Measurement of optical extinction coefficients in sapphire as a function of the shock pressure

Sapphire has been extensively used as a window material for both optical and thermal property measurements of shocked materials. Its optical extinction characteristic under dynamic compression is crucial in understanding the measured physical behaviors. Here we demonstrated a quantitative study of the dynamic optical extinction of sapphire in the shock pressure range from 72 to 183 GPa. Along its $<$1000$>$ orientation, the optical extinction coefficient ($\alpha )$ of sapphire crystal has been in-situ measured at several discrete wavelengths with an optical pyrometer incorporated with a shock-generated bright light source. The significant findings indicated that: (i) the $\alpha $ coefficient increases with the shock pressure but decreases with the wavelength, showing a quite different behavior from that observed in the low-pressure compression in the literature; (ii) the obtained linear relationship between the square-root of $\alpha $ coefficient and the photon energy can be well described by a Mie scattering calculation assuming a particle radii of $\sim $140nm, which suggests that the optical extinction of sapphire is attributed to a light scattering mechanism related with the shock-induced crack-net distribution in this strong brittle material. These quantitative extinction data have provided new insight into the decay nature of thermal radiance histories previously concerned at a metal/sapphire interface, which is crucial for inferring an interfacial equilibrium temperature.