Theory of Brillouin scattering from opaque media

Abstract

The general theory of Brillouin scattering from the surfaces of opaque media is developed using a total field solution approach. Acoustical modes appropriate to a stress-free surface are found from solutions to the acoustical-wave equation and boundary conditions for a finite isotropic medium. Two light-scattering mechanisms, namely, the surface corrugation and bulk elasto-optical effects, were analyzed by deriving optical fields which satisfy both the acousto-optically driven wave equation and the electromagnetic boundary conditions. The special case for a metal of scattering by acoustically created conductivity fluctuations was also investigated in a similar way. The Brillouin spectrum was obtained by summing the scattering from the individual acoustical modes over the density of phonon states. Excellent agreement was obtained with experiment for Brillouin scattering from the metals gallium (liquid) and aluminum (solid).