## 2 Absorption of Laser Beam Energy by Metals and Semiconductors  
The energy of the laser beam is first absorbed in the affected zone and then transferred to the bulk material through heat conduction processes by means of electron-phonon coupling [49; 50; 51]. The mechanisms of laser beam absorption by metals and semiconductors are quite different. The interaction of light with metals is determined by the presence of a large number of electrons so weakly bound to the crystal lattice that these electrons can be considered virtually free [52]. The electrostatic positive charge of the metal lattice ions compensates for the negative charge of these electrons. The free electrons or conduction electrons are commonly referred to as electron gas. In the field of the incident electromagnetic wave, the free electrons oscillate and emit secondary waves, which when combined produce a strong reflected wave [53]. The absorption of light by the conduction electrons is only possible through their interaction with the metal lattice and is therefore partially converted into heat [54; 55]. It should be noted that in a perfect conductor, where Joule heating [56] losses are absent, the absorption equals to zero and the incident light is completely reflected [57]. The absorption of light leads to an increase in the energy of the free electrons. Since the time for establishing equilibrium in an electron gas is much shorter than the time for establishing equilibrium between electrons and the lattice of atoms, two thermodynamic subsystems emerge in the metal with different temperatures: an electron subsystem and a phonon subsystem [58]. Part of the absorbed energy is transferred by the electrons to the lattice, but the transfer efficiency is low due to the large mass difference between electrons and ions. Therefore, at first the electron gas is significantly overheated compared to the lattice. However, the increase in temperature of the electron gas only occurs until the amount of energy transferred to the lattice is equal to the amount of energy received by the electrons from the electromagnetic wave. When processing in a continuous beam or in a pulsed mode with a duration up to the nanosecond range inclusive, the exposure time of the irradiation on the material is several orders of magnitude greater than the energy exchange time between the electrons and the lattice. In this case, their temperatures are considered equivalent [59]. However, when processing with ultrafast laser pulses, the characteristic energy exchange time between these subsystems exceeds the pulse time. This means that by the end of a femtosecond laser pulse, the energy absorbed by the matter is stored in the electronic subsystem, while the lattice remains practically at its original temperature. During the process of electron-lattice relaxation, which in metals occurs in times of the order of tens of picoseconds, lattice thermal conduction is virtually absent, which is a great advantage of femtosecond laser ablation that allows for precision processing of materials [60; 61]. The electrical and optical properties of semiconductors are related to the fact that the electron-filled energy levels in the valence band are separated from the conduction band by the energy gap or band gap where electron states are forbidden. Accordingly, a quantum approach is used, referring to light as a flow of photons with the energy of a light quantum. Semiconductors have a low concentration of free electrons and if the energy of the light quantum is less than the band gap width, their optical properties are similar to those of dielectrics [62; 63]. The surface of a semiconductor is usually a macroscopic discontinuity of the periodicity of the crystal lattice. The electronic processes on and near the surface are influenced not only by capture and recombination centres, but also by the state of the spatial charge region, which can be determined by the existence of local surface centres. Such centres can be related both directly to periodicity rupture and to adsorbed atoms and molecules.