Photoelasticity of the cuprous halides

Abstract

We present our acousto-optic measurements of the photoelastic tensor of the cuprous halides, CuCl, CuBr, and CuI. The data include the magnitudes and signs of the coefficients and their optical wavelength dependence. Several distinct trends are observed as a function of ionicity both within the halides and in the isoelectronic and isostructural sequence Ge, GaAs, ZnSe, CuBr. Particularly interesting results are found for the infinite-wavelength limits. We find that $p_{44}$ is very nearly the same in all the materials considered, whereas $\frac{(p_{11}-p_{12})}{2}$ grows monotonically on going from Ge to CuBr; $\frac{(p_{11}-p_{12})}{2}$ is approximately equal to $p_{44}$ in the halides, indicating isotropy of the direct photoelastic tensor. The hydrostatic coefficient goes monotonically from a large negative value in Ge to a positive value in the halides—the only known tetrahedrally coordinated binary semiconductors to have positive coefficients. We believe this implies that the deformation potential of the average optical gap is nearly zero.