Temperature dependence of ZnGeP2 birefringence using polarized light interference

We have measured the birefringence (Delta n) of ZnGeP2 throughout the 14-500 K temperature range using polarized light interference spectra obtained in transmittance from 0.7 to 12 mu m. It was found that Delta n continuously increases with temperature for all wavelengths but that the amount of increase is not constant with temperature as often assumed. Delta n increases by approximately 0.006 when the temperature changes from 14 to 500 K. The temperature derivative of the birefringence, d(Delta n)/dT, was determined at 50 K increments and its value was found to be quite dependent on the exact temperature range over which it was measured. It increases linearly with temperature, having a coefficient of 3.3x10(-8)/K. Actual d(Delta n)/dT values range from 5x10(-6)/K at the lowest temperature to 19x10(-6)/K at the highest. We conclude that the anomalously low phase matching angles observed for second-harmonic generation in ZnGeP2 are not associated with errors in birefringence values as previously speculated but instead are most likely due to nonlinear absorption phenomena.