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.

Pulse-width-dependent critical power for self-focusing of ultrashort laser pulses in bulk dielectrics.

Microscale filamentation of 0.25 NA-focused, linearly and circularly polarized 1030 nm and 515 nm ultrashort laser pulses of variable pulse widths in fused silica, fluorite, and natural and synthetic diamonds demonstrates the Raman-Kerr effect in the form of critical pulse power magnitudes, proportional to squared wavelength and inversely proportional to laser pulse width of 0.3-10 ps. The first trend represents the common spectral relationship between the quantities, while the second indicates its time-integrated inertial contribution of Raman-active lattice polarization, appearing in transmission spectra via ultrafast optical-phonon Raman scattering. The optical-phonon contribution to the nonlinear polarization could come from laser field-induced spontaneous/stimulated Raman scattering and coherent optical phonons generated by electron-hole plasma with its clamped density in the nonlinear focus. Almost constant product value of the (sub)picosecond laser pulse widths and corresponding critical pulse powers for self-focusing and filamentation in the dielectrics ("critical pulse energy") apparently implies constant magnitude of the nonlinear polarization and other "clamped" filamentation parameters at the given wavelength.