Laser-Induced Damage Threshold of Single Crystal ZnGeP2 at 2.1 µm: The Effect of Crystal Lattice Quality at Various Pulse Widths and Repetition Rates

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.