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.

In this work, the method of IR digital holography intended for detection of volumetric defects in ZnGeP₂ single crystals has been tested. The holographic method is verified by a comparison of the results obtained with the data obtained by other methods. The spatial resolution of the experimental setup is ~15–20 µm. The volumetric defects of the ZnGeP₂ crystal structure (in samples with thickness up to 50 mm) such as growth striations, dislocation chain, and inclusions of the second phase (Zn₃P₂) shaped as needles up to ~100 µm long and ~10 µm wide have been visualized by the method of IR digital holography.

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.

Polycrystalline zinc selenide (ZnSe) and Cr²⁺ or Fe²⁺ doped ZnSe are key optical elements in mid-infrared laser systems. The laser-induced damage of the optical elements is the limiting factor for increasing the power and pulse energy of the lasers. In the present work, the optical damage of the ZnSe, Cr²⁺:ZnSe, and Fe²⁺:ZnSe samples induced by a repetitively pulsed Ho³⁺:YAG laser at 2091 nm was studied. The probability of the optical damage and the laser-induced damage threshold (LIDT) were determined for the samples manufactured using different processing techniques. The highest LIDT was found in ZnSe samples annealed in an argon atmosphere. It was also found that the samples annealed in a zinc atmosphere or with hot isostatic pressing resulted in a decrease in the LIDT. The Cr²⁺-doped ZnSe had the lowest LIDT at 2.1 µm compared to Fe²⁺-doped or undoped ZnSe. The LIDT fluence of all tested ZnSe samples decreased with the increase in the pulse repetition rate and the exposure duration. The results obtained may be used to improve the treatment procedures of ZnSe, Cr²⁺:ZnSe, and Fe²⁺:ZnSe polycrystals to further increase their LIDT.

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.