Temperature dependence of ZnGeP2 birefringence using polarized light interference

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Abstract
We have measured the birefringence (Δn) of ZnGeP2 throughout the 14–500 K temperature range using polarized light interference spectra obtained in transmittance from 0.7 to 12 μm. It was found that Δn continuously increases with temperature for all wavelengths but that the amount of increase is not constant with temperature as often assumed. Δn increases by approximately 0.006 when the temperature changes from 14 to 500 K. The temperature derivative of the birefringence, d(Δn)/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 3.3×10−8/K. Actual d(Δn)/dT values range from 5×10−6/K at the lowest temperature to 19×10−6/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.