Ordinary refractive index of sapphire in uniaxial tension and compression along the c axis

Abstract

The ordinary refractive index of sapphire, at 532 nm, was measured in single crystals subjected to elastic, uniaxial tensile (and compressive) strain along the c axis in plate impact experiments. The refractive index under both tension and compression was determined from the difference between the uncorrected particle velocities from velocity interferometer measurements and the known particle velocities in symmetric impact experiments. Results to a peak stress of 100 kbar in tension and compression show that the refractive index has an overall monotonic, nonlinear dependence on density that is asymmetric about the ambient density. In compression, the refractive index decreases by an extremely small amount; in contrast, the change is relatively larger in tension. These refractive index results at large tensile stresses suggest that sapphire polarizability is quite different under compression and tension. Compression measurements by themselves can be approximated well by a linear refractive index–density relation, resulting in a window correction (at 532 nm) that is a constant fraction of the true particle velocity up to at least 115 kbar longitudinal stress. The present data also provide high-precision shock velocity results in compression and demonstrate the appropriateness of using second and third order elastic constants for calculating the elastic response of sapphire under uniaxial tensile strain loading.