Residual absorption in infrared materials

The sources of the residual absorption of ir materials in their transparent region between 2microm and 10microm are reviewed with emphasis upon recent developments. On the longer wavelength side of this spectral region, many insulating crystals are limited by intrinsic multiphonon absorption, and, on the shorter wavelength side, all insulating crystals are limited by extrinsic effects associated with impurities, defects, and surfaces. During the last few years, the general character of the intrinsic multiphonon absorption has become sufficiently well understood so that its frequency and temperature dependence can be accounted for in both ionic and covalent crystals. The nature of extrinsic absorption is more complicated as it arises from a number of sources. In some cases, the measured absorption coefficient can be attributed with some certainty to a specific origin. In others, especially for the ionic crystals at shorter wavelengths, the origin of the extrinsic residual absorption is not known and may be limited by the experimental measurement techniques, at least in some of the better crystals.