Ionic contributions to the Raman tensor of insulators

Abstract

Earlier authors have pointed out that at infrared frequencies, ionic contributions to the nonlinear optical susceptibilities of crystals can become important. Of particular interest is their frequency variation, which may exhibit resonances when the frequency of one or more optical beams coincides with that of an optical phonon. We present a simple theory of the nonlinear optical response of insulators in the infrared region which incorporates both the electronic and ionic contributions to the Raman tensor. The method is easily extended to the discussion of other nonlinear mixing or harmonic-generation processes in the infrared region. Primary application is to crystals of the zinc-blende structure, and the processes examined explicitly are the generation of optical phonons by resonances with the difference frequency of two infrared waves mixed in the crystal, and the ionic Raman effect.