Raman Scattering from Localized-Mode Polaritons

In a crystal containing a finite concentration of impurities which give rise to localized vibration modes, the coherent motion of the impurity system can result in the generation of a macroscopic electric field, if the local modes are infrared-active. In this case, the true normal modes of the impurity system are collective modes with a wavelike character. We show that the transverse local modes have the character of polariton modes, with properties very similar to the properties of the phonon-polariton modes of pure crystals. We also present the theory of the Raman scattering of light from local-mode polaritons. We estimate the Raman efficiency for the scattering of light from local-mode polaritons, and make comparisons between the scattering from these excitations and the scattering from phonon polaritons in pure crystals.