Multiphonon infrared absorption in the transparent regime of alkaline-earth fluorides

Abstract

We present an experimental investigation of the frequency and temperature dependence of the multiphonon infrared absorption coefficient in the alkaline-earth fluorides ${\mathrm{CaF}}_2$, ${\mathrm{SrF}}_2$, and ${\mathrm{BaF}}_2$. Over the range investigated, a nearly structureless, exponential-like behavior is found for the frequency dependence of the absorption, and a Bose-Einstein-like multiphonon behavior for the temperature dependence. The data are analyzed employing a simplified model of multiphonon absorption; the overall agreement with experiment is quite good, although various discrepancies are present. We conclude that alkaline-earth fluorides do indeed display intrinsic multiphonon absorption over an extended frequency and temperature range, and we determine the parameters governing this behavior.