Temperature-Dependent Phase Matching for Far-Infrared Difference-Frequency Generation in InSb

The efficiency of difference-frequency generation in InSb at 100 ${\mathrm{cm}}^{-1\mathrm{}}$, using a C${\mathrm{O}}_2$ laser as input, was found to be strongly temperature dependent. This dependence is shown to be due to phase matching resulting from an increase with temperature of the short-wavelength refractive indices. A value of 4.03 is derived for the refractive index at 100 μ. The effect of astigmatism in the input optics is pointed out.