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 inciden t on the samples, birefringence in the material is manifested as a beat fre quency in the probe light that is returned from the specimens. Proper proce dures for interpreting the velocity interferometry data for various polariz ation conditions were developed. The refractive indices at 514.5 nm wavelen gth decreased linearly with the density. The data were analyzed to yield th ree photoelastic coefficients: p(12), p(31), and p(41). Calibration is deve loped for any polarization state of the probe light. Particle velocity wave forms are consistent with elastic behavior up to 170 kbar shock stress, an d evidence of deviation from elastic behavior is present at 190 kbar impact stress. High precision shock velocity measurements are reported to 170 kba r stress along the a axis. (C) 2001 American Institute of Physics.