**1. Introduction**

Interest in the study of fundamental processes of interaction of powerful optical radiation with matter and their practical applications arose simultaneously with the invention of lasers. Depending on the characteristics of the laser radiation (intensity, wavelength, pulse duration, polarization and coherence), the processes of interaction and the results of interaction may be

© 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. © 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.

different. At low intensities and high duration of the interaction of the radiation with the solid absorbing target, only heating of the target occurs. Increasing the intensity of the radiation can lead to the reaching of the melting temperature and to the further evaporation of the substance of the target. Under powerful pulse laser radiation, the evaporation of the substance occurs with the formation of plasma cloud that contains ions, clusters and small particles. For such an "explosive" evaporation, the term "laser ablation" (from Lat. "ablatio"—taking, removal) is used. Sometimes in the literature, the term "ablation" is interpreted more widely, covering the removal of substances from the surface as a result of any physical and chemical processes that occur under high-energy impact on the object. In the present chapter, the term "ablation" will be used only forthe threshold process of explosive vaporization of material from a surface of solid targets with the formation of gas (vapor) plasma cloud via the rapid absorption of energy of high-power laser pulses in a limited volume.

First of all, the process of pulsed laser ablation (PLA), when micro-, nano- and later femtosecond pulses are exposed to the target, had found a successful application in laser materials processing for punching, marking and precise removal of layers of material in electronics [1]. Next, the PLA in gas phase and vacuum became successfully used for atomization of targets in mass spectrometry, obtaining thin films and ultrafine powders [2–4].

Using PLA of bulk targets in liquids to obtain nanoparticles (NPs) was initiated by the rapid development of nanotechnology in the 1990s of the twentieth century. The first deliberate use of this technique for the synthesis of nanocolloids took place in 1993 when the dispersions of Ag, Au, Pt, Pd and Cu nanoparticles in water and organic solvents for the surface-enhanced Raman scattering (SERS) spectroscopy were obtained [5].

Over the past two decades, pulsed laser ablation in liquids (PLAL) has become an effective and popular tool for obtaining nanosized materials. The absence of a mechanical interaction in the synthesis process and the ability to prepare "pure" nanoparticles without additional chemicals in the pure solvents straightaway in the form of stable colloids make this method very attractive for biological and medical applications. Catalysis, electronics and nonlinear optics are also the areas where the nanocolloids synthesized by the PLAL method and nanocrystalline powder obtained via further drying of the dispersions are used. The relatively simple experimental technique and the possibility of obtaining different types of nanoparticles (from metals to ceramics and polymers) on the same installation make this method a suitable tool for obtaining nanomaterials for the study of fundamental properties of substances in the nanostate. An opportunity to change the parameters of laser pulses and use various solvents with additives of precursors provides additional options to vary the composition, structure and dimensional characteristics of the nanoparticles obtained.

As of now, thousands of original research works, reviews and monographs on various aspects of the PLAL for the synthesis of nanostructures have been published [6–8]. They consider common mechanisms of PLAL and obtaining and characterization of specific nanomaterials as well. However, the interest in such research work continues unabated. On the one hand, this is due to the demand for nanomaterials with specific functional properties for various applications. On the other hand, there are three main aims that have not been achieved yet: effective control of structure and dimensional characteristics of nanoparticles obtained; obtaining of multicomponent particles and particles with complex structure, such as core shell; and initiation of the required chemical reactions in the synthesis process.

Another important point for PLAL as a synthesis method of nanoparticles is its low productivity caused by the physical mechanisms of the ablation process itself. It hinders the broad practical use of this technology. That is why the optimization of the experimental conditions to achieve maximum yield of nanoparticles in liquid is important. Individual factors that influence the productivity of particle synthesis via PLAL are, for example, the thermophysical characteristics of the target and its optical properties and secondary interaction of radiation with a plasma torch and with particles in a colloid. Developed theoretical models of the ablation process, to some extent, take into account the parameters of the medium and the characteristics of the laser irradiation. So far, however, focused systematic experimental investigation of the influence of the factors on the productivity of synthesis has not been carried out. In addition to solving a practical task of increasing the productivity of the method, these studies are important for obtaining new fundamental knowledge about the processes taking place in a reactor under the repetitively pulsed irradiation of the target in the presence of the concentrated colloids of particles with high chemical activity stimulated by laser radiation. Moreover, in addition to the changing of the effectiveness of the PLAL process, finite properties of nanomaterials may vary significantly.

In this section, the basic experimental aspects influencing the process of oxide nanoparticles obtaining under nanosecond-pulsed laser ablation of some reactive metals in liquids are discussed and the characteristics and some applications of such prepared nanoparticles are considered.
