The ZnGeP₂ crystal is a material of choice for powerful mid-IR optical parametric oscillators and amplifiers. In this paper, we present the experimental analysis of the optical damage threshold of ZnGeP₂ nonlinear crystals induced by a repetitively-pulsed Ho³⁺:YAG laser at 2091 nm. Two types of ZnGeP₂ crystals grown under different conditions were examined using the laser and holographic techniques. The laser-induced damage threshold (LIDT) determined by the pulse fluence or peak intensity was studied as a function of the pulse repetition rate (PRR) and laser exposure duration. The main crystal structure factor for a higher LIDT was found to be a reduced dislocation density of crystal lattice. The ZnGeP₂ nonlinear crystals characterized by the high structural perfection with low density of dislocations and free from twinning and stacking faults were measured to have a 3.5 J/cm² pulse fluence damage threshold and 10.5 MW/cm² peak intensity damage threshold at 12 kHz PRR; at 40 kHz PRR the pulse fluence damage threshold increased to over 6 J/cm², but the peak intensity damage threshold dropped to 5.5 MW/cm².

A multispectral anti-reflective coating of high radiation strength for laser applications in the IR spectrum for nonlinear ZnGeP₂ crystals has been developed for the first time. The coating was constructed using YbF₃/ZnS. The developed coating was obtained by a novel approach using ion-beam deposition of these materials on a ZnGeP₂ substrate. It has a high LIDT of more than 2 J/cm². Optimal layer deposition regimes were found for high film density and low absorption, and good adhesion of the coating to the substrate was achieved. At the same time, there was no dissociation of the double compound under high-energy ions.

In this work, the effect of the defect structure and the parameters of antireflection interference coatings based on alternating layers of Nb₂O₅/Al₂O₃ and Nb₂O₅/SiO₂ layers on the laser-induced damage threshold of ZGP crystals under the action of Ho:YAG laser radiation at a wavelength of 2.097 μm was determined. Coating deposition was carried out using the ion-beam sputtering method. The laser-induced damage threshold of the sample with a coating based on alternating layers Nb₂O₅ and SiO₂ was W_0d = 1.8 J/cm². The laser-induced damage threshold of the coated sample based on alternating layers of Nb₂O₅ and Al₂O₃ was W_0d = 2.35 J/cm². It has been found that the presence of silicon conglomerates in an interference antireflection coating leads to a decrease in the laser-induced damage threshold of a nonlinear crystal due to local mechanical stresses and the scattering of incident laser radiation.

In this work, the parameters of antireflection interference coatings based on alternating layers of ZnS/Al₂O₃ on the laser-induced damage threshold (LIDT) of ZGP crystals under the action of Ho:YAG laser radiation at a wavelength of 2.097 μm were determined. The coating deposition was carried out using the ion-beam sputtering method. The LIDT of the sample with a coating based on alternating layers ZnS and Al₂O₃ was equal to $W_o^E$ = 3.45 J/cm², and the LIDT of the uncoated sample was equal to $W_o^E$ = 2.23 J/cm². An increase in the optical breakdown threshold by ~55% was observed after the deposition of an AR coating based on ZnS and Al₂O₃ materials. An assumption was made about the absence of local fluctuations in the composition and mechanical stresses in the case of the coated sample, namely that this leads to good adhesion of the multilayer coating to the polished surface of the crystal, and as a result to an increase in the optical breakdown threshold as compared to the uncoated sample due to closure of the dangling chemical bonds and bulk defects emerging on the polished surface.

Magnetorheological processing was applied to polish the working surfaces of single-crystal ZnGeP₂, in which a non-aqueous liquid with the magnetic particles of carbonyl iron with the addition of nanodiamonds was used. Samples of a single-crystal ZnGeP₂ with an Angstrom level of surface roughness were received. The use of magnetorheological polish allowed the more accurate characterization of the possible structural defects that emerged on the surface of a single crystal and had a size of ~0.5–1.5 μm. The laser-induced damage threshold (LIDT) value at the indicated orders of magnitude of the surface roughness parameters was determined not by the quality of polishing, but by the number of point depressions caused by the physical limitations of the structural configuration of the crystal volume. These results are in good agreement with the assumption made about a significant effect of the concentration of dislocations in a ZnGeP₂ crystal on LIDT.

The observation and study of defects of single-crystal multicomponent optical material is necessary to determine the qualitative characteristics and optical properties of a material and to diagnose its manufacturing procedures. This paper utilizes the digital IR-holography to measure the geometrical parameters, shape, and location of defects as well as to characterize them. The paper illustrates the examples of physical, chemical, and optical inhomogeneities. Also, the paper presents the results of the study of dynamic processes in optical elements under the influence of laser radiation with high power density. The possibility of using the digital holographic technology to determine the dynamics of optical breakdown in the Z n G e P 2 single crystal is illustrated, namely, to estimate the speed and time of breakdown development, which can be used to interpret the mechanisms of breakdown development.