**2. Laser-material interaction**

During laser-material interaction, the product materialises directly after laser irradiation onto the surface of the solid target material by condensing the plasma plume. After laser-pulse deposition onto the surface of the target material, the laser-pulse energy will heat the target, leading to an increase in the temperature of the materials. The temperature propagates in the axial and radial directions in a specific area. In the case of a perpendicular laser beam on a flat target material, the temperature propagation as a function of time (*t*) and depth (*x*) can be written as follows [14]:

$$\Delta T(\mathbf{x}, t) = \mathbf{2}(1 - R)\alpha I\_o \left(\frac{t}{\pi kpc}\right) \text{ierfc}\left(\frac{\mathbf{x}}{2\sqrt{kt/\rho c}}\right) \tag{1}$$

where *t*, *R*, *α*, *I*o, *k*, *ρ* and *c* are irradiation time, reflectivity, absorptivity, spatial distribution of laser intensity, thermal conductivity, target material density and velocity of light, respectively. When > 2 the surface temperature can be simplified as follows:

Although some laser beam parameters such as laser fluence, wavelength and pulse duration are important for controlling material processing, the ablation environment is also an important factor in laser-material interaction. For example, Zhu et al. [9] concluded that the ablation rate of a Si target material is greatly enhanced by using a water confinement regime (WCR) at

178 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

amplitude of the acoustic waves is approximately 25% higher than that in an ambient environment. Besner et al. [10] used a femtosecond laser for ablation in a vacuum, air and water for surface modifications; they showed that the threshold values of the Si and Au target materials were almost identical in all three environments. The values of Si in the single and

, respectively. Patel et al. [11] showed that the laser-ablation process is more efficient in water than in air, which depends on the thermal properties of the materials. It was also concluded that laser-ablation production in water is more suitable for the production of uniform nanoparticles and the mass production of nanoparticles. In addition to the aforementioned environments for laser ablation, Lindley et al. [12] studied the laser-ablation plume of an aluminium target material in a plasma environment, as well as in a vacuum and argon gas. It was concluded in this study that the laser-ablation plumes in the plasma expand and dissipate slightly faster than in the gas and the vacuum. Iqbal et al. [13] showed the effects of the laser fluence and ablation environments (vacuum and hydrogen) on the micro- and nanostructure of a Ge target material. It was shown in their study that the formation and growth of laser-induced periodic surface structures (LIPSS), cones and micro-bumps strongly depended on the laser fluence and environmental conditions. Hence, the growth, size and shape

In this chapter, a critical comparison of laser ablation in different environments such as a vacuum, ambient air, different liquid environments and different background gases is presented. The optimal medium and laser-beam parameters for laser ablation will be desig-

During laser-material interaction, the product materialises directly after laser irradiation onto the surface of the solid target material by condensing the plasma plume. After laser-pulse deposition onto the surface of the target material, the laser-pulse energy will heat the target, leading to an increase in the temperature of the materials. The temperature propagates in the axial and radial directions in a specific area. In the case of a perpendicular laser beam on a flat target material, the temperature propagation as a function of time (*t*) and depth (*x*) can be

> ( , ) 2(1 ) <sup>2</sup> *<sup>o</sup> t x T x t R I ierfc kpc kt c* a

p

D =- ç ÷ ç ÷ è ø è ø

r

(1)

æ ö æ ö

. They also found that in water, the first peak-to-peak

; for Au, their values were 0.9 and 0.3 J/

laser fluence ranges from 2.0 to 5.0 J/cm2

cm2

nated.

multi-pulse irradiation regime were 0.4 and 0.2 J/cm2

of these structures strongly depended on the laser fluence.

**2. Laser-material interaction**

written as follows [14]:

$$
\Delta T(t) = \frac{2\alpha I\_o \sqrt{t}}{\sqrt{\pi k \rho c}} \tag{2}
$$

Two main things which have an effect on the laser ablation of materials are: laser-beam parameters such as laser fluence, laser wavelengths and laser-pulse duration, and factors in the experimental set-up, such as the type of environment and the solution. Laser-ablation mechanisms and their products are different depending on whether the laser-material interaction is produced by nanosecond, picosecond or femtosecond lasers [4]. In the laserablation process, groove width and depth are two important factors with which to characterise the results (see **Figure 1**) [15].

**Figure 1.** Groove width, groove depth and heat-affected zone of a laser-ablated material target.

Laser ablation in the target material occurs when the laser intensity reaches its threshold value. The laser-intensity threshold for material removal (*I*th) can be written as follows [15]:

$$I\_{\rm th} = I\_o \exp\left(\frac{-2G\_w^2}{d\_b^2}\right) \tag{3}$$

where *Io*, *Gw*, and *db* are laser-beam intensity, groove width and laser-beam diameter, respectively. This equation can be written as follows:

$$\ln\left(\frac{I\_o}{I\_{\text{th}}}\right) = \left(\frac{2G\_w^2}{d\_b^2}\right)^{\rho} \tag{4}$$

where *β* is an empirical coefficient which may be added to the formula due to some effects, such as recoil pressure, plasma-shielding effect, dynamics of vaporisation and multiple reflection of the laser beam in the cut channel. Thus, the groove width (*Gw*) equation can be written as a general equation as follows:

$$G\_w = \sqrt{\frac{d\_b^2}{2} \left[ \ln \left( \frac{I\_o}{I\_{th}} \right) \right]^{l\rho}}\tag{5}$$

As shown in the following equation, the predictive model for groove depth (*Gd*) can be calculated based on 'Furzikov's study and the *b* coefficient applied to the logarithmic function of the laser intensity ratio' [15]:

$$G\_d = \left[ \gamma \frac{\lambda}{\sqrt{n}} \left( \frac{1}{\alpha} + \sqrt{\frac{4kd\_o}{\nu}} \right) \right]^{l2} \left[ \ln \frac{I\_o}{I\_{th}} \right]^{l/\rho} \tag{6}$$

where *γ* is another empirical coefficient, *λ* is the laser wavelength, *n* is the refractive index of the target material, *α* is the absorption coefficient of the target material, *k* is the thermal diffusivity of the target material and *ν* is the laser-traverse speed. The last empirical coefficient (*γ*) is related to the degree of laser absorption and plasma formation in different environments such as air, water and ethanol, in which their penetration depths to produce a cut are different. In polymethyl methacrylate (PMMA) the approximated values of the refractive index *n*, absorption coefficient *α* and the thermal diffusivity *k* are 1.4827, 500 cm−1, and 1.073 × 0−7 m2 /s. The HAZ formula can be written as [15]:

$$G\_{H\mathcal{K}} = \sqrt{\frac{d\_b^2}{8} \left[ \ln \left( \frac{I\_o}{I\_{th}} \right) \right]^{l\rho}} - \frac{G\_w}{2} \tag{7}$$

The groove formed by laser ablation of a Si target material in air and water is quite different. In water, the hole edge is very smooth, but in an ambient environment, the hole has bumps with a height of 3.6 mm. In addition, the hole diameter in water is larger than that in air, and the hole depth in water is several times greater than in air [8]. **Figure 2** shows the laser-ablated groove profile of a Silicon target material ablated in air and in water [8].

The ablation depth increases with increasing laser power, reducing spot size and decreasing scan speed [7]. In addition, the ablation rate significantly decreases with the depth of the hole [16].

During laser ablation, some of the laser energy will be lost in the ablation environment before it reaches the target material. The ratio of loss is higher in water than in air and in air, it is higher than in a vacuum. As shown in **Figure 3**, under the same laser-beam parameters the energy loss in air is higher than that in a vacuum. This is because for laser ablation in a vacuum, laser-induced air breakdown and ionisation do not exist; in addition, the ejected energetic electrons freely diffuse in the vacuum. The absorption of the laser beam which causes laser energy loss in the vacuum may only be caused by hot electron. Furthermore, absorption rate in a vacuum is very low because the collision frequency is low and the electron density is small [17]. In contrast, in air, laser-induced air breakdown does exist; as a result, laser absorption is increased. In water, laser absorption will be increased considerably due to the water level above the sample.

where *β* is an empirical coefficient which may be added to the formula due to some effects, such as recoil pressure, plasma-shielding effect, dynamics of vaporisation and multiple reflection of the laser beam in the cut channel. Thus, the groove width (*Gw*) equation can be

> <sup>1</sup> <sup>2</sup> ln 2 *b o*

é ù æ ö <sup>=</sup> ê ú ç ÷ ê ú ë û è ø

*th*

b

1 2 1

*th*

b

(5)

(6)

/s.

(7)

*I*

As shown in the following equation, the predictive model for groove depth (*Gd*) can be calculated based on 'Furzikov's study and the *b* coefficient applied to the logarithmic function

1 4 ln *b o*

where *γ* is another empirical coefficient, *λ* is the laser wavelength, *n* is the refractive index of the target material, *α* is the absorption coefficient of the target material, *k* is the thermal diffusivity of the target material and *ν* is the laser-traverse speed. The last empirical coefficient (*γ*) is related to the degree of laser absorption and plasma formation in different environments such as air, water and ethanol, in which their penetration depths to produce a cut are different. In polymethyl methacrylate (PMMA) the approximated values of the refractive index *n*, absorption coefficient *α* and the thermal diffusivity *k* are 1.4827, 500 cm−1, and 1.073 × 0−7 m2

> <sup>1</sup> <sup>2</sup> ln

é ù æ ö <sup>=</sup> ê ú ç ÷ - ê ú ë û è ø

*dI G <sup>G</sup> I*

8 2 *bo w*

b

*th*

The groove formed by laser ablation of a Si target material in air and water is quite different. In water, the hole edge is very smooth, but in an ambient environment, the hole has bumps with a height of 3.6 mm. In addition, the hole diameter in water is larger than that in air, and the hole depth in water is several times greater than in air [8]. **Figure 2** shows the laser-ablated

The ablation depth increases with increasing laser power, reducing spot size and decreasing scan speed [7]. In addition, the ablation rate significantly decreases with the depth of the hole

During laser ablation, some of the laser energy will be lost in the ablation environment before it reaches the target material. The ratio of loss is higher in water than in air and in air, it is higher than in a vacuum. As shown in **Figure 3**, under the same laser-beam parameters the

é ù æ ö é ù = + ê ú ç ÷ ê ú ë û è ø ë û

*w*

l

g

*HAZ*

groove profile of a Silicon target material ablated in air and in water [8].

*d*

*d I <sup>G</sup>*

180 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

*kd I <sup>G</sup> <sup>n</sup> v I*

a

written as a general equation as follows:

of the laser intensity ratio' [15]:

The HAZ formula can be written as [15]:

[16].

**Figure 2.** Laser-ablated region profile of a Si target after 1000-pulse irradiation in air (a) and water (b).

**Figure 3.** Pulsed laser-beam energy loss during laser ablation of aluminium target materials in air and in a vacuum. Laser wavelength (*λ*) was 800 nm and laser pulse duration (*τ*) was 100 fs.
