## 2 Absorption of Laser Beam Energy by Metals and Semiconductors
A distinction is made between intrinsic, intraband, impurity and lattice absorption of electromagnetic radiation by semiconductors [64]. Intrinsic or interband absorption of light [65; 66] occurs when the energy of the quantum is greater than the band gap. In this case, due to the internal photoelectric effect, electrons move from the valence band to the conduction band. The reflectivity of the semiconductor increases. At the same time, the absorption of radiation by free carriers enables the acceleration of electrons in the conduction zone, which leads to an increase in theirconcentration as a result of the thermal ionisation of the valence zone, i.e., a self-accelerating process of warming up the substance [67]. Intraband absorption or absorption by free carriers, which are electrons and holes, is inherently similar to free electron absorption in metals [68; 69]. The difference is the relatively low concentration of free carriers. Impurity absorption [70] is determined by the participation of carriers with energy states within the band gap. Lattice or residual absorption [71] takes place when the irradiation interacts directly with the ions of the semiconductor and the electron subsystem remains uninvolved. In this case, there is an interaction of photon with phonon, which has a quantum character with rather wide lines. The transfer of energy absorbed by the free carriers and converted into heat from the surface layers of the semiconductor through its volume is carried out by thermal conduction [72]. At the initial stage of the process, when the concentration of free electrons in the semiconductor is negligible, lattice heat conduction predominates. As the concentration grows, increasingly more energy is transferred by the conduction electrons, and they contribute essentially to the total thermal conductivity [73]. Energy transfer in semiconductors can also be achieved by recombination. In this case, semiconductors produce electron-hole pairs due to the absorption of laser irradiation; during recombination, these pairs transfer the energy to the crystal lattice [74]. The morphology of a metal surface also influences its optical absorption capacity. Depending on the degree of roughness, which can be either smaller or larger than the laser wavelength, the enhancement of laser beam absorption can be due to the following mechanisms. When the roughness is smaller than the wavelength, the light absorption can be due to the anti-reflective effect of sub-wavelength surface textures [75]. In the case when the roughness is larger than the wavelength, the enhancement may be due a multiple reflection at the air-metal interface [76]. The optical properties of nanomaterials are also different from their bulk counterparts [77]. An analytical model for calculating optical absorption by a rough metal surface is formulated in [78]. Figure 1 illustrates the laser beam absorption and reflection scheme and the model for absorption calculation. In the model, the roughness at the air-metal interface is treated as a slab of thickness d in fractional dimensional space (\(\mathbf{F^{\alpha}}\)), and the parameter \(0<\alpha<1\) is defined by the roughness value, where the value \(\alpha=1\) indicates its absence. This model [78] provides the possibility to pre-calculate the optical absorption capacity, up to high absorption values, in order to create a suitable surface roughness for metallic materials.