## 3 Results  
There are two main mechanisms of optical damage of nonlinear crystals accompanied by accumulative effects. The first is caused by thermal heating of the material when absorbing laser radiation with volumetric defects. The volumetric defects, as well as  
Figure 3: Reconstructed holographic image of GaSe (sample no. 1) in the best focus plane containing images of microcracks (**left**) and their magnified images (**right**). Figure 2: Schematic layout of the test stand. the defect-matrix medium boundary, may have an absorption coefficient significantly higher than the matrix medium, and may cause optical damage being heated to the melting point when absorbing laser radiation. Thus, at a high frequency of laser radiation pulses, the absorption heating may exceed the heat removal from the defect due to the thermal conductivity of the material and cause an \"accumulative effect\", leading to material breakdown. The second proposed mechanism is nonlinear absorption associated with the transition of charge carriers to impurity levels for example, formed by point defects, under the influence of laser radiation. The point defects caused by the incorporation of impurity atoms into the crystal lattice or the formation of vacancies and the substitution of atoms of the original semiconductor components may be the sources of impurity levels in the semiconductor. Due to the transition of free charge carriers to impurity levels the absorption may increase many times as the intensity of laser radiation increases. This may result in the material being heated to temperatures above the melting point and, as a result, to the optical damage. To assess the effect of volumetric defects on the optical damage threshold, we determined the presence or absence, as well as the geometry of volumetric defects, in all tested samples through using the digital holography. To assess the effect of point defects on the optical damage threshold, we measured the absorption at the wavelength of exposure radiation (since the presence of point defects leads to the formation of impurity levels in the band gap, which may significantly affect the absorption coefficient at the exposure wavelength). The study of LIDT was preceded by recording the digital holograms of the test samples using a 1.064 um digital holographic camera (DHC) manufactured by LLC Laboratory of Optical Crystals. Computer reconstruction of holographic images was carried out from the recorded holograms in order to determine internal inhomogeneities and volumetric defects. The limiting resolution of the method was 15 um. A detailed description of the digital holography technique and a description of the setup are given in [16,17]. According to the results of the holographic test, the GaSe (sample no. 1) did not contain any inclusions and volumetric defects except for the microcracks shown in Figure 3. However, volumetric inclusions with a size of \(\approx\)300 um were found in GaSe:In (sample no. 2) (Figure 4). Then, the transmittance of both samples was measured, and the absorption was calculated, taking into account multiple reflections from the plane-parallel faces of the plates at a wavelength of 2.091 um. The beam reflection losses in GaSe sample were calculated using dispersion equations for refractive index of ordinary and extraordinary waves obtained in the study [18]. The following results presented in Table 1 were obtained by determining the transmittance at a wavelength of 2.091 um and calculating the absorption. Figure 4: Reconstructed holographic image of GaSe:In (sample no. 2) in the best focus plane containing images of inclusions (**left**) and magnified images of regions containing inhomogeneities (**right**). The R-on-1 method [19] was used to determine LIDT since the aperture of the samples under study was limited in the given experiment.