<title>Electrothermal defect generation and optical damage in a transparent medium</title>

Abstract

A model description of the laser-induced free carriers influence upon the point defect generation and optical damage in the wide-gap semiconductors and die1ectrie i presented. The model is based on the following assertions:-the defect creation probability in solid considerably increases compared with its usual ternperathre fluctuation value if the free carriers are involved in defect generation process (so-called recombintion-stimu1ated defect reactions); -the point defect creation in crystal resulting in energy specirum transformation leads to free carriers generation in solid even without the lattice heating. The positive feedback between free carrier and point defect concentrations stimulates the fast increase both cainer and defect densities and under certain conditions results in optical damage of crystal.The conducted analysis makes it possible to assert that:-the optical damage of transparent solids is often due to the e1ectronstimulated defect reactions arising under the wide range oflight irradiation parameters; -the mentioned above process of the defect generation nrnst be taken into account in the analysis of 'aceumu1ative effects" in low absorbing media under multiple laser action with sub-threshold intensities.

Relaxation processes and formation of thermal shock waves during laser ablation

“…For example, bearing in mind data from [7] for fused silica and spherical model of focal area, one can obtain the following In such conditions temperature-induced variation of absorption becomes important factor affecting whole heating dynamics through formation of various feedbacks [8,16]. Its mechanisms [8,16] are connected with laser-induced generation of point defects resulting in appearing of defect energy levels within material band gap. Such energy levels increase probability of thermal electron transitions from valence to conduction band what leads to increasing of freeelectron density and absorption by the free electrons because that type of absorption is proportional to electron density ne [8, 1 6].…”

“…(8) Relaxation time can approximated by constant ; in certain interval of temperature [1 1, 18], then the system (7) Equations for phonon temperature does not include diffusion term and source of straight heating but it includes the term R1h describing energy flow coming from non-radiative plasma relaxation. Thus, energy comes to phonons from two sources -nonradiative relaxation of electron plasma and thermal conductivity from hot electrons.…”

“…Relaxation term turns usual heat-transfer equation into wave equation describing propagation of decaying temperature waves with speed depending on characteristic relaxation time. Temperature dependence of absorption and relaxation time makes the equation nonlinear (due to exponential dependence çf absorption on temperature [8], for example). The most interesting feature of obtained nonlinear equations is possibility of formation of shock temperature waves (STW).…”

“…Following general scheme from [1] and modifying it by introducing formation of electron plasma and connected processes, we briefly review possible laser-induced relaxation processes and mechanisms of temperature-induced variations of absorptioii [8]. Then a theoretical model for description of femtosecond heating of transparent materials is described.…”

Laser-induced instability in transparent materials induced by low-absorbing microinclusion

“…That model opened new direction for theoretical investigations and gave new possibilities for understanding of nature of LID. Recently the concept of LID initiating by instabilities was enriched by model of inclusion-induced field instability1 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In general case instability of a certain parameter A (temperature T in case of thermal instability and electric field strength E in case of field instability) induced by inclusion comes through the following" ''': either both the parameter A inside the inclusion and its derivation with respect to incident field amplitude E0 tends to infinity while incident field amplitude approaches finite threshold value E*o: ÔA * A-+cc, -+ciwith E -*E (1) 5E0 0 0 or the parameter tends to some limited value and its derivation tends to infinity:…”