Multiphonon absorption in highly transparent semiconducting crystals

We perform experimental and theoretical investigations of multiphonon infrared spectra in the three- and four-phonon regimes of semiconducting crystals, including GaAs, ZnSe, ZnS, and Si, over a range of frequencies and temperatures. The most striking aspect of these spectra is the persistence of structure at higher frequencies and elevated temperatures. A simple theoretical model which we develop and apply to interpret the data suggests that nonlinear moments and selection rules have a relatively weak effect on the observed spectra. Rather, the principal characteristics of the observed spectra appear to be consistent with highly simplified calculations emphasizing phonon density-of-states effects and multiphonon interactions induced by the anharmonic crystal potential.