TRANSIENT THERMAL LENS IN A ZNGEP2 CRYSTAL

The first observations, to the authors' knowledge, of transient thermal lensing in a ZnGeP2 crystal achieved with 2.09-mu m laser excitation of 800-mu s, 70-mJ pulses at 30 Hz and by burst-made Q-switched pumping are presented.. The laser power transmitted through an aperture was approximated by an adiabatic model with currently accepted values for the thermal properties of ZnGeP2 and corresponded to focal-length changes from infinity to 10 cm during each 800-mu s pulse. Similar results were seen when the crystal was operated as an optical parametric oscillator. This transient thermal lens severely limits ZnGeP2 as a material for use in optical parametric oscillators for these modulated, high-power operating conditions.