Pressure Dependence of the Refractive Indices of the Hexagonal Crystals Beryl,α-CdS,α-ZnS, and ZnO

The variation of the ordinary and extraordinary refractive indices of the hexagonal crystals beryl, $\alpha$-CdS, $\alpha$-ZnS, and ZnO with hydrostatic pressure to 7 kbar has been measured by an interferometric technique for $\lambda 5893$ Å. For beryl, both the refractive indices increase linearly with pressure with slopes of +0.072 × ${10}^{-3\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$ over the entire range investigated. For $\alpha$-CdS, $\alpha$-ZnS, and ZnO, the ordinary refractive indices decrease linearly with pressure to about 4 kbar with slopes of -1.38×${10}^{-3\mathrm{}}$, -0.25×${10}^{-3\mathrm{}}$, and -0.34×${10}^{-3\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$, respectively, while the corresponding extraordinary refractive indices also decrease linearly with pressure to about 4 kbar with slopes of -1.37×${10}^{-3\mathrm{}}$, -0.25×${10}^{-3\mathrm{}}$, and -0.35×${10}^{-3\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$, respectively. Above 4 kbar both the refractive indices of these crystals decrease nonlinearly with pressure as well as with strain, indicating the influence of the second-order piezo-optic coefficients. On release of pressure, no hysteresis effects were noticed, indicating that the above nonlinear piezo-optic behavior occurs in the elastic region. Finally, the results are interpreted according to Mueller's theory of photoelasticity.