Laser-induced damage threshold of ZnGeP2 crystal for (sub)picosecond 1-µm laser pulse

The laser-induced damage threshold (LIDT) was measured for a ZnGeP2 crystal exposed to 0.3–9.5 ps 1030-nm laser pulses. Single-pulse LIDT fluence was ∼0.22J/cm2 for the laser pulse widths of 0.3–3.5 ps and increased until 0.76J/cm2 for 9.5-ps pulses. Multi-pulse LIDT fluence for 0.3-ps pulses at repetition frequencies in the range of 100 Hz–1 kHz was ∼0.053J/cm2 and decreased further at higher, multi-kHz, pulse repetition frequencies. The coating of the ZnGeP2 crystal surface with an anti-reflection multi-layer thin film increased the multi-pulse LIDT by one order of magnitude, up to 0.62J/cm2 (about 2TW/cm2). The significant increase in LIDT coupled with a decrease in reflection losses provides a way to cardinally improve efficiency of frequency conversion of popular 1-µm ultrashort pulses into mid- and far-IR ranges with a thin AR-coated ZnGeP2 crystal sample.