Resonant Brillouin scattering by amplified phonons in CdS

Dispersion spectra of the Brillouin scattering cross section are investigated in the range of photon energy 2.0-2.4 eV at room temperature by making use of intense acoustic phonons amplified through the acoustoelectric effect in CdS. We have found a deep and narrow minimum of the scattering cross section at a photon energy of about 2.22 eV and a steep increase near the fundamental absorption edge. These features are explained in terms of resonant enhancement and cancellation based on Loudon's theory. Theoretical calculation by taking into account the exciton effect shows an excellent agreement with experimental data. Scattering of the virtual intermediate states associated with the $p$-like $A$, $B$, and $C$ valence bands was found to give rise to the resonant enhancement. The dominant contribution comes from the scattering process of the exciton of the $B$ state to the $A$ state by the deformation potential $C_6$ and the recombination of the $A$ state to emit the scattered photons in the present configuration.