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

The laser-induced damage threshold (LIDT) was measured for a <i>Z</i> <i>n</i> <i>G</i> <i>e</i> <i>P</i> ₂ crystal exposed to 0.3-9.5 ps 1030-nm laser pulses. Single-pulse LIDT fluence was ∼0.22<i>J</i>/<i>c</i> <i>m</i> ² for the laser pulse widths of 0.3-3.5 ps and increased until 0.76<i>J</i>/<i>c</i> <i>m</i> 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 ∼0.053<i>J</i>/<i>c</i> <i>m</i> ² and decreased further at higher, multi-kHz, pulse repetition frequencies. The coating of the <i>Z</i> <i>n</i> <i>G</i> <i>e</i> <i>P</i> ₂ crystal surface with an anti-reflection multi-layer thin film increased the multi-pulse LIDT by one order of magnitude, up to 0.62<i>J</i>/<i>c</i> <i>m</i> ² (about 2<i>T</i> <i>W</i>/<i>c</i> <i>m</i> 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 <i>Z</i> <i>n</i> <i>G</i> <i>e</i> <i>P</i> ₂ crystal sample.