Refractive indices of sapphire under elastic, uniaxial strain compression along the a axis

Sapphire crystals were shocked to 190 kbar along the a axis to characterize their use as optical windows, for velocity interferometry measurements, up to their Hugoniot elastic limit. When partially polarized light is incident on the samples, birefringence in the material is manifested as a beat frequency in the probe light that is returned from the specimens. Proper procedures for interpreting the velocity interferometry data for various polarization conditions were developed. The refractive indices at 514.5 nm wavelength decreased linearly with the density. The data were analyzed to yield three photoelastic coefficients: p(12), p(31), and p(41). Calibration is developed for any polarization state of the probe light. Particle velocity wave forms are consistent with elastic behavior up to 170 kbar shock stress, and evidence of deviation from elastic behavior is present at 190 kbar impact stress. High precision shock velocity measurements are reported to 170 kbar stress along the a axis. (C) 2001 American Institute of Physics.