, we propose the use of high- power water-window emission from laser-produced high-*Z* plasmas, analogous to the extending scheme of efficient, high-volume manufacturing EUV sources.

High-power EUV sources with high efficiency for semiconductor lithography at 13.5 [5] and 6.7 nm [6–8] based on laser-produced plasmas (LPP) have been demonstrated in high-volume manufacturing of integrated circuits (IC) having node sizes of 22 nm or less [9, 10]. The EUV emission at the relevant wavelength may be coupled with La/B4C or Mo/B4C multilayer mirror with a reflectivity of 40% to provide a source at 6.5−6.7 nm. Recently, a reflection coefficient of about 60−70% was shown to be feasible in a theoretical study [11]. Consequently, the devel‐ opment of a new wavelength EUV source for the next- generation semiconductor lithography, which can be coupled with an efficient B4C multilayer mirror, is particularly timely.

High-*Z* element plasmas of Sn and Gd produce strong resonant band emission due to 4*d*−4*f* and 4*p*−4*d* transitions around 13.5 nm and 6.7 nm, respectively, which are overlapped in adjacent ion stages to yield the intense unresolved transition arrays (UTAs) in their spectra. The in-band high-energy emissions are attributable to hundreds of thousands of neardegenerate resonance lines lying within a narrow wavelength range. Rare earth elements gadolinium (Gd) and terbium (Tb) produce strong resonant emission in an intense UTA around 6.5−6.7 nm [6–8]. The choice of these elements was prompted by the use of UTA radiation in tin (Sn) for the strong 13.5-nm emission, where *n* = 4−*n* = 4 transitions in Sn ions overlap to yield an intense UTA [12, 13], as the optimum source for 13.5 nm and the scaling of this emission to the shorter wavelength with increasing *Z*. Because the emitting ions in Gd and Tb plasmas have largely a similar electronic structure to Sn, they are expected to have a similar spectral behavior and emit an intense UTA due to 4*d*−4*f* and 4*p*−4*d* transitions at shorter wavelengths.

Plasmas of the rare earth elements gadolinium (Gd) and terbium (Tb) produce strong resonant emission due to the presence of an intense UTA around 6.5−6.7 nm in the spectra of their ions [6]. In tin (Sn), the presence of the corresponding feature at 13.5 nm prompted its selection as the optimum source material at that wavelength. The UTA emission scales to shorter wave‐ length with increasing atomic number, *Z*. Because the emitting ions in Gd (*Z* = 64) and Tb (*Z* = 65) plasmas have an electronic structure largely similar to Sn, they are expected to exhibit a similar spectral behavior and emit an intense UTA due to 4*d*−4*f* and 4*p*−4*d* transitions at shorter wavelengths. Recently, the suitability of Nd:yttrium-aluminum-garnet (Nd:YAG) LPP EUV sources based on Gd and Tb has been demonstrated for high- power operation [6]. Since at high plasma electron densities, the opacity effects reduce the intensity of the resonance lines thereby limiting the output power, methods of reducing the effects of reabsorption (opacity) were evaluated to achieve high energy conversion efficiency (CE) from the incident laser energy to the EUV emission energy and the spectral purity. The effect of optical thickness was evaluated by changing the laser wavelength to alter the plasma electron density [7, 14]. In order to increase the EUV energy CE and spectral efficiency (purity), the optical thickness in the dominant region of the EUV emission of high-*Z* highly charged plasmas should be controlled. To enhance the EUV emission from Gd plasmas, it is important to reduce reabsorption by the resonance lines and the emission from satellite lines that attribute to the long wavelength side of the array around 6.7 nm to improve the spectral purity as well as increase the resonance emission intensity [7]. In order to achieve this, we used low initial –density targets for the Nd:YAG LPPs [8]. In low-density, optically thin plasmas, a suppression of the reabsorption effect and the satellite emission, which originates from the high electron and ion density region, is expected, similar to the results obtained with low-density Sn targets used to optimize the emission from the Nd:YAG LPP EUV sources at 13.5 nm [15, 16]. It is known that optically thick plasmas can strongly self-absorb resonance emission. Optically, thin plasmas provide more efficient sources. Therefore, systematic LPP UTA source studies with up-to-date intense picosecond pulse lasers [17] or middle infrared laser, such as the CO2 laser [14], are needed to determine available light source wavelengths for future applications.

In this chapter, we show the efficient EUV and soft x-ray sources in the 2- to 7- nm spectral region related to the beyond extreme ultraviolet (BEUV) question at 6.*x* nm and a water window source based on laser-produced high-*Z* plasmas. Resonance emission from multiply charged ions merges to produce intense UTA spectral structure, extending below the carbon K-edge (4.37 nm). An outline of a microscope design for single-shot live cell imaging is proposed based on a high-*Z* plasma UTA source, coupled to x-ray optics. We discuss the progress and *Z*-scaling of UTA emission spectra to achieve lab-scale table-top, efficient, highbrightness high-*Z* plasma EUV–soft x-ray sources for *in vivo* bioimaging applications.

### **2. Characteristics of the Gd plasmas for BEUV source applications**

In order to increase the energy CE from the incident laser energy to the interested wavelength emission energy with the defined bandwidth, it is important to suppress not only the reab‐ sorption by assurance of the plasma is optically thin but also plasma hydrodynamic expansion loss, while maintaining a plasma electron temperature of *Te* = 100−120 eV [6, 17]. Lateral expansion of the plasma causes kinetic energy losses, which reduce the energy available for radiation and is particularly important for small focal spot diameters [6]. For practical EUV source development, it is important to establish the optimum plasma condition related to laser irradiation condition and construct a database of properties of the UTA plasma EUV sources. In addition, to compare with one-dimensional (1D) numerical simulation, it is important to produce 1D expansion plasmas by irradiating multiple laser beams based on the laser inertial confinement fusion (ICF) geometry [18]. Laboratory- scale experiments have, to date, only been studied under 2D conditions due to the use of a single laser beam and small focal spot diameters. Under multiple laser irradiation, it is expected that the highest CE will be achieved as plasma expansion loss can be neglected in plasmas from targets irradiated by solid-state laser pulses. In the database point of view, we demonstrate high CE for the EUV emission around 6.7 nm from multiple laser beam –produced 1D spherical plasmas of rare earth elements of Gd and Tb. The maximum in-band EUV CE at 6.7 nm within a 0.6% bandwidth (0.6%BW) in a solid angle of 2*π* sr was observed to be 0.8%, which is twice as large as that obtained by the use of a Joule-class laboratory- scale single laser beam with 2D or 3D plasma expansion losses. The CE value was one of the highest ever reported due to the reduction of the plasma expansion loss applying 12 laser beams under 1D plasma expansion condition.

A Nd:glass laser system, GEKKO-XII at the Institute of Laser Engineering (ILE) in Osaka University was used to produce the 1D expanding uniform plasma [19]. The GEKKO-XII laser facility consists of 12 laser beams each at a wavelength of 1.053 µm and a constant 1 J pulse energy, irradiating a total energy of 12 J, with a temporal Gaussian- shaped pulse width of 1.3 ns [full width at half maximum (FWHM)]. The 12 laser beams were located at 12 faces of a regular dodecahedron to irradiate spherical targets uniformly. A thick metallic layer of 2 µm was coated onto spherical polystyrene balls for providing targets. The laser power imbalance was monitored to be within ± 6.3% of the average. Then, the laser beams were uniformly irradiated onto the target, to provide a 1D plasma expansion with low expansion loss.

Figure 1 shows the temporal history of the in-band emission around 6.7 nm with the bandwidth of 0.6% from Gd plasmas observed by the x-ray streak camera to provide 1D time-resolved imaging. The red and blue lines are the EUV emission at the optimum intensity of 1 × 1012 W/ cm2 and the maximum intensity of 3 × 1013 W/cm2 , respectively. Under optimum irradiation conditions with the highest CE, the temporal profile of the EUV emission was similar of that of the laser pulse shown by the dashed line and reached a maximum a little later. On the other hand, the behavior of the EUV emission profile at 3 × 1013 W/cm2 initially rose faster, but the peak was delayed by comparison with that obtained under optimum conditions. The initial steep rise indicates that the electron temperature quickly reaches a value necessary for the inband EUV emission. The final electron temperature is expected to be higher than optimum, so that higher charge state ions higher than q = 28 are produced, which predominantly emit shorter- wavelength out-of-band emission around 2−4 nm. After the maximum electron temperature is attained, plasma recombination proceeds accompanied by adiabatic expansion, resulting in cooling. The in-band emission from ionic charge states of *q* ≈ 20 arises in the recombination phase. Then, the time- resolved emission consists of a fast rising component and a delayed peak relative to the laser pulse. This measurement suggests that the temporal shape of the in-band emission should essentially behave similarly to the laser pulse shape under optimum laser irradiation conditions.

The in-band EUV CEs were evaluated at *λ* = 6.7 nm within a bandwidth of 0.6% for Gd and Mo and at *λ* = 6.5 nm with the bandwidth of 0.6% for Tb. The CEs were maximized at 0.8% in both Gd and Tb at *IL* = 1 × 1012 W/cm2 , and the observed maximum CE was almost in agreement with the theoretical evaluation of 0.9% obtained from a collisional–radiative (CR) and modified 1D hydrodynamic code numerical simulation [21]. It is noted that the wavelength of 6.6 nm, predicted in the work, is slightly different compared to our spectral peaks at 6.5 and 6.7 nm. A decrease in CE was also observed at the laser intensity higher than 1 × 1012 W/cm2 . Around these intensities, the rare earth highly charged plasmas are overheated, the average ionization stage increases and the population of relevant ions with *q* ≈ 20 decreases. Then, the CE decreases due to the increase in electron temperature [20].

**Figure 1.** Time-resolved spectral images at two different laser intensities of (a) 1 × 1012 W/cm2 and (b) 3 × 1013 W/cm2 , respectively. (c) Temporal histories of the EUV emission at 6.7 nm from Gd plasmas at two different laser intensities of 1 × 1012 W/cm2 (red) and 3 × 1013 W/cm2 (blue), together with a temporal profile of the laser pulse (dashed). At an opti‐ mum laser intensity of 1 × 1012 W/cm2 , the temporal behavior of the in-band emission is essentially the same as that of the laser pulse. It should be noted that intensities are normalized for timing comparison [20].

In addition, it is important to understand the physics of the EUV emission and transport in laser-produced dense high-*Z* plasmas. In order to achieve an efficient light source, or to diagnose complex highly charged ion (HCI) plasmas, the evaluation of plasma parameters is of fundamental importance in order to benchmark radiation hydrodynamic simulation codes. One matter of fundamental physics is the relationship between the electron density profile and the dominant EUV emission region. In general, dense high-*Z* plasmas are optically thick in the EUV spectral region, and the EUV emission originates from regions of reduced electron density where there is not only sufficient emissivity but also lower effects from opacity. We describe the results of measurements of the electron density profile of a laser produced isotropically expanding spherical Gd plasma using a Mach-Zehnder interferometer, as shown in Figure 2 [22]. The interferometry was performed at a wavelength of 532 nm to enable penetration of the plasmas to a high -density region, which has a maximum density close to the critical density of 1 × 1021 cm−3 as set by the plasma initiating laser wavelength of 1.053 µm. The EUV emission was observed using a monochromatic EUV pinhole camera. We present benchmark data for the electron density profile with the dominant EUV emission at 6.7 nm occurring in a region with an electron density close to 1019 cm−3 [14], which was corresponded to the critical density of the CO2 (carbon dioxide) laser LPP, as shown in Figure 3 [22].

**Figure 2.** Schematic diagram of the experimental setup. Interferograms were produced by a Mach-Zehnder interferom‐ eter by the use of a Nd:YAG laser at a wavelength of 532 nm with a pulse duration of 6 ns (FWHM) [22].

The production of low-density plasma by the use of CO2 LPPs has been proposed, because the critical electron density *n*ec depends on the laser wavelength, *λL*, i.e., *n*ec ∝ *λ<sup>L</sup>* −2. The critical density at a laser wavelength of *λL* = 10.6 µm for a CO2 laser is two orders of magnitude smaller than at *λL* = 1.06 µm for the solid-state laser. Then, a suppression of reabsorption and satellite emission in the wavelength region longer than 6.*x* nm is expected in CO2 LPPs due to the lower plasma electron density. By extending efficient CO2 laser–produced Sn plasma EUV sources around 13.5 nm, the CE and spectral efficiency, which is important when considering out-ofband spectral suppression, should be increased in an optically thin plasma. In order to ascertain the applicability of a CO2 LPP EUV source at 6.*x* nm, its behavior needs to be clarified in a manner similar to the work performed on CO2 LPP EUV sources at 13.5 nm.

We characterize the EUV emission from CO2 laser–produced plasmas (CO2-LPPs) of the rare earth element of Gd. The energy CE and the spectral purity in the CO2-LPPs were higher than that for solid-state LPPs at 1.06 µm, because the plasma produced is optically thin due to the lower critical density, resulting in a maximum CE of 0.7% at 6.76 nm with 0.6% bandwidth in the solid angle of 2π sr. The peak wavelength was fixed at 6.76 nm for all laser intensities. The plasma parameters at a CO2 laser intensity of 1.3 × 1011 W/cm2 was also evaluated using the hydrodynamic simulation code to produce the EUV emission at 6.76 nm.

Figure 4(a) shows time-integrated EUV emission spectra from the Nd:YAG-LPPs at different laser intensities ranging from 9.7 × 1011 to 6.6 × 1012 W/cm2 . The peak wavelength shifts from 6.7 to 6.8 nm and is mainly due to *n* = 4−*n* = 4 (Δ*n* = 0) transitions in HCIs with an open 4*f* or 4*d* outermost subshell. The sharp peak at 6.65 nm and the dip structure below 6.59 nm first appear at a laser intensity of 2.4 × 1011 W/cm2 . The emission at wavelengths less than 6 nm, increases with increasing laser intensity, and according to numerical evaluation, lines in the *λ* = 2.5−6 nm (*hν* = 207−496 eV) spectral region originate from Gd ionic charge states between Gd19+ and Gd27+ and arise from *n* = 4−*n* = 5 (Δ*n* = 1) transitions [14].

In the case of CO2-LPPs, the main spectral behaviors near 6.7 nm, on the other hand, are narrower than for Nd:YAG laser irradiating plasma, as shown in Figure 4(b). The CO2 laser intensity was varied from 5.5 × 1010 to 1.2 × 1011 W/cm2 . The spectral structure was dramatically different to that from the Nd:YAG-LPPs. The peak wavelength of 6.76 nm remains constant with the increase of the laser intensity. Moreover, the emission intensity of the peak at 6.76 nm increases more rapidly with laser intensity than the emission in the ranges of *λ* = 3−6.6 nm and *λ* = 6.8−12 nm, respectively. Under the optically thin plasma conditions imposed by the CO2- LPPs, this peak, which is mainly due to the 4*d*<sup>10</sup> <sup>1</sup> *S*0−4*d*<sup>9</sup> 4*f* <sup>1</sup> *P*<sup>1</sup> transition of Pd-like Gd18+ overlapped with 2 *F*−<sup>2</sup> *D* lines of Ag-like Gd17+, known to lie around 6.76 nm shows that these ions are indeed present in the plasma. Similar structure has been also observed in a dischargeproduced plasma, which has low density and is optically thin like the CO2-LPP. It is noted that

**Figure 3.** Profiles of the radial electron density (solid line) and radial EUV emission (dashed line) at the time of three different peak laser intensities of (a) 1 × 1012 W/cm2 , (b) 7 × 1012 W/cm2 , and (c) 1 × 1014 W/cm2 , corresponding to laser focal spot and target diameters of (a) 500 µm, (b) 200 µm, and (c) 50 µm [22].

the peak wavelength of 6.76 nm was constant with high spectral efficiency (purity) and energy CE in optically thin CO2-LPPs of Gd [14].

**Figure 4.** (a) Time-integrated EUV emission spectra from the Nd:YAG LPPs at different laser intensities of 9.7 × 1011, 2.2 × 1012, and 6.6 × 1012 W/cm2 , respectively. The peak wavelength shifts from 6.7 to 6.8 nm with increasing the laser inten‐ sity. (b) Time-integrated EUV emission spectra from the CO2 LPPs at different laser intensities of 5.5 × 1010, 8 × 1010, 9.8 × 1010, and 1.3 × 1011 W/cm2 , respectively. The peak wavelength of 6.76 nm remains constant with increasing the laser intensity [14].

In order to infer the laser parameters that maximize 6.*x*-nm Gd-LPP emission, direct compar‐ ison between emissions from a laser-produced Gd plasma and that of Gd ions from welldefined charge states is necessary, as the charge state dependence of the emission at 6.*x* nm is defined by the electron temperature. We present a study of the charge state–defined emission spectra to explain the laser power density dependence of the Gd-LPP spectra and to evaluate the charge states contributing to the 6.*x*-nm emission.

The profile of the intense emission at 6.*x* nm becomes broader, and its peak wavelength shifts to longer wavelength with increasing laser power density, as shown in Figure 5(a). However, the range of wavelengths involved is quite small, and the peak lies between 6.7 and 6.8 nm over this entire range of power densities. The emission from each of these peak wavelengths within a 0.6%BW becomes more intense with increasing laser flux. This behavior causes difficulty in fixing the precise wavelength of 6.*x* nm and optimization of the spectral efficiency while simultaneously maximizing the CE. The spectral efficiency denotes the ratio of the inband energy at 6.70 nm within a 0.6%BW to that in the spectral ranging from 3 nm to 12 nm. An increase in laser power density raises the electron temperature which, in turn, implies an increase of both the highest charge state and the abundance of higher charge states. This change in the ion population must cause the observed shift of the peak wavelength for Gd-LPPs. Up to now, there was no direct experimental evidence that changes in emitting ion populations were responsible for this shift.

**Figure 5.** (a) The wavelength of the emission peaks near 6.*x* nm as a function of the Nd:YAG laser power density. The dotted line is a fitted curve. (b) EUV emission spectra of Gd ions with electron beam energies (*Ee*) of 0.43−0.92 keV. In the case of *Ee* = 0.43 keV, the compact EBIT with lower resolution was employed, while the Tokyo-EBIT was used in other cases. (c) Calculated *gA* values for 4*d*−4*f* transitions of the corresponding highest charge states (*q*max) from Figure 5(b). The ground configuration of Gd18+ is [Kr]4*d*10 [23]. Note that *q* is the charge state of Gd in Figures 5(b) and 5(c).

To verify the above explanation, charge-defined emission spectra were measured with the EBITs for different highest charge states. EUV emission spectra from EBIT experiments are shown in Figure 5(b), and calculated *gA* values of 4*d*−4*f* transitions for corresponding highest charge states are shown in Figure 5(c) to compare the charge state dependence of the emission near 6.*x* nm. The *gA* values are the transition probabilities from excited states multiplied by their statistical weights and thus are proportional to the emission intensities of the transitions. Note that the EBIT spectra include a subset of all possible radiative transitions that are predominantly resonant transitions to the ground state. For Pd-like Gd18+, only one strong line is predicted corresponding to the 4*d*<sup>10</sup> <sup>1</sup> *S*0−4*d*<sup>9</sup> 4*f* <sup>1</sup> *P*1 at 6.7636 nm, and this is clearly seen in the spectrum. In the absence of the configuration interaction (CI), according to the UTA model, the position of the intensity-weighted peak of the 4*dN*−4*dN*−14*f* array depends directly on the occupancy of the 4*d* subshell, *N*, and the Slater-Condon *Fk* (4*d*,4*f*) and *Gk* (4*d*,4*f*) parameters. In the present case, the values of *Fk* and *Gk* change little with ionization stage, and therefore, the position of the array moves to lower energy with decreasing *N*. The presence of CI causes this shift to be reduced, but nevertheless, the overall trend is to move to longer wavelength with increasing ionization stage. The dominant emissions around 7 nm in the EBIT spectra indeed move to longer wavelengths with an increase of the highest charge state. The EBIT can thus generate charge- defined emission spectra, which are essential for both analysis of plasma emission spectra and the benchmarking of theoretical calculations [23].

### **3. Quasi-Moseley's law for the UTA emission**

In this section, we show that the strong resonance UTAs of Nd:YAG LPPs for elements with *Z* = 50−83 obey a quasi-Moseley's law [24]. A 150-ps Nd:YAG laser with a maximum energy of 250 mJ at *λL* = 1.064 µm and an 8-ns Nd:YAG laser giving 400 mJ at *λL* = 1.064 µm were employed to provide the desired variation of laser intensity. The laser beam was incident normally onto planar high-*Z* metal targets in vacuo. The expected focal spot size, produced by an anti–reflection- coated plano-convex BK7 lens with a focal length of 10 cm, had a FWHM of approximately 50 µm. The laser was operated in single shot mode, and the target surface was translated to provide a fresh surface after each laser shot. A flat-field grazing incidence spectrometer (GIS) with an unequally ruled 2400 grooves/mm grating was placed at 30° with respect to the axis of the incident laser. Time-integrated spectra were recorded by a Peltiercooled back-illuminated charge-coupled device (CCD) camera and were corrected by its quantum efficiency. The typical resolution was better than 0.005 nm (FWHM). The Large Helical Device (LHD) is one of the largest devices for magnetically confined fusion research and is described in detail elsewhere. The LHD plasmas were produced by the injection of a small amount of target elements into the background hydrogen plasma. The plasma density is about 1013 cm−3, much lower than that in a LPP, and guarantees an optically thin condition. Emission spectra were recorded by a 2-m grazing incidence Schwob-Fraenkel spectrometer with a 600 grooves/mm grating. The exposure time of the detector was set at 0.2 s, and the spectral resolution is about 0.01 nm (FWHM).

Figures 6(a) −6(k) show LPP emission spectra from high-*Z* metal targets. The main UTA peak at 8.17 nm in the case of Nd clearly shifts to shorter wavelength with increasing atomic number, 3.95 nm in the case of Bi. This movement indicates the availability of a wide wavelength range for a LPP light source. While the main UTA peaks correspond to 4*p*<sup>6</sup> 4*dN*−4*p*<sup>6</sup> 4*dN*−14*f* transitions, the 4*p*<sup>6</sup> 4*dN*−4*p*<sup>5</sup> 4*dN*+1 UTAs were also observed around them, at 4 nm for the LPP of Pt, in the case of 150-ps LPPs. Optically thinner LHD plasma spectra are shown in Figures 6(l) −6(q). It should be noted that the electron temperatures of LHD plasma were relatively low, ≤ 1 keV, but higher than in 150-ps LPPs [24].

As a result, we have not observed significant emission of the type 4*f <sup>N</sup>*−4*f <sup>N</sup>*−15*l* from stages with open 4*f* valence subshells in LHD spectra. Comparing LPP and LHD spectra, the UTA widths in LHD spectra are relatively narrower than in LPPs especially for lighter elements. This arises as a result of a number of factors: the increased contributions from ions with an outermost 4*d*104*f <sup>N</sup>* configuration from transitions of the type 4*d*104*f <sup>N</sup>*−4*d*<sup>9</sup> 4*f <sup>N</sup>*+1 in LPP spectra and the differences in opacity that reduce the intensity of the strongest lines and the increased contribution from satellite emission. In addition, earlier research demonstrated that if the majority of radiation originates from open 4*f* subshell ions, whose complexity inhibits the emission of strong isolated lines, then no strong isolated lines are expected to appear through‐ out the EUV emission, which is clearly seen for the LPP spectra in Figure 6. Moreover, selfabsorption effects are clearly observed in the case of 10-ns LPP for Nd due to optical thickness. Although the *n* = 4−*n* = 4 UTA transition peak was observed at 8.05 nm in the LHD spectrum, the strongest 4*d*−4*f* transitions essentially disappear in the 10-ns LPP owing to self-absorption. Because of their large transition probabilities, resonant lines that are strong in emission also strongly absorb in underdense (*ne* < *n*ec) or optically thick plasma conditions. An optically thinner plasma reduces the self-absorption effects and increases the spectral efficiency of *n* = 4−*n* = 4 UTA emissions.

Figure 7 shows the atomic number dependence of the observed peak wavelength of *n* = 4−*n* = 4 UTAs. The solid line is an approximated curve for 150-ps LPPs with a power-law scaling of the peak wavelength given by *λ* = *aR*<sup>0</sup> −1(*Z* − *s*) −*b* in nm, where *a* = 21.86 ± 12.09, *b* = 1.52 ± 0.12, *s* = 23.23 ± 2.87 is the screening constant while Slater's rule gives *s* = 36−39.15 for 4*d* electrons, and *R*0 is the Rydberg constant. This empirical law is surprisingly similar to Moseley's law, where *a* = 4/3, *b* = 2, and *s* = 1 were used to give the transition wavelength of the *Kα*-line of characteristic x-rays. It is noted that the Moseley's law derived from the Bohr model gives *λ* = 0 for Δ*n* = 0 transitions in terms of the energy difference. It can, however, be fitted as a quasi-Moseley's law because there are energy differences between Δ*n* = 0 levels due to different angular momentum quantum numbers [24].

**Figure 6.** Time-integrated EUV emission spectra of the Nd:YAG LPPs for (a) 83Bi, (b) 82Pb, (c) 79Au, (d) 78Pt, (e) 75Re, (f) 74W, (g) 73Ta, (h) 68Er, (i) 65Tb, (j) 64Gd, and (k) 60Nd targets with 150-ps laser (red, solid line) and 10-ns laser (blue, dotted line), respectively. Typical laser power densities were 2.5 × 1014 W/cm2 for ps-laser illumination and 5.6 × 1012 W/cm2 for 10-ns laser irradiation. The measured LHD spectra (green, solid) for (l) Bi, (m) Pb, (n) Au, (o) W, (p) Gd, and (q) Nd targets, respectively. An emission line at 3.4 nm is from impurity carbon ions. Intensities were normalized at each maximum of the *n* = 4−*n* = 4 UTAs. Solid arrows indicate peak position of *n* = 4−*n* = 4 UTAs of 150-ps LPP and LHD spectra. An open arrow indicates structure due to self-absorption [24].

We propose here a pathway to produce feasible laboratory-scale high-*Z* LPP sources for a wide range of applications. For efficient UTA emission, plasmas of higher-*Z* elements need highelectron temperatures to produce higher charge state ions contributing to the 4*p*<sup>6</sup> 4*dN*−4*p*<sup>6</sup> 4*dN* −14*f* UTAs. The electron temperature, *Te*, rises with increasing laser intensity, such as *Te*, ∝ (*ILλ<sup>L</sup>* 2 ) 0.4, where *IL* and *λL* are the laser intensity and wavelength, respectively [25]. On the other hand, an optically thin plasma has a low electron density, *ne*, which decreases with increasing *λL*. In terms of these features, use of a longer laser wavelength is necessary to generate the brightest LPP, such as a CO2 laser operating at 10.6 µm due to the low critical density of 1 × 1019 cm−3 attainable with a pulse duration sufficiently short to give a laser intensity of the order of 1013 W/cm2 but sufficiently long to permit excitation to the appropriate ionization stages, i.e., ~ 1 ns. Moreover, we can also obtain longer wavelengths, > 10.6 µm, with a Raman conversion system.

Figure 7. Atomic number dependence of the peak wavelength of *n* = 4−*n* = 4 UTAs in 150-ps LPP (red, circles), 10-ns LPP (blue, squares), and LHD (green, diamonds) spectra. Calculated peak **Figure 7.** Atomic number dependence of the peak wavelength of *n* = 4−*n* = 4 UTAs in 150-ps LPP (red, circles), 10-ns LPP (blue, squares), and LHD (green, diamonds) spectra. Calculated peak wavelengths with GRASP are also shown (black, crosses). Sn spectra are not shown in Figure 6. The solid line is an approximated curve for *n* = 4−*n* = 4 UTAs in 150-ps LPPs with a power-law scaling [24].

wavelengths with GRASP are also shown (black, crosses). Sn spectra are not shown in Figure 6. The

#### solid line is an approximated curve for *n* = 4−*n* = 4 UTAs in 150-ps LPPs with a power-law scaling **4. Water window soft x-ray source by high-***Z* **ions**

**4. Water window soft x-ray source by high-***Z* **ions**

[24].

#### **4.1. Spectroscopy of low electron temperature in lab-scale laser-produced ions**

**4.1. Spectroscopy of low electron temperature in lab-scale laser-produced ions**

wavelengths of 79Au, 82Pb, and 83Bi reach the water window soft x-ray spectral region.

According to the quasi-Moseley's law in Figure 7, the elements from 79Au to 83Bi are one of the candidates for high-flux UTA source in water window soft x-ray sources for single-shot (flash)

According to the quasi-Moseley's law in Figure 7, the elements from 79Au to 83Bi are one of

Figures 8(a)−8(c) show time-integrated spectra from Au, Pb, and Bi plasmas at a laser

the candidates for high-flux UTA source in water window soft x-ray sources for single-shot (flash)

bio-imaging in the laboratory size microscope, because the UTA emission is essentially high-power

emission due to much resonant lines around the specific wavelength (photon energy). The UTA peak

intensity of the order of 10<sup>14</sup> W/cm<sup>2</sup> with a pulse duration of 150 ps (FWHM). The time-integrated

19

bio-imaging in the laboratory size microscope, because the UTA emission is essentially highpower emission due to much resonant lines around the specific wavelength (photon energy). The UTA peak wavelengths of 79Au, 82Pb, and 83Bi reach the water window soft x-ray spectral region.

Figures 8(a) −8(c) show time-integrated spectra from Au, Pb, and Bi plasmas at a laserintensity of the order of 1014 W/cm2 with a pulse duration of 150 ps (FWHM). The time-integrated soft x-ray spectra between 1 and 6 nm from each element display strong broadband emission around 4 nm, which is mainly attributed to the *n* = 4−*n* = 4 transitions from HCIs with an open 4*f* or 4*d* outermost subshell with the broadband emission of 2−4 nm originating from the *n* = 4−*n* = 5 transitions from HCIs with an outermost 4*f* subshell. The intensity of the *n* = 4−*n* = 4 UTA emission was higher than that of the *n* = 4−*n* = 5 transition emission. The atomic number dependence of the spectral structure is shown in Figure 8(d). The predicted emission photon energy of each peak photon energy was shifted to higher photon energy with the increase of the atomic number. Neither the emission spectra nor the plasma electron temperatures, however, have been optimized, as shown below. However, the emission intensity of the *n* = 4−*n* = 5 transitions was compared with that of the *n* = 4−*n* = 4 transitions of the UTA emission [26].

We compared the results of numerical calculation for some different experimental tempera‐ tures with the observed spectra as shown in Figure 9(a). Four regions corresponding to emission peaks were identified. The emission in the region of "1" results primarily from the 4*f*−5*g* transitions in HCIs with an open 4f subshell, i.e., the stages lower than 35+ Bi ions. The emission in regions of "2" and "3" originates from 4*p*−4*d* and 4*d*−4*f* transitions with an open 4*d* subshells of Bi36+−Bi45+, and numerical calculations show that the higher- energy region results from the more highly ionized species higher than Bi42+. The emission in the region of "4" was also associated mainly with the 4*d*−4*f* transition emission from lower ionic charge stages with an open 4*f* outmost subshell. As a result, the bulk of the emission, especially from regions of "1" and "4", was associated with the recombining phase of the expanding plasma plume. We evaluate for comparison spectra calculated for steady-state electron temperatures of 180 and 700 eV, while the higher temperatures were required to produce the emission in the region of "2", the calculations verify that both the longer and shorter wavelength features were consistent with much lower plasma electron temperatures [26].

In Figure 9(b), evaluated spectra at different electron temperatures higher than 900 eV were shown. Numerical calculations show that high-*Z* plasmas at an electron temperature lower than 700 eV, as shown in Figure 6(a), radiate strongly around 3.9 nm. In the case of higher electron temperatures from 800 to 1500 eV, the strongest emission, however, is expected at around 3.2 nm, suitable for coupling with Sc/Cr multilayer mirrors. Therefore, for an optimized source, we should produce a plasma at high electron temperature of around 1 keV. The emission intensity of the Bi plasma was compared with 2.48-nm nitrogen line emission from a Si3N4 planar target, in the same experimental setup, and was observed to be 1.2 times higher within a bandwidth of 0.008 nm (FWHM) even though the plasma electron temperature was much lower than the optimum value [26].

**Figure 8.** Time-integrated spectra from the picosecond-laser–produced high-*Z* plasmas by the use of Au (a), Pb (b), and Bi (c), and the atomic number dependence of the photon energies of the peak emission of the *n* = 4−*n* = 4 transition (circles) and the *n* = 4−*n* = 5 transition (rectangles) (d) [26].

**Figure 9.** (a) The comparison between the observed spectrum with numerical calculation under assuming steady-state electron temperatures of 190 and 700 eV. (b) Calculated spectra for electron temperatures higher than 900 eV. [26]

### **4.2. Toward the laboratory water window soft x-ray microscope**

Because of the broadband features of the emission, the zone plate components cannot be used, so one of the possible solutions would be to use a transmission planar x-ray nano-waveguide to image the sample. In order to achieve high resolution in the recorded image, we should also replace the recording device from the x-ray CCD camera to the sensitive EUV resist to overcome the resolution limitation of the CCD pixel size, coupling with the Schwarzschild optics, consisting of Sc/Cr multilayer mirrors. Although our proposal is based on a simple microscope construction, the key component is the UTA emitted from a hot dense Bi plasma point source, combined with Sc/Cr MLMs and sensitive EUV resists based on the photochem‐ ical reaction [26].

### **5. Summary**

We have shown EUV and soft x-ray sources in the 2- to 7- nm spectral region related to the BEUV question at 6.*x* nm and a water window source based on laser-produced high-*Z*plasmas. The efficient 6.*x*-nm BEUV sources have been demonstrated at the CE of 0.7% due to the high spectral purity by the optically thin plasmas after the database experiments. According to the atomic number dependence of the UTA emission, so- called quasi-Moseley's law, the Bi HCI plasma source is one of the solutions in the laboratory single-shot (flash) bio-imaging by extending the UTA light source feature.

### **Author details**

Takeshi Higashiguchi1\*, Padraig Dunne2 and Gerry O'Sullivan2

\*Address all correspondence to: higashi@cc.utsunomiya-u.ac.jp

1 Department of Electrical and Electronic Engineering, Faculty of Engineering, Utsunomiya University, Utsunomiya, Tochigi, Japan

2 School of Physics, University College Dublin, Belfield, Dublin, Ireland

### **References**

[1] Gorniak T, Heine R, Mancuso A. P, Staier F, Christophis C, Pettitt M. E, Sakdinawat A, Treusch R, Guerassimova N, Feldhaus J, Gutt C, Grübel G, Eisebitt S, Beyer A, Gölzhäuser A, Weckert E, Grunze M, Vartanyants I. A, Rosenhahn A: X-ray holo‐ graphic microscopy with zone plates applied to biological samples in the water win‐ dow using 3rd harmonic radiation from the free-electron laser FLASH. Optics Express. 2011;19:11059–11070. DOI: 10.1364/OE.19.011059]


Dunne P, O'Sullivan G, Yanagida T, Azechi H, Nishimura H: Density and x-ray emission profile relationships in highly ionized high-*Z* laser-produced plasmas. Ap‐ plied Physics Letters. 2015;106:121109. DOI: 10.1063/1.4916395


## **Laser-Induced Plasma and its Applications**

Kashif Chaudhary, Syed Zuhaib Haider Rizvi and Jalil Ali

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61784

#### **Abstract**

The laser irradiation have shown a range of applications from fabricating, melting, and evaporating nanoparticles to changing their shape, structure, size, and size distribution. Laser induced plasma has used for different diagnostic and technological applications as detection, thin film deposition, and elemental identification. The possible interferences of atomic or molecular species are used to specify organic, inorganic or biological materials which allows critical applications in defense (landmines, explosive, forensic (trace of ex‐ plosive or organic materials), public health (toxic substances pharmaceutical products), or environment (organic wastes). Laser induced plasma for organic material potentially provide fast sensor systems for explosive trace and pathogen biological agent detection and analysis. The laser ablation process starts with electronic energy absorption (~fs) and ends at particle recondensation (~ms). Then, the ablation process can be governed by thermal, non-thermal processes or a combination of both. There are several types of mod‐ els, i.e., thermal, mechanical, photophysical, photochemical and defect models, which de‐ scribe the ablation process by one dominant mechanism only. Plasma ignition process includes bond breaking and plasma shielding during the laser pulse. Bond breaking mechanisms influence the quantity and form of energy (kinetic, ionization and excitation) that atoms and ions can acquire. Plasma expansion depends on the initial mass and ener‐ gy in the plume. The process is governed by initial plasma properties (electron density, temperature, velocity) after the laser pulse and the expansion medium. During first mi‐ crosecond after the laser pulse, plume expansion is adiabatic afterwards line radiation be‐ comes the dominant mechanism of energy loss.

**Keywords:** Laser Induced Plasma, Plasma Diagnostics, Plasma Applications

### **1. Introduction**

Laser-induced plasma (LIP) formation is a rapid process that is under investigation for several decades due to its versatile and complex nature. Intense laser pulse delivers energy to the target surface for a very short interval of time that instantly excites, ionizes and vaporizes the material

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

into an extremely hot vapour plume also called as 'plasma plume'. It has three main regions as depicted in Figure 1. Near the target surface is the hottest and the densest part of the plasma called core; in this region, the material is mostly found in the ionized state because of high temperatures. Within the core adjacent to the target surface there exists a so-called *Knudsen layer* having a thickness equal to a few mean free paths. *Knudsen layer* is defined as the region in which a particle achieves an equilibrium velocity distribution from non-equilibrium distribution within a few mean-free paths [1]. In the mid-region of plasma, ions and neutrals (atoms + molecules) coexist due to the ongoing ionization and recombination processes. The outermost region of the plasma is relatively cold, where the population of neutrals dominates and may absorb the radiations coming out of the core and mid-regions of the plasma. Beyond it, there is a shock wave front produced due to the explosive expansion of the plasma, and travels ahead of the plasma plume.

**Figure 1.** Schematic illustration of laser-induced plasma

In this chapter, the interaction of short (~nanosecond (ns)) and ultrashort (~picosecond (ps) and femtosecond (fs)) laser pulses with a variety of media, pulsed laser ablation (PLA), laserinduced plasma, plasma diagnostic techniques and applications of LIP is discussed. Out of the large canvas, the description about standard plasma diagnostic techniques encompassing optical emission spectroscopy (OES), laser-induced fluorescence (LIF), Langmuir probe, Faraday's cups and solid-state nuclear track detectors (SSNTDs) is provided. As the last section of the chapter, extensive literature review is presented that elaborates applications of the laserinduced plasma. Applications of laser-induced plasma in two main fields, that is, laser-induced breakdown spectroscopy (LIBS) and thin-film or pulsed laser deposition (PLD), are discussed in diverse fields of science and technology.

### **2. Laser–matter interaction**

Lasers are unique energy sources characterized by their spectral purity, spatial and temporal coherence and high intensity. When laser interacts with matter, it can reflect, scatter, absorb or transmit depending upon the material characteristics (i.e. composition, physical, chemical and optical properties) and laser parameters. Laser energy, wavelength, spatial and temporal coherence, exposure time/pulse duration, etc. are the key influential parameters in laser– matter interaction.

The laser basically interacts with electrons in the material by energy transfer and excitation. Free electrons absorb energy and accelerate, whereas bound electrons excite to higher energy levels if laser photons match the bandgap energy of atoms. This absorbed energy is later released by the excited/accelerated electrons as electromagnetic radiation or dissipate into lattice as heat energy.

Different types of interactions of laser with the matter have found various applications. However, this chapter focuses on the laser-induced plasma that is, primarily, the consequence of laser energy absorption in the material. The interaction of laser is different for different materials.

In *metals*, there are a large number of free electrons available; the laser energy excites these electrons to various energies. The excitation of free electrons is taken as the increase in their kinetic energy. As free electrons are excited, the collision rate increases and the energy is transferred from the excited free electrons to lattice phonons.

Since *semiconductors* and *insulators* have very few free electrons, the laser interacts with bound electrons in atomic/ionic shells and generates electron–hole pairs. The generation of electron– hole pair depends upon the laser photon energy. If it matches the material's bandgap '*E*g', that is, the energy difference between the highest level in valence band and lowest level in the conduction band, an electron–hole pair is generated. If it is lower than the *E*g, the photon may not absorb or cause low-energy intra-band electronic excitations in vibrational states.

In *dielectrics*, the laser energy causes ionization and liberates electrons within the material. Once the free electrons are available, collisional ionization starts that is termed as impact or avalan‐ che ionization. It forms a dense cloud of ions near the surface that is opaque to the laser and absorbs later part of the laser pulse. In dense *ceramics*, the laser interaction phenomenon is same as in dielectrics; however, in the porous *ceramics*, the laser is mostly scattered due to the random distribution of particle size.

### **3. Pulsed laser ablation**

*Pulsed laser ablation* is a high-photon-yield sputtering phenomenon in which the material is removed from a solid surface by the interaction of high-intensity short laser pulses. It is consequence of the conversion of laser-induced electronic and vibration excitation into the kinetic energy of nuclear motion. The material removal rate typically exceeds one-tenth of a monolayer per pulse that alters the surface composition at mesoscale. If particles are ejected out of the surface but there is no detectable modification in the surface composition, it is commonly referred to as *laser-induced desorption*. Being a mesoscale phenomenon, PLA is influenced by bulk properties of the material, for example, elasticity and compressibility. Moreover, the laser ablation need not involve massive, catastrophic destruction of the surface.

In pulsed laser ablation,


**Figure 2.** Illustration of nanosecond pulsed-laser ablation

PLA is a useful technique for surface machining. Although microcracks and heat-affected zone (HAZ) are formed as a result of thermal stresses and heat conduction into the bulk medium as illustrated in Figure 2, PLA provides an advantage of minimizing the heat-affected zone under certain circumstances. In the case of nanosecond laser pulses, the heat-affected zone will be minimal if the ablation depth per pulse *Δh* is comparable to the thermal penetration depth *l* <sup>t</sup>≈2 *Dτ*<sup>l</sup> or the optical penetration depth *l* <sup>α</sup> <sup>≈</sup>*<sup>α</sup>* <sup>−</sup><sup>1</sup> [2]:

$$
\Delta h \approx \max \left\{ l\_{\alpha}, l\_{\text{t}} \right\} \tag{1}
$$

where *D* represents thermal diffusivity of the material, *τ*<sup>l</sup> is the laser pulse duration and *α* is the optical absorption of the material. In case of ablation with ultrashort laser pulses, HAZ is nearly absent or is negligible due to non-thermal mechanisms governing the ablation.

#### **3.1. Mechanisms of laser ablation 3.1. Mechanisms of Laser Ablation**

Laser ablation mechanisms are different for short (~ns) and ultrashort (~ps and fs) laser pulses due to the difference in laser coupling with matter at different timescales involved. Short pulse, that is, nanosecond pulse ablation, can be characterized through thermal, non-thermal and combination of both mechanisms. These are also referred to as photo-thermal, photo-chemical and photo-physical mechanisms, respectively. However, for certain experimental parameters and materials, the ablation can be described through one dominant mechanism. Laser ablation mechanisms are different for short (~ns) and ultrashort (~ps and fs) laser pulses due to the difference in laser coupling with matter at different timescales involved. Short pulse, that is, nanosecond pulse ablation, can be characterized through thermal, non-thermal and combination of both mechanisms. These are also referred to as photo-thermal, photo-chemical and photo-physical mechanisms, respectively. However, for certain experimental parameters and materials, the ablation can be described through one

If the excited material quickly converts the absorbed laser energy into heat, high temperature can build up near surface stresses in the material, which can lead to material ablation off the surface. Such an ablation is governed by the photo-thermal mechanism. dominant mechanism. If the excited material quickly converts the absorbed laser energy into heat, high temperature can build up near surface stresses in the material, which can lead to material

If the incoming laser photons have sufficiently high energy, the absorption can induce defects in the material or cause direct bond breaking of atoms, ions or molecules. Defects and bond breaking can individually cause ablation of the material. Such an ablation is termed as photochemical ablation. However, if the ablation occurs as a result of both thermal and non-thermal mechanisms, it is termed as photo-physical ablation. Figure 3 graphically illustrates the laser ablation process through thermal, non-thermal and photo-physical processes. ablation off the surface. Such an ablation is governed by the photo-thermal mechanism. If the incoming laser photons have sufficiently high energy, the absorption can induce defects in the material or cause direct bond breaking of atoms, ions or molecules. Defects and bond breaking can individually cause ablation of the material. Such an ablation is termed as photo-chemical ablation. However, if the ablation occurs as a result of both thermal and nonthermal mechanisms, it is termed as photo-physical ablation. Figure 3 graphically illustrates

the laser ablation process through thermal, non-thermal and photo-physical processes.

Figure 3. Graphical illustration of mechanisms leading to pulsed laser ablation **Figure 3.** Graphical illustration of mechanisms leading to pulsed laser ablation

In case of ultrashort laser pulses, the mechanisms of photon absorption, heat transfer within the material and hydrodynamic ablation take place at different timescales independent of each other [3]. The phenomena that are not important or less significant in short-pulse laser ablation become dominant and significant because of ultrashort timescales and extreme laser intensities involved. Nonlinear absorption, Coulomb explosion and overcritical heating are the dominant phenomena in ultrashort pulsed laser ablation. At low laser intensities, close to In case of ultrashort laser pulses, the mechanisms of photon absorption, heat transfer within the material and hydrodynamic ablation take place at different timescales independent of each other [3]. The phenomena that are not important or less significant in short-pulse laser ablation become dominant and significant because of ultrashort timescales and extreme laser intensities involved. Nonlinear absorption, Coulomb explosion and overcritical heating are the dominant phenomena in ultrashort pulsed laser ablation. At low laser intensities, close to the ablation threshold, Coulomb explosion governs the ablation. The absorbed laser energy accelerates electrons, and if the energy is sufficient to overcome the binding energy and work function of the material, electrons eject from the surface and form a charged cloud (of electrons) above the material surface leaving behind a charge density of ions on the surface. It creates a space charge separation; the electric field within the charge separation can be so strong that it can pull ions out of the material resulting in removal of top few monolayers from the surface [4]. Because of enormous intensities, non-linear multiphoton absorption phenomenon increases the laser energy absorption in the material that leads to cascade ionization. At ultrashort timescales (~pico- and femtoseconds), the material cannot either transfer energy to the lattice or evaporate instantly, rather the absorbed laser energy causes extreme excitation and heating of the material above critical temperatures. Consequently, a rapidly expanding mixture of liquid droplets and vapours is observed above the target surface [5].

### **4. Laser-induced plasma formation**

#### **4.1. Plasma ignition**

Laser ablation can be divided into three phases


The laser ablation process starts with electronic energy absorption (~fs) and ends at particle recondensation (~ms). Plasma ignition process includes bond breaking and plasma shielding during the laser pulse interaction with the material surface. Bond-breaking mechanisms influence the quantity and form of energy (kinetic, ionization and excitation) that atoms and ions can acquire.

For nanosecond laser pulses of irradiance < 108 W/cm2 , the dominant mechanism of plasma ignition is thermal vaporization [6], whereas for picosecond laser pulses with irradiances between 108 and 1013 W/cm2 both thermal and non-thermal mechanisms contribute to the plasma ignition. However, with femtosecond laser pulses of irradiance > 1013W/cm2 , the main bond-breaking (plasma ignition) mechanism is non-thermal, that is, Coulomb's explosion [6].

#### **4.2. Plasma expansion and cooling**

Plasma expansion depends on the initial mass and energy in the plume. The process is governed by initial plasma properties (electron density, temperature and velocity) after the laser pulse and the expansion medium. During the first microsecond after the laser pulse, plume expansion is adiabatic; afterwards, line radiation becomes the dominant mechanism of energy loss.

#### **4.3. Particle formation and condensation**

Condensation or particle formation takes place during the decay/cooling process of plasma. It starts when plasma temperature reaches the boiling point of the material and stops at the condensation temperature of the material. Exfoliation can also be observed as a result of high thermal stresses within the material due to fast heating. Thermal stresses produced in the material can break it into irregularly shaped particles that may eject from the surface. This phenomenon is called as exfoliation.

### **5. Plasma diagnostic techniques**

Plasma is a rich source of radiations. A wide spectrum of electromagnetic radiation ranging from IR through X-rays is emitted by excited species of the plasma that is of practical interest. Various specialized techniques are utilized for the diagnostics of specific radiations emitting from the plasma. Diagnostic techniques can largely be categorized in optical diagnostic techniques, electrical diagnostic techniques and diagnostics using solid-state detectors. Optical diagnostics of the plasma gives indirect estimation of plasma conditions based on character‐ istics of plasma emission spectrum. However, electrical diagnostic techniques provide direct measurements of plasma characteristics and X-rays, electron and ion emissions from the plasma. For energy distribution of particles emitting from plasma, solid-state detectors are brought into use.

### **5.1. Optical diagnostics**

Due to fast process and short lifetime, not all optical diagnostic techniques are useful to study laser-induced plasma that are used for persistent plasmas like inductively coupled plasma (ICP) and plasma focus (PF). The most common technique used for optical diagnostics of laserinduced plasma is emission spectroscopy. Radiations emitted from the plasma are collected by a spectrometer or a monochromator depending on the nature of study. Spectrometer captures a range of electromagnetic spectrum by a charge-coupled device (CCD) detector or with intensified CCD for even better results, whereas in monochromator a specific wavelength or a band is studied in high resolution with a CCD detector of a photomultiplier tube (PMT).

The spectrum is obtained as a combination of characteristic line emissions and continuum emissions. These spectra carry loads of information about the plasma conditions and compo‐ sition. Various features of the spectrum can be utilized to obtain specific information about the plasma. For example, Doppler broadening of an emission line is related to velocity of the emitting particle, intensity of the emission line is proportional to the quantity of the emitter, Stark broadening of the line tells about electron density in the plasma while the ratio between intensity of emission line and continuum can provide us with temperature of the plasma, etc.

Another optical technique that is used for highly localized study of laser-induced plasma diagnostics is the laser-induced fluorescence. A tunable laser tuned to a specific wavelength, which is passed through the plasma to excite specific elements for certain transitions. In turn, upon de-excitation, radiations of a different frequency are obtained that help in studying the species of interest in the plasma. However, such a system is less popular because of the difficult set-up and expensive equipment. A combined system for plasma diagnostics through optical emission spectroscopy and laser-induced fluorescence is schematically shown in Figure 4.

**Figure 4.** Schematic illustration of a combined set-up of laser-induced fluorescence and optical emission spectroscopy of laser-induced plasma

#### *5.1.1. Electron density measurements*

Electron density in the plasma can be measured through Stark broadening of an emission line or through the intensity ratio of two different emission lines of the same element. Utilizing the Stark-broadening method is considered more reliable because broadening of the emission line due to the Stark effect is a direct consequence of the presence of charged particles around the emitter.

For estimation of electron density in the plasma by making use of Stark broadening of an emission line, correct measurement of the Stark broadening is important. The emission line is normally broadened by a combination of three broadening mechanisms, that is, natural broadening, Doppler broadening that is caused by thermal motion of the emitter and Stark broadening caused by splitting of energy level because of the electric field strength of charged particles near the emitter. Doppler broadening is prominent at high temperatures, whereas the Stark broadening, which is also called as collision or pressure broadening, dominates at high densities of charged particles in the plasma.

*Natural line broadening* is related to naturally existing split in an energy level that makes an energy band. It can be related to the uncertainty in the energy of an excited state 'Δ*E*' for a limited excitation time 'Δ*t*' of an electron through Heisenberg's uncertainty principle as ~Δ*E*Δ*t* >ℏ. Normally, natural broadening is so small that it is undetectable by the spectrome‐ ters typically used in plasma diagnostics.

*Doppler line broadening* is a consequence of thermal motion of the emitter along the line of observation. The variation in the wavelength is explained on the basis of the Doppler effect. If movement of the emitter is towards the detector, a slightly shorter wavelength is recorded, and if the movement is away from the detector, a slightly longer wavelength is observed by the detector. Consequently, a broader emission line with a Gaussian profile is observed. For an emitter of atomic mass *M*, Doppler broadening Δ*λ*D of an emission line at wavelength *λ* for a particular electron temperature *T* can be calculated as

$$
\Delta \mathcal{J}\_{\rm D} = 2 \mathcal{J} \sqrt{\frac{2kT \ln 2}{Mc^2}} \tag{2}
$$

In addition to the above-described broadenings, the emission line is superimposed by another broadening contributed by the spectrometer itself that is referred to as *instrumental broaden‐ ing*. It can be determined by using a narrow line laser beam. Typically, Doppler and Stark are the main competing broadening mechanisms. At high temperatures, Doppler broadening can be dominant, whereas at high densities Stark can be a dominant broadening mechanism. When Doppler broadening is negligible and Stark broadening mechanism is the main broadening mechanism, Stark broadening Δ*λ*stark can be isolated from the instrumental broadening Δ*λ*stark by deconvolution:

$$
\Delta \mathcal{J}\_{\text{stack}} = \Delta \mathcal{J}\_{\text{total}} - \Delta \mathcal{J}\_{\text{instantal}} \tag{3}
$$

Stark broadening is directly linked with the electron density through electron impact param‐ eter *w*FWHM, by the following relation:

$$
\Delta \mathcal{J}\_{\text{stack}} = \mathcal{Z}w\_{\text{FWHM}} \left( \frac{n\_{\text{e}}}{10^{16} \text{cm}^{-3}} \right) \tag{4}
$$

Once the Stark broadening is obtained, the electron density *ne* can be estimated by using this relation (eq. 4) if the electron impact parameter *w*FWHM is known, which can be found in the literature. Electron density in the plasma depends upon a number of experimental parameters, for example, laser energy, background gas, ambient pressure and characteristics of the target itself. However, it represents a temporal profile that follows an exponential decay as a function of plasma lifetime [7, 8].

#### *5.1.2. Plasma temperature measurements*

Plasma temperature can be determined by several spectroscopic methods, for example, the line-pair intensity ratio method, Boltzmann plot, line-to-continuum ratio method, etc. One method may be more suitable than others under specific conditions. For example, diagnostics of the early-state plasma line-to-continuum ratio method is more suitable because during initial few hundred nanoseconds, both continuum and line intensities are of comparable strength. However, after a few microseconds, the continuum fades away and line intensities dominate in the spectrum; in such a situation, the line-pair intensity ratio or Boltzmann plot method would be more suitable. These methods are described as follows.

#### *5.1.2.1. Line-pair intensity ratio method*

Assuming that the plasma is in local thermodynamic equilibrium (LTE), the temperature of the plasma can be calculated through the intensity ratio of a pair of spectral lines of atom or ion of same ionization stage. In LTE, the level population supposedly obeys the Boltzmann distribution. Then the integrated intensity of a transition (*j*→*i*) can be represented as [9]:

$$I\_{\rm i\downarrow} = n\_{\rm i}^\* A\_{\rm i\downarrow} \tag{5}$$

Here, *n*<sup>i</sup> s represents the population density of species '*s*' in level '*i*' given as

$$m\_i^s = \frac{\mathcal{G}\_1}{\mathcal{U}^s(T)} m^s e^{\mathcal{E}\_i/kT} \tag{6}$$

Therefore, intensity *I*ij can be written as

$$I\_{\mathbb{H}} = \frac{g\_{\mathbb{H}} A\_{\mathbb{H}}}{\mathcal{U}^\*(T)} n^\* e^{\mathbb{E}/kT} \tag{7}$$

where *gi* is the partition function of level '*i*', *A*ij represents the transition probability of *i*→*j* transition, *n*<sup>s</sup> and *U*<sup>s</sup> *(T)* are total number density of species in the plasma and partition function of the species '*s*', respectively. *E*<sup>i</sup> , *k* and *T* are energies of the upper level '*i*', Boltzmann constant and plasma temperature, respectively.

Now, consider another spectral line of the same species but originated from a different transition, that is, *m*→*n*. Such that the upper energy level is different (i.e. *E*<sup>i</sup> ≠ *E*m) and the lower energy level may or may not be the same. By taking intensity ratio of these two spectral lines, the plasma temperature can be calculated as follows:

$$\frac{I\_{\rm ij}}{I\_{\rm mn}} = \frac{\frac{\mathcal{S}\_l A\_{\rm q}}{\mathcal{U}^\*(T)} u^\* e^{x\_l \mathcal{U}^{\rm tr}}}{\frac{\mathcal{S}\_m A\_{\rm mn}}{\mathcal{U}^\*(T)} u^\* e^{x\_l \mathcal{U}^{\rm tr}}}\tag{8}$$

Rearranging

#### Laser-Induced Plasma and its Applications http://dx.doi.org/10.5772/61784 269

$$T = \frac{E\_\mathrm{i} - E\_\mathrm{m}}{k \ln\left(\frac{I\_{\mathrm{mm}} A\_\mathrm{i} g\_\mathrm{i}}{I\_\mathrm{i} A\_\mathrm{m} g\_\mathrm{m}}\right)}\tag{9}$$

It is advisable to choose lines that are as close in wavelength and as far in upper-level energy as possible. It will limit the device response variation in measurement of spectral line intensi‐ ties. Assuming that the experimental error influences only intensities of the spectral lines, error in the temperature measurement is given as [10]

$$\frac{\Delta T}{T} = \frac{kT}{\Delta E} \frac{\Delta R}{R} \tag{10}$$

Here, Δ*E* =*E*i− *E*m energy difference between upper energy levels and *R* measures the intensity ratio, that is, *R* = *I*ij/ *I*mn. ∆*R* is the uncertainty associated with ratio and Δ*T* is the corresponding uncertainty in temperature measurement.

#### *5.1.2.2. Boltzmann plot method*

Boltzmann plot is a reliable method for calculation of plasma temperature that has been trusted by many researchers in the latest literature.

The emission intensity of a spectral line is represented as [11]

$$I\_{\mathbb{H}} = \frac{\hbar c}{4\pi} \frac{A\_{\mathbb{H}} g\_{\mathbb{H}}}{\mathcal{A}\_{\mathbb{H}} \mathcal{U}(T)} n e^{-E\_{\mathbb{H}} f \mathbb{H}} \tag{11}$$

Here, *h*, *c*, *k*, *T* and *U(T)* are the Plank's constant, speed of light, Boltzmann constant, plasma temperature and partition function, respectively, whereas *A*ij, *g*<sup>i</sup> , *E*<sup>i</sup> , *λ*ij and *n* are the transition probability, degeneracy of upper level, upper-level energy, emission wavelength and total population density of the emitting species, respectively.

Taking natural logarithm and re-arranging the eq. (17), we obtain

$$\ln\left(\frac{I\_{\rm y}\mathcal{X}}{g\_{\rm i}A\_{\rm y}}\right) = \frac{-E\_{\rm i}}{kT} + \ln\left(\frac{hcn}{4\pi\mathcal{U}(T)}\right) \tag{12}$$

This is the equation of a straight line that arises as a result of plot between ln( *<sup>I</sup>*ij*<sup>λ</sup> g*i *A*ij ) and upper-

level energy *E*<sup>i</sup> . The slope of this plot is equal to *−1/kT*. From this slope, plasma temperature '*T*' can easily be estimated [12, 13]. The further apart the upper-level energies of the selected lines, the better would be the measurement of the slope. The Boltzmann plot method is considered to be more precise for using several lines that averages out uncertainties involved in meas‐ urement than the intensity ratio method, which makes use of a pair of emission lines only [9]. The value of plasma temperature depends upon laser–target interaction, laser energy and characteristics of the ambient environment. However, it shows exponential decay during the lifespan of laser-induced plasma [7, 8].

#### **5.2. Electrical diagnostics 5.2. Electrical Diagnostics**

**5.2.1. Langmuir Probe** 

A number of electrical diagnostic tools are used to characterize the laser-induced plasma. Few of them are briefly discussed in this section. A number of electrical diagnostic tools are used to characterize the laser-induced plasma. Few of them are briefly discussed in this section.

#### *5.2.1. Langmuir probe*

The Nobel laureate Irving Langmuir has invented the Langmuir probe to determine the electron temperature, density and the space potential *V*sp in cold low-density plasmas. The local plasma parameters can be measured using stationary or slow time-varying electric (and/ or magnetic) fields to collect or emit charged particles from the plasma. The charge particles, electrons, ions or both, are collected through the electric field established between the bulk plasma and the metallic surface of the probe. The simplest Langmuir probe consists of a metallic electrode, a bare wire or metal disk, with well-defined dimensions such as planar, cylindrical or spherical as shown in Figure 5. Langmuir probe is electrically biased with respect to a reference electrode to collect electron and/or positive ion currents. The dimensions of the probe as length, diameter and the thickness (or spatial extension) of the plasma sheath attached to the collecting surface play an important role in the collection of charge particles [14, 15]. The Nobel laureate Irving Langmuir has invented the Langmuir probe to determine the electron temperature, density and the space potential *V*sp in cold low-density plasmas. The local plasma parameters can be measured using stationary or slow time-varying electric (and/or magnetic) fields to collect or emit charged particles from the plasma. The charge particles, electrons, ions or both, are collected through the electric field established between the bulk plasma and the metallic surface of the probe. The simplest Langmuir probe consists of a metallic electrode, a bare wire or metal disk, with well-defined dimensions such as planar, cylindrical or spherical as shown in Figure 5. Langmuir probe is electrically biased with respect to a reference electrode to collect electron and/or positive ion currents. The dimensions of the probe as length, diameter and the thickness (or spatial extension) of the plasma sheath attached to the collecting surface play an important role in the collection of charge particles [14, 15].

**Figure 5.** Schematic diagram of the Langmuir probe with different dimensions (a) cylindrical, (b) spherical and (c) pla‐ nar

Figure 5. Schematic diagram of the Langmuir probe with different dimensions (a) cylindrical, (b) spherical and (c) planar The metallic probe is immersed into the plasma and a potential *V*p is provided to the probe using an external circuit, which bias the probe *V = V*p*−V*sp with respect to the local space The metallic probe is immersed into the plasma and a potential *V*<sup>p</sup> is provided to the probe using an external circuit, which bias the probe *V = V*p*−V*sp with respect to the local space plasma potential *V*sp. The drained current called probe current *I*<sup>p</sup> *= I(V*p*)* for different probe potentials *V*p is utilized to measure the different plasma parameters via voltage–current (IV) characteristic

plasma potential *V*sp. The drained current called probe current *I*p *= I(V*p*)* for different probe potentials *V*p is utilized to measure the different plasma parameters via voltage–current (IV) characteristic curves [14]. Figure 6 shows typical voltage–current (IV) curve of the Langmuir probe. The potential *V*sp presents the localized electric potential at the point within plasma curves [14]. Figure 6 shows typical voltage–current (IV) curve of the Langmuir probe. The potential *V*sp presents the localized electric potential at the point within plasma where the probe is immersed. As the probe does not emit particles, the drained current '*I*p' is the combination of ion current '*I*<sup>i</sup> ' and electron current '*I*e', that is, '*I*p*= I*<sup>i</sup> *+ I*e'.

For very negative bias voltages where *V*<sup>p</sup> is much less than *V*sp (i.e. *V*<sup>p</sup> *<< V*sp at the left of point A as shown in Figure 6), the electrons are repelled, while ions are attracted by the probe. In this region, the characteristic curve is designated as the 'ion accelerating region'. The probe potential is sufficiently negative that only positive ions contribute to the probe current. The drained ion current from the plasma is limited by the electric shielding of the probe and *I*<sup>p</sup> decreases slowly. The current *I*<sup>p</sup> *≃ I*is is the denominated ion saturation current.

The voltage corresponding to zero value of current is termed as the 'floating potential'. The bias potential *V*F where *I*<sup>p</sup> = 0 is the floating potential (point B) where the contributions of the ion and electron currents are equal.

After the floating potential, the *I–V* trace takes a sharp turn upwards. The bend is also referred to as the 'knee'. At this point, probe has the plasma potential. In the region, where *V*p is much higher than *V*sp (i.e. *V*<sup>p</sup> *≫ V*sp at the right of point C as shown in Figure 6), the ions are repelled and the electrons are the attracted charges. In this case, the electrons are responsible for the electric shielding of the probe, and *I*<sup>p</sup> *≃ I*es is called the electron saturation current. As the positive bias is increased on the probe, eventually the collected electron current reaches a saturated value, that is, maximum electrons collected by the probe [14].

**Figure 6.** Typical voltage current (IV) curve of the Langmuir probe

For the qualitative interpretation, an idealized non-equilibrium collisionless, Maxwellian and unmagnetized plasma conditions are considered. Under the Maxwellian distribution, current density follows the Boltzmann law [16]:

$$J = J\_\circ \exp(-eV \mid kT\_\circ) \tag{13}$$

where '*k*' is the Boltzmann constant, '*T*' is the temperature and '*J*' is the current density at any potential '*V*' and *J*o is the current density at zero potential:

$$I = I\_o \exp(-eV/kT\_\iota) \tag{14}$$

If '*T*e' is the electron temperature, '*V*a' is the applied biasing voltage; '*I*p' is the probe current at any biasing voltage and '*I*o' is the probe current at zero biasing:

$$I\_p \, / \ I\_o = \exp\left(eV \, / kT\_e\right) \tag{15}$$

$$\begin{aligned} \ln(I\_p \mid I\_o) &= eV \;/\; kT\_o\\ T\_e &= eV \;/\; k.1 \;/\ln(I\_p \;/\; I\_o) \end{aligned} \tag{16}$$

Electron density of the laser-induced plasma can be calculated by the relation

$$m\_e = I\_o \;/\; A\_e \text{.} \left[m\_e / 2\varepsilon\_0 kT\_e\right]^\mu \tag{17}$$

where *T*e is the electron temperature; *I*o is the probe current at *V*a = 0; *A*e is the probe surface area; *n*<sup>e</sup> is the electron density; *m*<sup>e</sup> is the mass of electron; *k* is the Boltzmann constant; and *ε*<sup>0</sup> is the permittivity of free space.

Debye's length can be calculated as

$$\mathcal{A}\_{\rm D} = \left[ \varepsilon\_{o} k T\_{e} \;/\; e^{2} n\_{e} \right]^{\psi} \tag{18}$$

Plasma frequency can be calculated as

$$
\rho \rho\_p = \left( \mathfrak{n}\_\varepsilon \mathfrak{e}^2 / \ \mathfrak{s}\_\circ \mathfrak{m}\_\mathfrak{e} \right)^\vee \tag{19}
$$

#### *5.2.2. Faraday cup*

Faraday cup (FC) is a metal cup with small aperture and a deep collector, which is designed to collect charged particles. When charged particles hit the metal, the metal gains a small net charge. The metal can then be discharged to determine a small current equivalent to the number of impinging charged particles as ions or electrons. A Faraday cup is used as a measurement tool for charged-particle-beam parameters such as current and current density profile. In the laser-induced plasma, the FC is used to measure the charged-particle parameters such as charge particle density, energy (using time of flight technique), the plasma angular distribution and distance charge dependence during the plasma-free expansion into the vacuum [17, 18].

The typical FC configuration is schematically shown in Figure 7, which mainly consists of two major parts: an inner cup for the charged particle collection and the grounded shield to measure the current *Ic* produced by the primary charged particle striking the cup. The electric current in the FC depends on the incident particle beam [19, 20].

**Figure 7.** Schematic sketch of the Faraday cup

A bias voltage is applied to the cup to prevent secondary emission and electrons from escaping. The FC is connected to the measuring device as ammeter or oscilloscope to display the signal across a resistor from the conducting lead to ground [21, 22].

### *5.2.3. Scintillators and photomultiplier tubes*

Radiation detectors play an important role in the non-destruction inspection and analysis in different field of studies. Photomultipliers are used to identify low-energy photons in UV–Vis range; high-energy photons (X-rays and gamma rays) and ionizing particles are identified using scintillators. The combination of photomultiplier tube and scintillator is one of the effective tools for the non-destruction characterization, which has several advantages such as fast time response, high detection efficiency, wide detection area and availability of different types of detective materials over number of other detection methods. A photomultiplier with scintillator converts radiations to an electrical signal, which is amplified to a useful level by emission of secondary electrons [23]. Due to high selectivity of photomultiplier tube, it is used to characterize the X-ray radiations ranging from 10 to 10 keV (soft and hard X-rays) [24, 25].

Figure 8 shows the schematic sketch of the photomultiplier system, which consists of a photocathode (scintillator) to convert radiation flux into electron flux, electron-optical input system to focus and accelerate the electron flux, electron multiplier comprises a series of secondary-emission electrodes (dynodes) to amplify the electric signal and an anode to collect the electron flux from the multiplier and displays the output signal.

**Figure 8.** Schematic sketch of the photomultiplier system

The two fundamental phenomena to operate a photomultiplier are photoemission and secondary emission. Photoemission is due to a fraction of the incident photons that impart all their energy to bonded electrons of the photocathode material, giving some of the sufficient energy to escape, which are then amplified by dynodes and displayed as the output [23, 26].

#### **5.3. Solid-state detectors**

Solid-state nuclear track detectors (SSNTDs) are used to detect and estimate the energy of the charged particles in various disciplines. The solid-state nuclear track detection is an important and useful method to detect charged particles with energy ranging from tens of kilo-electron volts up to few hundred mega-electron volts. For instance, the detector of CR-39 is suitable for the detection of protons, deuterons and alpha detection due to high sensitivity of CR-39 material towards these particles. Once the SSNTD material is exposed to charged particles, these particles leave narrow damage trails on their passage through SSNTDs [27]. These damage trails represent regions of enhanced chemical activity as compared with bulk material due to disorders in the structure caused by the charged particles, which in turn are associated with large free energy. These trails are too small to be seen even with the help of a microscope. Therefore, the exposed detector material containing damage trails is subjected to the etching process in next stage to remove the material from the track.

To make the tracks visible, the exposed detectors are etched in the chemical solution to a certain temperature and for specific time. The etched detectors were then examined using an optical microscope or scanning electron microscope [28]. An optical micrograph for the carbon ions on the CR-39 material is shown in Figure 9.

On treating with suitable chemical reagents, the detector material along the damage trails interact much faster as compared with the undamaged bulk material. The resulting etch pits can be approximated by geometrical cones with the damage trails as axes. The study of etchpit geometry and determination of the range of particles in the detector provides the identifi‐ cation of the particles creating the tracks and energies of particles [27].

The diameter of the etched tracks is used to evaluate the energy of the ion using diameter energy relationship given as

$$D = 1.8409 E^{0.1624} \tag{20}$$

where *D* is the track diameter in micrometers and *E* is the energy in kilo-electron volt [29].

**Figure 9.** Optical micrograph of carbon ions track on CR-39 material

### **6. Applications of laser-induced plasma**

Due to versatility of laser-induced plasma, it has been found to be suitable for numerous applications in various fields of science and technology. The utility of powerful lasers in remote, physically inaccessible locations makes it a special tool for applications in hostile environments. Laser-induced plasma has been utilized for lateral and depth elemental analysis, thin-film deposition, X-ray production, ion production, identification of materials, etc. Further details on applications of different aspects of laser-induced plasma are described in the following sections.

#### **6.1. Laser-induced breakdown spectroscopy**

The optical emission spectroscopy of laser-induced plasma is named as laser-induced break‐ down spectroscopy (LIBS) or sometimes laser-induced plasma spectroscopy (LIPS). It is a powerful and versatile analytical technique for elemental analysis, which could be utilized for an enormous range of materials. Spectral features such as emission lines, peak intensity and integrated intensity, etc. are used for the determination of elemental concentration of the target or discriminate one material, organic or inorganic, from another through their unique spectral signatures. The basic set-up of LIBS is very simple (Figure 10); however, for higher signal intensities and lower limit of detections (LOD), double-pulse configuration of LIBS is preferred that has two variants, that is, collinear and orthogonal as shown in Figure 11. Reheating or preheating modes of operation can be utilized in both of these configurations. In the reheating mode, the laser pulse 1 ignites plasma plume, whereas laser pulse 2 (after a certain time delay) further excites plasma species to obtain larger amount of radiations. While in preheating mode, the laser pulse 1 heats the atmosphere above target surface and lets the plasma, generated by laser pulse 2, expand in hot and rare ambience.

**Figure 10.** Schematic diagram of a typical laser-induced breakdown spectroscopy set-up

LIBS provides a huge advantage of requiring no specific sample preparation procedure to follow. Ideally, it can be utilized for solid, liquid and gaseous types of materials. Unique features of LIBS are stand-off and remote analyses that are typically useful when dealing with hazardous materials, toxic environments and humanly inaccessible locations. These charac‐ teristics are unique for LIBS because other chemometric techniques do not offer such utility. In the following sections, we discuss versatile applications of LIBS in various fields of science in the recent years.

**Figure 11.** Two different modes of double-pulse laser-induced breakdown spectroscopy (DP-LIBS)

### *6.1.1. Geochemical applications*

LIBS has proved to be useful in *geochemical* studies. The spectral signatures obtained from laserinduced plasma of targets have made investigations of geological materials possible. It has been taken up as a new technique for geological studies to determine the origin of petroge‐ netically important minerals and archeologically important rocks and stones on the basis of their physical and chemical properties. It includes not only the typical elemental measurements but also material identification with and without using machine-learning software [30-34].

With the capability of highly localized analysis (on the scale of µm), LIBS has made the spatial profiling of minerals possible in geological samples. Prochazka et al. [35] utilized LIBS for 3D profiling of minerals in geological samples performed at a resolution as high as 100 µm. They practised the advantage of no pretreatment of samples and found very good results.

#### *6.1.2. Environmental applications*

Another aspect of LIBS applications is *environmental studies*. It has widespread applications in environmental sciences for real-time elemental detection and analysis outside the laboratory. Its portability, simple execution, no requirement of sample preparation and simultaneous multi-element identification are attractive attributes for on-site analysis in terms of environ‐ mental health and safety. It covers a wide range of samples from industrial waste water to aerosols in the air.

LIBS has been successfully used for the identification and quantification of various pollutants and elements including carbon (C) in samples of diverse nature and origin [36-39]. Carbon is a good indicator of CO2 gas in the environment, which is a greenhouse gas and influences the climate change. Since CO2 cycles between atmosphere, vegetation and soil, it is quite possible that it increases the content of C in terrestrial carbon sinks, for example, soil. Therefore, the increase in the C content of terrestrial carbon sinks is an indicator of increasing CO2 in the environment. Several studies have been performed that demonstrate the inherent advantages and capabilities of LIBS for quantification of the C content in soil from various origins [40-42]. Martin *et al.* [43] successfully attempted to differentiate the organic and inorganic C in the selected set of soil samples by the application of chemometric techniques of partial least squares (PLS) and principal component analysis (PCA) on LIBS spectra. By obtaining the correct signal intensity, linearity of their calibration curves significantly improved, that is, indicator of improvement accuracy of measurements. Nguyen *et al.* [44] determined the C concentration as an indicator of CO2 greenhouse gas. The challenge regarding overlapping of C emission lines was handled by optimizing the time window of data collection for minimum broadened lines to mitigate interference effects. An accuracy of 95% was ensured in measure‐ ments of C in forest soils, wetland soils and sediment samples.

In addition to greenhouse gases, heavy metals are of major concern for human health and safety. The presence of toxic heavy metals like arsenic, lead, antimony, zinc and cadmium in natural resources of the environment, for example, water, soil, air etc. or synthetic products that come in direct human contact, such as paint, are of risk for living organisms. Because of the toxicity of Pb, which could be poisonous for the human body [45], its detection and achieving low limits of detection have always been the matter of interest. Analysis of liquid samples with LIBS is challenging; short lifetime of plasma, liquid–laser coupling and splashes resulting from plasma shockwaves raise the difficulty of LIBS measurement and increase the limit of detection for liquid samples. However, new techniques for LIBS are improving the analysis of liquid samples. Järvinen *et al.* [46] incorporated evaporation pre-concentration for the measurement of Zn and Pb in water and at limits of detection as low as 0.3 and 0.1 ppm, respectively.

#### *6.1.3. Agricultural applications*

LIBS has a prominent potential for agricultural studies. Portability and no sample preparation features again provide LIBS an advantage over conventional analytical techniques. Soil, crops, plants, fruits, grains and irrigation water can be tested in field. Although the portable systems for agricultural applications are not very mature, the pace of technological advancement suggests that some decent portable LIBS systems for a variety of applications would be available soon.

LIBS has successfully been utilized for agricultural studies from various aspects. Nutrients and minerals in the soil are of great importance from agricultural point of view. There are many studies referring to the employment of LIBS for measurement of macro- and micronutrients in soil and crops. Naturally, organic samples are not very homogeneous in elemental compo‐ sition. It is sometimes difficult to analyse heterogeneous samples with LIBS because it only provides information about a highly localized region on the sample surface. The determination of Fe, Mn, Mg, Ca, Na and K in heterogeneous soil samples was performed by Nguyen [44] by using laser ablation–LIBS (LA–LIBS) without sample preparation. Unnikrishnan *et al.* [47] demonstrated the measurement of trace amounts of nutrients Cu, Zn and Ca in soil samples with limits of detection of 4.8, 4.4 and 6 ppm, respectively. In addition to mare elemental analysis, a recent publication by Ferreira *et al.* [48] shows that LIBS can potentially be effective in determination of pH of soil.

LIBS has been employed for studying diverse plants for practical applications and for explo‐ ration and development [49-51]. It is well known that the deficiency of macro-/micronutrients can adversely affect the crop yield and degrade the nutritious quality of the product. Therefore, monitoring of essential elements in crop plants, fruits and vegetables is vital for maintaining the nutritional status and food security. Numerous studies can be listed in which LIBS is featured as an investigation tool for nutritious quality of diverse range of organic samples. For instance, Arantes de Carvalho *et al.* [52] studied macro- (Ca, Mg, P) and micronutrient (Cu, Fe, Mn and Zn) concentrations in a variety of crop samples that include sugarcane, soy, citrus, coffee, maize, eucalyptus, mango, bean, banana, lettuce, brachiaria, pearl millet, grape, rubber tree and tomato. Kim *et al.* [53] investigate the quantity of macronutrients (Mg, Ca, Na and K) in spinach and rice samples and apply discriminatory operations upon LIBS spectra to discriminate the samples contaminated with pesticides from clean ones. LIBS has also been utilized for the determination of toxic heavy metals and contaminations residing on the surface of fruits because of environmental pollution and pesticide sprays [54, 55]. The list of publica‐ tions regarding LIBS applications for investigations of agricultural products encompassing diverse variety of samples is long. One can simply deduce its applicability in almost all parts associated with agriculture, ranging from irrigation water to ripe crops, fruits and vegetables.

#### *6.1.4. Stand-off and remote applications*

Stand-off and remote analysis capabilities are unique for LIBS. In stand-off application, a powerful laser pulse is fired at a distant (approximately several meters) target and emissions from the microplasma at the target surface are collected through a telescope. Schematic diagram of a typical stand-off LIBS system is provided in Figure 12. In remote studies, a fibreoptic probe is commonly utilized to reach remote and humanly inaccessible locations. This probe delivers the laser to the target and transfers the plasma radiations from target to the spectrometer back in the system. Schematic illustration of a typical remote LIBS system is depicted in Figure 13. Portability of LIBS system has elevated the potential of LIBS capability in the investigation of quarantined regions, potentially hostile and physically inaccessible locations. Applications of stand-off and remote LIBS can be found in identification of bioma‐ terials at far distance, investigation of deep sea objects, studying radioactive and explosive materials from a safe distance, monitoring the composition of alloys in production line, Mars and exploration, etc.

**Figure 12.** Schematic illustration of a typical stand-off LIBS system

**Figure 13.** Schematic illustration of a typical remote LIBS system

At a distance of ~6.2 m, Vítková *et al.* [56] performed LIBS analysis on a variety of biological samples that included mortar, sea shell, human tooth, swine bone, soil pellet and a brick. Each of these samples was classified on the basis of their spectral signatures by complementing LIBS with supervised machine-learning methods, such as linear discriminant analysis (LDA) and artificial neural networks (ANN). In another study, Vítková *et al.* [57] demonstrated the LIBSbased discrimination of historically important bricks according to their archaeological localities with the help of LDA at a stand-off distance of ~6.2 m. Another application of standoff LIBS is presented for the analysis of soil and vegetation powders for studying the content of heavy metals and toxic trace elements in targets [58].

An interesting underwater application of stand-off LIBS is demonstrated by Fortes *et al.* [59]. To test the feasibility of underwater LIBS at a stand-off distance of 0.8 m from sensor to the target deeply submerged in the water, fundamental investigations were performed. A similar study has reported [60] the application of remote LIBS for studying underwater shipwreck. In such applications, the advantage of remote over stand-off approach is that the laser beam interaction with ocean water is minimal and the laser energy losses are likewise minimum. However, the advantage of stand-off approach is that it is not required to bring the LIBS probe to the target in deep waters, rather, measurements can be taken from far.

Stand-off LIBS is capable of investigating nuclear materials from a safe distance, for instance, Gong *et al.* [61] has demonstrated the investigations made on cerium oxide (CeO2) and potassium chloride (KCl) at a stand-off distance of 1.45 m through the protection shield. Standoff LIBS is particularly suitable for industrial applications like real-time monitoring of product quality in production line without interfering the production process [62].

LIBS has gained popularity among the geologists since the NASA has equipped the Curiosity Mars rover with a ChemCam LIBS system. A field test of this system with a remote analysis probe was published as a meeting abstract [63]. The ChemCam LIBS system has proved useful and reliable in Mars exploratory mission since the Curiosity has landed the surface of Mars in year 2012 [64].

### **6.2. Pulsed laser deposition**

Pulsed laser deposition is a versatile technique. It makes use of the laser's ability to ablate and excite almost every type of material and, therefore, has been utilized to deposit an extensive range of thin films may it be metallic, polymeric or composite.

A high-power laser pulse, typically ≥ 108 W/cm2 , is utilized to evaporate and ionize the material into a plasma plume that expands in the ambient environment perpendicular the target surface. This evaporated and highly accelerated material is collected on an appropriately positioned substrate upon which it condenses and grows a thin film. The size and expansion velocity of the plasma plume depend largely upon nature and density of the ambient atmos‐ phere along with laser pulse duration and irradiation. The plasma plume has found to expand with velocities of the order of 106 cm/s under vacuum conditions (~10−6 mbar) for typical laser fluences ~109 −1010 W/cm2 and increases for higher fluences. Plume dimensions and expansion velocity are also inverse functions of the ambient pressure and time [65, 66]. When the plasma species strike surface of the substrate with such high velocities, sufficient adhesion is obtained to keep particles sticking with the surface for gradual growth of the film. The film growth and quality depend upon characteristics of substrate, substrate temperature and energy of the plasma species, that is, atoms, ions and clusters striking the surface of substrate that in turn depends upon laser parameters. Surface morphology and texture of thin films are strongly influenced by substrate temperature, nature and pressure of the ambient gas. By controlling the ambient atmosphere, we can actually control mechanical, optical, surface morphological properties of the deposited thin film. The choice of laser and laser parameters are vital for PLD. The selection of wavelength and energy of laser depends upon the coupling efficiency with target material to be used. Nanosecond Nd:YAG and excimer lasers are the most widely utilized lasers for PLD as found in the published literature.

PLD is principally a simple technique and versatile in terms of utilizing target and substrate materials. A typical PLD arrangement is schematically illustrated in Figure 14. Laser ablation is considered to be a stoichiometric phenomenon, which means that plasma plume has the same composition as that of the target that is delivered onto the substrate surface as a thin film. It is normally referred to as 'stoichiometry transfer' from target to film on substrate surface that is difficult to achieve with other techniques. It, therefore, permits the control of thin-film composition by appropriate selection of target composition. Multilayers of different materials can be deposited on a single substrate by using multiple targets during the PLD procedure. Since only small substrate areas (typically ~1 × 1 cm2 ) can be targeted by the laser-induced plasma, it allows the preparation of complex samples enriched with isotopes for laboratoryscale research purposes. Being a short-pulsed phenomenon makes PLD a flexible operation, because operational parameters can be varied from pulse to pulse according to the requirement of the application.

Despite numerous advantages of the PLD technique, it is not yet well adopted in the industrial manufacturing process because of the uncontrolled plasma process and small-area deposition. In addition, the deposition may not be of uniform thickness because of strong forward peaking nature of the laser-induced plasma. However, it has gained popularity among the scientists who are convinced and exploring its enormous potential in laboratories and discovering new possibilities and procedures for extensive applications in diverse fields of science and tech‐ nology. Amorphous, polycrystalline, epitaxial thin films can be grown, depending on the nature of the substrate and its temperature. There are a large number of applications in which PLD has shown great promise, for example, photovoltaics, superconductor technology, nanostructured thin films, optical coatings, etc.. Following is given a brief review of versatile applications of PLD.

PLD has been found to be very suitable for deposition of thin films for *photovoltaic* applications. Sekiguchi *et al.* [67] were perhaps the first to demonstrate the use of PLD for epitaxial growth of CZTS films on GaP substrates heated at 350–400 °C. Stoichiometric crystalline thin films with an appropriate optical band gap of ~1.5 eV were observed. However, the first-ever publication about Sn-rich CZTS thin film deposition by PLD, particularly for solar cell applications was reported by Moriya *et al.* [68]. Films showed an optical band gap of ~1.5 eV, consequently prepared solar presented a conversion efficiency of 1.74% for an active area of 0.092 cm2 .

Pulsed laser deposition has also found application in depositing high-quality superconducting thin films. Superconductivity has found to be thickness dependent; therefore, precise control of thin films is crucial. Here, PLD provides an advantage of controlled growth of thin films through variation in experimental parameters [69, 70], which can improve the quality of thin films in individual applications. There are many recent studies found in literature that demonstrate the exploration of experimental parameters of PLD for diverse types of highquality superconducting thin films. Oshima [70] report the deposition of LiTi2O4 thin films of various thicknesses by employing the PLD method using krypton fluoride laser (KrF) excimer

**Figure 14.** Schematic illustration of pulsed-laser deposition system

laser. Superconducting transition was observed around 12 K at a film thickness of 9 nm. One of the benefits of PLD method is that quality and thickness of thin films can be controlled by varying deposition time, growth rate (determined by pulse repetition rate) and substrate temperature during the film growth [69, 70].

PLD method is highly suitable for growing *composite thin films* of a wide range of materials over diverse substrates. Sharma *et al.* [71] report on composite thin films 0.9BiFeO3-0.1YCrO3 on Pt, TiO2, SiO2 and Si substrates by sequential deposition of BiFeO3 and YCrO3 targets by the PLD method. Consequently obtained thin films represented fine, particle-free smooth surfaces with a crystal structure as a mixture of BiFeO3 and YCrO3 crystals. Chowdhury *et al.* [72] deposited SnO2-Fe2O3 composite thin films by the PLD method with the aim to investigate structural, morphological, optical and electrical properties that revealed their suitability for optoelectronic applications.

An interesting possible application of PLD can be the coating of surgical instruments with antimicrobial composite thin films. Several studies have demonstrated the growth of antifun‐ gal and antibacterial composite thin films on various substrates. For instance, Ag-SiO2 antibacterial composite thin films were grown on Si (111) substrate by the PLD method using KrF Excimer laser [73] at room temperature in the oxygen environment. In another study, Pradhaban *et al.* [74] demonstrated the deposition of antibacterial Ag:ZrO2 composite films on stainless steel substrates by the PLD technique using Nd:YAG laser. Similary, Eraković *et al.* [75] utilized PLD to grow antifungal Ag:HA (hydroxyapatite) composite thin films on titanium (Ti), anodized Ti and Si (111) substrates.

PLD is still a laboratory-scale method for thin-film deposition. Large amount of data published on investigation of the PLD method for a variety of applications is an evidence of intensive efforts being made by the scientists. However, it will take time to see the PLD method being used for industrial-scale applications.

### **7. Conclusions**

This chapter encompasses fundamentals of laser-induced plasma from laser–matter interac‐ tion to radiation emission from plasma and deposition of the ablated material on a distant substrate. LIP is a rapid process and fairly complex in nature. It starts with absorption of pulsed-laser energy by the material that breaks it down and evaporates it in the form of plasma that eventually decays after few tens of microseconds. During its lifetime, the plasma temper‐ ature, particle density and continuum and line emissions show temporal fluctuations. Characteristics and emissions of laser-induced plasma strongly depend upon laser parameters, nature of the target and ambient conditions. There are several methods used for estimation of plasma parameters based on spectroscopic data. Among them, the Boltzmann plot method for plasma temperature measurements and the Stark broadening method for estimation of electron density are the most commonly used methods. Besides spectroscopic diagnostics, the most well-known electrical diagnostic tools include Langmuir probe, Faraday cups and scintillators and photomultipliers, which have been widely used to study plasma parameters, energy of charged particles and X-ray emissions from plasma, respectively. A physical method of measuring the density, energy and distribution of charged particles is the use of solid-state nuclear track detectors. By investigating the density and dimensions of the tracks produced on SSNTDs, the density, energy of the particles emitting at various angles of the plasma can be obtained.

Being a rich source of radiation and particle emission, LIP has found a variety of applications in diverse areas. Line emissions from the laser-induced plasmas inform about elemental composition of the material that has opened up several fronts of applications. It is termed as LIPS (laser-induced plasma spectroscopy). It has been utilized for numerous laboratory and in-field applications from food safety to explosive detection, soil studies to Mars exploration, archaeology to artefact studies, micro to bulk exploration and many more. The samples that could be studied with LIPS have virtually no limits, from solids to gasses and aerosols have been investigated that encompass organic materials to nuclear and explosive materials. In addition to spectroscopic investigations, the excited material of LIP has been utilized for thinfilm deposition on a wide range of substrates for diverse applications. It is commonly termed as pulsed-laser deposition. Through PLD, films of various materials including conductors, insulators, dielectrics, in general, have been deposited on a variety of substrates. Deposition of complex hybrid and multilayer films, a combination of multiple materials, have also been successfully demonstrated. The quality, morphology and structure of films depend upon the ambient atmosphere and substrate temperature besides laser parameters.

Because of numerous intrinsic benefits of LIP for various applications over other conventional techniques, some commercial products, specifically LIPS systems, can be seen in the market. However, the uncontrolled nature of LIP hinders its wider deployment for industrial appli‐ cations. Most of the potential applications are still in infancy and are being explored at the laboratory level. Keeping in mind the current pace of advancement in technology and enormous potential of LIP, in near future, we can expect to see some products in the market that are based on LIP for innovative applications.

### **Author details**

Kashif Chaudhary\* , Syed Zuhaib Haider Rizvi and Jalil Ali

\*Address all correspondence to: kashif@utm.my

Laser Center, Ibnu Sina Institute for Scientific & Industrial Research (ISI-SIR), Universiti Te‐ knologi Malaysia (UTM), Malaysia

### **References**


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src="/images/tex/24016.gif" alt="\hbox {NdGaO}\_{3}"> </inline-formula> (001) Sub‐ strates*.* Applied Superconductivity, IEEE Transactions on. 25(3): 1-4.


**Section 4**

**Plasma Diagnostics**
