**2.1. Plasma generation in high-density media**

fabrication of thin films (metallic, oxide, and complex heterostructures) and has become a

Pulsed laser ablation (PLA) in liquids, which consists in the irradiation of a solid target placed inside liquids, has enabled the functionalization and fabrication of a wide range of nanomaterials. The main advantage of PLA is that it enables the fabrication of nanoparticles that cannot be obtained easily by other methods. Moreover, PLA realized in high-density media—highpressure gases, liquids, and supercritical fluids—has opened up new possibilities for controlling nanomaterials fabrication by PLA and has also lead to the discovery of new phenomena

In the context of this chapter, we define high-density media as gases, liquids, or supercritical fluids at and above atmospheric pressures, that is, at pressures 0.1 MPa and densities *ρ*≳ 1 kg m-3. While there have already been extensive reviews on PLA in liquids and their applications for nanomaterials processing [2, 3] including high-density media [4], the main aim of the present chapter is to present the main differences between PLA in atmospheric pressure liquids and PLA in high-density media—high-pressure gases, pressurized liquids, and supercritical fluids—and to point out possible advantages of using PLA in such media,

The chapter is structured as follows: The first part (section 2) gives a brief overview on the phenomena related to the formation of PLA plasmas in dense media. We introduce the techniques used for generating plasmas by PLA in high-density media, and the fundamental phenomena encountered under such conditions. This part is also devoted to the description of the experimental instrumentation and analysis techniques. In the third part (section 3) gives an overview of the use of pulsed laser ablation to nanomaterials' synthesis and the characterization of nanoparticle growth processes using X-ray scattering techniques. Finally, in the last two parts (sections 4 and 5), we present briefly other possible applications of PLA in high-

As mentioned above, while PLA in liquids or in atmospheric pressure gases has been investigated extensively, PLA in high-pressure/high-density media has not yet received as much attention, but we hope that the present review can help researchers gain an overview on

This section gives a brief overview on the formation mechanism of PLA plasmas in highdensity media, and the main experimental equipment required for carrying out such experi-

The following section is devoted to introducing techniques for characterizing PLA plasmas in high-density fluids—direct imaging, shadowgraph and Schlieren imaging, laser scattering,

both for fundamental research and applications in materials science and processing.

density media and close the chapter with a short conclusion.

**2. Pulsed laser ablation plasmas in high-density media**

optical emission spectroscopy (OES) [5], and Raman scattering [6].

mainstay for realizing high-quality thin films of different compositions [1].

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

related to plasma-fluid interactions.

this field.

ments.

To realize the pulsed laser plasma, different types of lasers can be used. The most common are second harmonic (532 nm) Nd:YAG pulsed laser sources (e.g., Ref. [7]) that are operated with a repetition rate of a few Hz and pulse widths of a few nanoseconds (∼ 5 to 10 ns) and fluencies of a few tens of milli-Joules per pulse.

To realize the high-pressure conditions of the fluid, it is necessary to employ high-pressure pumps, for example, those used for high-performance liquid chromatography (HPLC) [8], or another method that allows compressing the gas or the liquid to be used. One simple approach is liquefaction that consists in cooling the source gas liquid nitrogen and then introducing it into the high-pressure cell, as this was used for PLA in supercritical xenon [7]. To keep the conditions in a well-defined state, the temperature and pressure in the cell are monitored by temperature and pressure sensors. Adjusting of the temperature can be achieved by using a heating/cooling circuit connected to the high-pressure cell.

One advantage of carrying out PLA in high-density and pressurized media is that varying the pressure and/or temperature of the medium allows further adjustment of the nanoparticle size and chemical composition (in addition to the laser fluence and pulse width).

For characterizing the plasma formation and the fluid evolution including the dynamics of the cavitation bubble, different fast imaging techniques are employed, the most common being shadowgraph and schlieren imaging. Finally, direct imaging can be used to generate the plasma formed by the laser irradiation. In shadowgraph imaging (cf. setup of **Figure 1**), the

**Figure 1.** Schematic of experimental setup for direct and shadowgraph imaging in high-pressure conditions. Temperature and pressure indicators (TI, PI) are used to monitor the fluid conditions inside the high-pressure chamber. Figure adapted with permission from Ref. [10].

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

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 reported [9].

**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 5000 nm.

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

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 presented in sections 2.2.2 and 2.2.3.
