Laser interferometer measurements of refractive index in shock-compressed materials

Laser interferometer systems provide a means for probing the refractive index of transparent specimens subjected to dynamic compression. Previous interferometer measurements of optical properties under shock loading are reviewed for polymethyl methacrylate, fused silica, sapphire, nitromethane, and an aqueous solution of zinc chloride; various degrees of departure from Gladstone-Dale behavior are noted for these materials. In addition, a detailed summary of recent optical studies of lithium fluoride (LiF) is provided. Interferometer data from plate-impact experiments verify sustained LiF transparency for Hugoniot stresses to at least 160 GPa, and establish the variation of LiF refractive index for shock amplitudes ranging from 1.58 to 115 GPa. The refractive-index data for LiF agree with earlier static and shock-wave data, and exhibit a pronounced deviation from predictions based on the Gladstone-Dale, Lorentz-Lorenz, and Drude relations. A modified form of the Gladstone-Dale relation is presented which correctly models the latest LiF measurements. Potential applications of LiF and other window materials to dynamic high-pressure experimentation are discussed.