Refractive index of sapphire at 532 nm under shock compression and release

Abstract

The refractive index of sapphire at 532 nm has been determined along the Z axis over a range of conditions produced in shock compression and release experiments. In these experiments, a laser interferometer is used to measure particle velocity at an interface with a sapphire window. Values for refractive index are found from velocity corrections that must be made to account for refractive-index changes in the window due to shock wave motion. Early studies found that Z-cut sapphire windows resulted in much larger corrections than other window materials. These studies examined refractive-index changes at a wavelength of 633 nm, corresponding to the helium-neon lasers typically used in interferometers at that time. Because of the high shock impedance of sapphire, its use as an interferometer window since the early studies has been limited. Recent interest in the shock response of high-impedance ceramics, however, has resulted in considerable use of sapphire windows in experiments using laser interferometers operating at a wavelength of 532 nm. The current study is a careful re-examination of the refractive index of Z-cut sapphire under shock compression and release at this wavelength. As in the earlier work, symmetric-impact experiments were conducted on a gas gun facility using sapphire discs cut with their faces normal to the Z axis of the crystal. Laser interferometer measurements were made under known conditions to find the necessary velocity corrections as a function of the shocked state. A method used previously for analyzing refractive-index effects was used to determine refractive index as a function of the shocked state during single-shock motion, then extended to later events involving additional wave motions within the window. The current refractive-index measurements show a simple, linear dependence on density in the shocked state, in contrast to a more complicated dependence found previously. This difference is shown to be a consequence of small changes in the measured velocity corrections. The polarizability behavior is also examined using the present results, indicating a near balance between refractive-index changes due strictly to increasing density and those resulting from polarizability changes in the shocked state. An analysis using available sapphire data on refractive-index changes during hydrostatic compression and isobaric heating indicates that the temperature dependence of the polarizability has a relatively small effect on the changes in refractive index during shock compression. Together with the linear index–density relation, this indicates that a simple velocity correction found for single-shock motion can be accurately applied to arbitrary wave motions within sapphire windows.