Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 μm using polarized light interference

The birefringence (Δn) of ZnGeP2 has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 μm wavelength range from samples of six different thicknesses. The Δn values were determined from the positions of fringe maxima (Δn=kλ/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of Δn accurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase-matching angles for second-harmonic generation applications.