**1. Introduction**

The surface nanostructuring and the generation of nanoparticles by laser ablation with nanosecond lasers are subjects that have gained importance in the last 15 years [1]. These issues were present since the early experiments with pulsed lasers because they are inherent to pulsed laser interaction with matter, and researchers have made great efforts to get rid of them. In laser ablation micromachining, these effects are known as HAZ (heat affected zone) and are undesirable [1, 3]. It was not until the explosive spring forth of nanotechnology in technologi‐ cal applications in several areas such as medicine and microelectronics among many others that the appearance of different nanostructures in a wide variety of materials started to be report‐ ed in the literature.

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In this chapter, we will present a review of the nanostructures found by our research group during the quality control of the surfaces of micromachined devices. On the other hand, a method for generating metallic nanoparticles which was developed on the basis of the analysis of the material ejected during the micromachining ablation process will be reported. Nano‐ particles were then generated in different media such as air, deionized water, isopropyl alcohol and sodium dodecyl sulfate (SDS) solution. Preliminary results of the generation of Fe@Au nanoparticles (core–shell nanoparticles) will be also discussed.

A brief summary of the fundamentals of laser ablation of solid substrates and of the theory of the resulting ablation plume will be given in the next section. A large variety of studies in this field including basic research and applications have been carried out worldwide by many researchers. However, we will focus on the area of micromachining by laser ablation [1–4].

#### **1.1. Laser ablation**

The laser ablation mechanism is one of the most complex phenomena observed when laser radiation interacts with a solid material [1]. Ablation may be produced either with pulsed or with intense continuous wave (cw) lasers [1, 3]. For a given type of material, the onset of ablation takes place around a threshold fluence (∅ *th*), which depends on absorption mecha‐ nisms and particular properties of the material, such as its microstructure, its morphology, and the presence of defects as well as on laser parameters (intensity, wavelength, and pulse duration) [1, 2, 5, 6]. When irradiation is performed with several consecutive pulses impinging on the same sample's area, ∅ *th* can vary due to the accumulation of defects in the previously irradiated zone. At a given wavelength, the amount of material removed per pulse usually shows a logarithmic increase with fluence in accordance with the Lambert–Beer law. On the other hand, Villagran-Muniz et al. [7] reported heuristic equations to describe the fluence dependence of the amount of removed material per pulse of a given substrate with parameters that can be related to physical properties of the substrate.

In particular, for laser pulse durations larger than 10 ps, the laser beam will also interact with the ablation plume as in the case of 10 ns Nd:YAG lasers. In these conditions, the plume will also absorb and scatter radiation. In consequence, the amount of energy reaching the substrate will be less since part of it will be absorbed by the plasma, generating a hot plasma. This excited plasma will then expand and create shock waves in the molten substrate. Frozen shock waves can be observed in the material when the irradiated zone cools [8]. Another effect produced by the excited plume is the explosion of the remnant molten material, which produces splashes. The expelled liquid will inevitably solidify around the irradiated area together with plume's material condensation. In addition, the rapid generation of large thermal gradients may induce excessive thermal stress and thermoelastic excitation of acoustic waves. These stresses may add hardening work, buckling, or cracking [1] to the response of the material, effects which must be taken into account in each particular application.

However, this is not a general feature of all materials. The dynamics of the laser–matter interaction starts with the electronic excitation and relaxation. For weak electric fields, ionization of an atom occurs when the energy of the incident photon exceeds the binding energy of the valence electron. Thus, unlike metals, in a large variety of materials with valence bands, such as dielectrics, electrons are not directly excited into the conduction band. In Chapter 8 of reference [1], an extensive explanation of the differences in the absorption processes of between metals and dielectrics is given.

Only the final results of the ablation process and perhaps some insights of the different phenomena that can possibly occur are experimentally observed. When the material's processing involves a laser, the light–matter interaction mechanisms that simultaneously occur will depend on the pulse duration. Thus, the most relevant mechanisms that take place on a substrate according to the laser pulse duration are as follows: material melting with cw and millisecond pulsed lasers, material vaporizing with nanosecond pulses, and finally, sublima‐ tion of material with femtosecond pulses [1]. It is worthy to note that short pulses (1 ns–1 μs) emitted by lasers with Q-switch devices also reduce the thermal impact on the material.
