Observation of Forbidden Brillouin Scattering near an Exciton Resonance

Abstract
We have observed resonant Brillouin scattering by TA phonons near the A exciton of CdS in a usually forbidden backscattering configuration. This effect, attributed to electron-phonon piezoelectric coupling, is equivalent to the Fröhlich-interaction-induced forbidden LO scattering. A numerical estimate of the scattering strength of the forbidden TA scattering accounts for the intensity of the observed lines.