PRESSURE DEPENDENCE OF REFRACTIVE INDICES OF HEXAGONAL CRYSTALS BERYL, ALPHA-CDS, ALPHA-ZNS, AND ZNO

The variation of the ordinary and extraordinary refractive indices of the hexagonal crystals beryl, α-CdS, α-ZnS, and ZnO with hydrostatic pressure to 7 kbar has been measured by an interferometric technique for λ5893 Ã. For beryl, both the refractive indices increase linearly with pressure with slopes of +0.072 × 10-3 kbar-1 over the entire range investigated. For α-CdS, α-ZnS, and ZnO, the ordinary refractive indices decrease linearly with pressure to about 4 kbar with slopes of -1.38×10-3, -0.25×10-3, and -0.34×10-3 kbar-1, respectively, while the corresponding extraordinary refractive indices also decrease linearly with pressure to about 4 kbar with slopes of -1.37×10-3, -0.25×10-3, and -0.35×10-3 kbar-1, 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. © 1969 The American Physical Society.