*2.2.1. Plasma formation and characteristics*

The formation of a plasma through laser irradiation is a highly complex process that involves several stages, which are illustrated in the case of a liquid in **Figure 2**. The laser irradiation and absorption of the laser energy by the target leads to the explosive vaporization of the target material and the formation of a cavitation bubble and subsequent nucleation and growth of nanoparticles that are ejected into the fluid.

While the detailed mechanisms are still not understood yet, depending on the type of laser, continuous wave (CW), nanosecond or pico-, respectively, and femtosecond, the mechanisms leading to the removal of material and plasma formation are different. In the case of CW lasers, material is removed primarily by melting, which creates a large heat-affected zone (HAZ), and material ejection is mainly dominated by thermal processes [11]. In nanosecond lasers, there are three main stages that lead to the formation of a plasma. In the first, laser photons couple both with electrons and phonons of the target material. The photon–electron coupling then results in an immediate rise of the electron temperature, leading to vaporization of the target. Compared to CW lasers, the HAZ created by nanosecond pulsed lasers is smaller.

With ultrafast pico- and femtosecond pulses, the laser pulse duration is much shorter than the timescale for energy transfer between free electrons and the material lattice, and electrons are excited to only a few or few tens of electron volts. Consequently, the lattice temperature of the target remains unchanged, and the main amount of the laser pulse energy is primarily absorbed in a thin layer of only a few microns close to the surface, where extremely high pressures and temperatures can be attained. The absorbed energy heats the material very quickly past the melting point, directly to the vapor phase with high kinetic energy, and the material is removed by vaporization. Consequently, in the case of pico- and femtosecond pulsed lasers, mainly the photon absorption depth governs the heated volume, the influence of thermal diffusion depth being smaller.

zone to be irradiated by the laser is illuminated from behind using a bright light source (a flash lamp or a laser source). The change in the fluid density leads to refraction of the light beams from the light source on the detector, which results in the formation of brighter and darker

PLA leads to the formation of shockwaves inside the target and the fluid. For 2D shocks produced in liquid water, pressures of up to 30 GPa and velocities up to Mach 6 have been

**Figure 1** illustrates the main components for conducting PLA in high-density media. The equipment consists of a reaction vessel capable of withholding pressures up to several megapascals. Usually, the reactor vessels are made of stainless steel (typically SUS316) or, in case of highly corrosive fluids such as supercritical water, other highly corrosion-resistant materials, mainly Ni-based alloys, for example, Hastelloy™, are used. As viewports, usually sapphire is used because of its superior hardness, high thermal conductivity, and chemical resistance. Another advantage is the large domain of optical transmission, from about 150 to

To characterize the evolution of the plasma and the cavitation bubble, different types of imaging methods are used: The simplest is direct imaging, which is used for temporal and spatial evolution of the plasma and, when using bandpass filters of specific wavelengths, the

Finally, shadowgraph and Schlieren imaging allow the observation of changes in the fluid density, and optical emission spectroscopy can be used to characterize the plasma. Examples of these techniques employed for the characterization of PLA in high-density media will be

The formation of a plasma through laser irradiation is a highly complex process that involves several stages, which are illustrated in the case of a liquid in **Figure 2**. The laser irradiation and absorption of the laser energy by the target leads to the explosive vaporization of the target material and the formation of a cavitation bubble and subsequent nucleation and growth of

While the detailed mechanisms are still not understood yet, depending on the type of laser, continuous wave (CW), nanosecond or pico-, respectively, and femtosecond, the mechanisms leading to the removal of material and plasma formation are different. In the case of CW lasers, material is removed primarily by melting, which creates a large heat-affected zone (HAZ), and material ejection is mainly dominated by thermal processes [11]. In nanosecond lasers, there are three main stages that lead to the formation of a plasma. In the first, laser photons couple both with electrons and phonons of the target material. The photon–electron coupling then results in an immediate rise of the electron temperature, leading to vaporization of the target.

Compared to CW lasers, the HAZ created by nanosecond pulsed lasers is smaller.

spatial distribution of emissions corresponding to certain species can be monitored.

**2.2. Characteristics of pulsed laser ablation plasmas in high-density media.**

zones on the detector and correlates with the fluid density gradient.

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

reported [9].

5000 nm.

presented in sections 2.2.2 and 2.2.3.

*2.2.1. Plasma formation and characteristics*

nanoparticles that are ejected into the fluid.

With nano-, pico-, and femtosecond pulsed lasers becoming more and more available, PLA has opened a wide range of new possibilities for materials processing: deposition of thin solid films, nanocrystal growth, surface cleaning, and the fabrication of microelectronic devices.

The evolution of the cavitation bubble is described in more detail in the next section.

**Figure 2.** Schematic of the different events occurring after pulsed laser irradiation of a target inside a fluid. The rapid heating of the target and subsequent plasma formation leads to vaporization of the fluid and the formation of a cavitation bubble. Nanomaterial (NM) nucleation and growth occurs in the later stages, at *t* ∼ 10∼<sup>6</sup> − 10−4 s. Figure adapted with permission from Ref. [12].

### *2.2.2. Evolution of cavitation bubbles*

One characteristic that distinguishes PLA in liquids or in pressurized media from PLD or PLA in vacuum is the confinement of the plasma plume by the surrounding fluid. As a consequence of this confinement and the large temperature rise of the target and the medium in the vicinity of the plasma plume, PLA in dense media is accompanied by the formation of a cavitation bubble. It is a region whose internal conditions—pressure and temperature—are different from those of the surrounding medium. Depending on the fluid conditions, expansion and compression of the cavitation bubble occur in several steps. It has been suggested that the bubble formed during pulsed laser ablation plays an important role in nanoparticle formation, as it confines the primary particles and redeposits them to the substrate.

In the first step, the cavitation bubble grows, until its internal pressure *p*<sup>b</sup> becomes equal to the external pressure, *p*0, after which the cavitation starts shrinking. Depending on the conditions of the medium, the expansion and shrinking can occur over several cycles.

To study the evolution of the cavitation bubble as a function of pressure, experiments in pressurized distilled water up to 3.5 × 107 Pa on Ti targets at pulse widths of 10 ns and a laser fluence of 22 mJ pulse-1 were conducted [13]. **Figure 3** shows the variation of the size of the

**Figure 3.** Variation of the length of the first cavitation bubble in the direction perpendicular to the target surface, *L*p, as a function of the delay time *t*d. **(a)** Water ambient pressure *p* = 3 × 106 Pa. **(b)** Water ambient pressure *p* = 1 × 105 Pa. The inset indicates the geometry of the cavitation bubble and the measure of *L*p, and the shaded area labelled "a" indicates the x- and y-axes scales. Data adapted with permission from Ref. [13].

cavitation bubble in a direction perpendicular to the target surface as a function of time and for two different water pressures.

*2.2.2. Evolution of cavitation bubbles*

One characteristic that distinguishes PLA in liquids or in pressurized media from PLD or PLA in vacuum is the confinement of the plasma plume by the surrounding fluid. As a consequence of this confinement and the large temperature rise of the target and the medium in the vicinity of the plasma plume, PLA in dense media is accompanied by the formation of a cavitation bubble. It is a region whose internal conditions—pressure and temperature—are different from those of the surrounding medium. Depending on the fluid conditions, expansion and compression of the cavitation bubble occur in several steps. It has been suggested that the bubble formed during pulsed laser ablation plays an important role in nanoparticle formation, as it

In the first step, the cavitation bubble grows, until its internal pressure *p*<sup>b</sup> becomes equal to the external pressure, *p*0, after which the cavitation starts shrinking. Depending on the conditions

To study the evolution of the cavitation bubble as a function of pressure, experiments in pressurized distilled water up to 3.5 × 107 Pa on Ti targets at pulse widths of 10 ns and a laser fluence of 22 mJ pulse-1 were conducted [13]. **Figure 3** shows the variation of the size of the

**Figure 3.** Variation of the length of the first cavitation bubble in the direction perpendicular to the target surface, *L*p, as

inset indicates the geometry of the cavitation bubble and the measure of *L*p, and the shaded area labelled "a" indicates

Pa. **(b)** Water ambient pressure *p* = 1 × 105

Pa. The

a function of the delay time *t*d. **(a)** Water ambient pressure *p* = 3 × 106

the x- and y-axes scales. Data adapted with permission from Ref. [13].

confines the primary particles and redeposits them to the substrate.

of the medium, the expansion and shrinking can occur over several cycles.

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

PLA in liquids has enabled the formation of a large variety of materials. A large variety of metallic nanoparticles [3], diamonds [14], and other carbon nanostructures [15].

The basis for calculating the variation of and inside a cavitation bubble can be estimated by using the the Rayleigh–Plesset equation:

$$\frac{p\_b(t) - p(t)}{\rho} = r\_b(t) \frac{\mathbf{d}^2 r\_b(t)}{\mathbf{d}t^2} + \frac{3}{2} \left(\frac{\mathbf{d} r\_b(t)}{\mathbf{d}t}\right)^2 + \frac{4\nu}{r\_b(t)} \frac{\mathbf{d} r\_b(t)}{\mathbf{d}t} + \frac{2S}{\rho\_1 r\_b(t)}\tag{1}$$

where *r*b*(t)* is the radius of the cavitation bubble at time *t, p*b*(t)* the pressure inside the bubble, and *p(t)* the pressure at a distance far from the bubble, *ρ* the density, *ν* the kinematic viscosity, and *S* the surface tension of the surrounding fluid. The value of *p*b at time *t* is given by

$$p\_{\rm b}(t) = p\_{\rm v}(T\_{\rm b}) + p\_{\rm G0} \left(\frac{T\_{\rm b}}{T\_{\rm o}}\right) \left(\frac{r\_{\rm o}}{r\_{\rm b}(t)}\right)^{\ddagger \gamma} \tag{2}$$

Here *p*v(*T*b) is the vapor pressure at *T*b, and *T*<sup>0</sup> is the temperature at a large distance from the cavitation bubble, γ = *Cp* / *Cv* is the isentropic expansion factor, and *p*G0 is the pressure of a bubble present in the fluid before laser irradiation and *r*<sup>0</sup> the corresponding radius, related by *p*G0 = *p*0 - *pv*(*T0*) + 2*S*/ *r*0. The temperature in the bubble is given by

$$T\_{\mathbf{b}}(r) = \frac{T\_0 r\_0^{\mathfrak{A}(r^{-1})}}{(r^3 - a^3)^{r^{-1}}} \tag{3}$$

with *a = r*0*/8.86* the hard core radius of the bubble. To improve the original Rayleigh–Plesset model, the authors took into account the hemispherical nature of the cavitation bubble and the effect of the contact angle between the bubble, the target, and the water [16]. **Figure 4** shows the variations of the pressure and temperature at the time of collapse inside the first cavitation bubble as a function of the external water pressure, *p*0. As can be seen from the graph, the values of *p*<sup>b</sup> reach values up to several TPa, with *p*<sup>b</sup> > 10 at the time of collapse for *p*<sup>0</sup> > 10 MPa. On the other hand, the temperature at the collapse decreases only weakly with *p*0, with values varying between 7000 − 13000 K.

The variation of the volume as a function of water pressure = 0.1, 10, 20 and 30 MPa is illustrated in **Figure 5**, which displays the variation of the cavity volumes as a function of time up to 2 μs following the laser irradiation. The volumes were estimated by measuring the extension of the cavitation bubbles from the shadowgraph images, assuming half of an oblate spheroid ( <sup>=</sup> <sup>4</sup> <sup>2</sup> <sup>⋅</sup> <sup>3</sup>*πa2 b*, where is the semi-major axis and the semi-minor axis). As can be

**Figure 4.** Dynamics of cavitation bubble and variation of pressure and temperature inside the bubble as a function of water pressure. **(a)** Experimental values of the evolution of the cavitation bubble radius as a function of time in comparison with theoretical model. **(b)** Variation of the bubble pressures and bubble temperatures at the time of collapse of the first cavitation bubble as a function of the water pressure *p*0, using a modified Rayleigh–Plesset model. The dashed lines connecting the data points act as guides to the eye. Data adapted with permission from Ref. [16].

seen in **Figure 5** up to = 600 ns, the volumes of the cavities are practically independent of the hydrostatic pressure. This is an indication that during the initial stages after laser irradiation, other mechanisms dominate the transient pressure profile surrounding the laser-ablated region [17].

The evolution of the cavitation bubble has been found to play an important role in the formation of nanoparticles (cf. Section 3.2). This control of the cavitation bubble dynamics can be achieved by several methods. One is by changing the laser fluence [18], or the viscosity of the medium [19]. Finally, another possibility is to pressurize the solution used for the PLA. By tuning these different parameters, the size, the chemical composition, and the type and concentration of defects in nanoparticles can be modified.

By changing the pressure of the surrounding medium, the plasma becomes confined and is restricted by the surrounding fluid.

**Figure 6** shows a series of shadowgraph images obtained for a cavitation bubble generated in supercritical CO2 (cf. **Figure 1** for a possible experimental setup for realizing shadowgraph images of PLA in high-density media). Near the critical point and depending on the fluid conditions, not only a single cavitation bubble but also a structure resembling a double-bubble can be observed [10, 20].

Pulsed Laser Ablation in High-Pressure Gases, Pressurized Liquids and Supercritical Fluids: Generation... http://dx.doi.org/10.5772/65455 229

**Figure 5.** Evolution of the volume of the cavitation bubbles generated by single pulse irradiation on a brass target immersed in water pressurized at 0.1, 10, 20, and 30 MPa. Data adapted with permission from Ref. [17].

seen in **Figure 5** up to = 600 ns, the volumes of the cavities are practically independent of the hydrostatic pressure. This is an indication that during the initial stages after laser irradiation, other mechanisms dominate the transient pressure profile surrounding the laser-ablated

lines connecting the data points act as guides to the eye. Data adapted with permission from Ref. [16].

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

**Figure 4.** Dynamics of cavitation bubble and variation of pressure and temperature inside the bubble as a function of water pressure. **(a)** Experimental values of the evolution of the cavitation bubble radius as a function of time in comparison with theoretical model. **(b)** Variation of the bubble pressures and bubble temperatures at the time of collapse of the first cavitation bubble as a function of the water pressure *p*0, using a modified Rayleigh–Plesset model. The dashed

The evolution of the cavitation bubble has been found to play an important role in the formation of nanoparticles (cf. Section 3.2). This control of the cavitation bubble dynamics can be achieved by several methods. One is by changing the laser fluence [18], or the viscosity of the medium [19]. Finally, another possibility is to pressurize the solution used for the PLA. By tuning these different parameters, the size, the chemical composition, and the type and

By changing the pressure of the surrounding medium, the plasma becomes confined and is

**Figure 6** shows a series of shadowgraph images obtained for a cavitation bubble generated in supercritical CO2 (cf. **Figure 1** for a possible experimental setup for realizing shadowgraph images of PLA in high-density media). Near the critical point and depending on the fluid conditions, not only a single cavitation bubble but also a structure resembling a dou-

concentration of defects in nanoparticles can be modified.

restricted by the surrounding fluid.

ble-bubble can be observed [10, 20].

region [17].

**Figure 6.** Series of shadowgraph images acquired for PLA on a Ni target in high-pressure liquid CO2. The snapshots show different instants from *t* = 1 μs to 600 μs near the critical point (*T* = 302.0 K, *p* = 7.30 MPa) of CO2. After the laser pulse, one can see the formation of a shockwave that emanates from the target (*t* = 3–7 μs). Then, from about 7 μs until 17 μs, the cavitation bubble exhibits the formation of a particular structure that consists of an inner bubble and an outer, partly transparent shell. Figure adapted with permission from Ref. [10].

The light source (e.g., a flash lamp or a light-emitting diode) is synchronized with a fast detector, for example, an intensified charge-coupled device (ICCD) or a streak camera. Flash lamps or diodes allow illumination times of typically Δ 10 − 100 μs while Nd:YAG lasers allow higher fluencies and permit illuminations at pulse durations of Δ 3 − 10 ns.

### *2.2.3. Optical emission characteristics*

**Figure 7(a)** shows plasma emission images of PLA realized on a Ti target immersed in water at 0.1 and 30 MPa, respectively [21]. A crosssection of the emission intensity in a direction normal to target, in the middle of the plasma, is presented in **Figure 7(b)**. In both cases, the maximum emission is at a small distance from the target, and as the pressure is increased, the extension of the plasma becomes squeezed along the normal to the target.

**Figure 7.** Optical emission intensities for pulsed laser ablation plasma in ambient and pressurized water. **(a)** Optical emission image observed at 0.1 MPa. **(b)** Optical emission image observed at 30 MPa. **(c)** Cross-sections of optical emission images in and along a direction normal to the target surface. Data adapted with permission from Ref. [21].

**Figure 8.** Optical emission spectra measured in CO2 at *p* = 0.1 and 7.4 MPa for PLA on a Ni target. The dashed boxes indicate the domains of the spectrum where Ni lines are dominant. The inset on the left shows a close-up in the wavelength range between 235 and 255 nm, where peaks that can be attributed to atomic and ionized C can be found. The inset on the right shows the detailed spectrum in the region around 777 nm containing lines of atomic O. Data adapted with permission from Ref. [5].

As the pressure of the medium is increased, discrete peaks of emitting species are broadening.

**Figure 8** shows two examples of OES data series recorded for PLA in atmospheric pressure and supercritical CO2 ( = 7.4 MPa) [5]. In this work, the authors also found that the total emitted intensity reached a maximum near the critical point of CO2, which was attributed to the maximum of the density fluctuation.

An additional spectroscopic technique that could be used for gaining information about the plasma characteristics is Raman spectroscopy. So far it has been used for characterizing laserinduced breakdown in water [6].
