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

The laser-induced damage threshold (LIDT) was measured for a Z n G e P ₂ crystal exposed to 0.3-9.5 ps 1030-nm laser pulses. Single-pulse LIDT fluence was ∼0.22J/c m ² for the laser pulse widths of 0.3-3.5 ps and increased until 0.76J/c m ² 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/c m ² and decreased further at higher, multi-kHz, pulse repetition frequencies. The coating of the Z n G e P ₂ crystal surface with an anti-reflection multi-layer thin film increased the multi-pulse LIDT by one order of magnitude, up to 0.62J/c m ² (about 2T W/c m ²). 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 Z n G e P ₂ crystal sample.