Raman Scattering by Coupled Plasmon-Longitudinal-Optical-Phonon Modes in Zincblende-Type Crystals

A phenomenological theory of Raman scattering by the $q\approx 0$ coupled plasmon-(LO) longitudinal-optical-phonon modes, which takes into account the electro-optic effect of the longitudinal electric fields associated with the coupled modes, is presented. Estimates of the scattering efficiency of the plasmon-like mode at high carrier densities, for which the major contribution comes from the electro-optic effect, are obtained using values of the electro-optic coefficient derived from second-harmonic light-generation data. For $n$-type GaAs with a carrier density of 3×${10}^{18}$/${\mathrm{cm}}^3$, the scattering efficiency of the plasmon-like mode is found to be 1×${10}^{-5\mathrm{}}$. The theoretical treatment is also applicable to Raman scattering by the collective cyclotron excitation modes of free carriers in a magnetic field and by their coupled modes with LO phonons.