Nonlinear absorption and laser damage threshold measurements of doped ZnGeP2

Zinc germanium diphosphide (ZnGeP2) has proven to be an important nonlinear optical crystal for the generation of mid-wave infrared radiation, especially in an optical parametric oscillator configuration pumped by a Ho:YAG laser operating at 2.09 micrometers. Future applications will require higher intensity levels of the laser pump beam which are limited by the crystal's laser damage threshold and nonlinear absorption. These two quantities were measured for silver-doped ZnGeP2 samples of which one was uncoated, two had conventional anti-reflection coatings, and one had an AR coating with a quintic refractive index profile. Prominent nonlinear absorption was observed in some of the crystals; the nonlinear absorption coefficient was found to be anisotropic and a weak correlation between the values of the linear and the nonlinear absorption coefficient was observed. The values of the nonlinear optical coefficients measured in these crystals for ordinary and extraordinary polarizations of incident light are reported along with the measured values of the laser damage threshold of these newly grown crystals at 2.09 micrometers, both in the AR coated and uncoated forms.