Brillouin scattering is the inelastic interaction between light and thermal acoustic excitations in dielectric materials. The physical reason for light scattering is the presence of inhomegeneities of the medium due to fluctuations in the dielectric constant, which are caused by thermally excited phonons. The coupling between thermal induced strains and fluctuations of the dielectric constants is given by the elasto-optic 4th rank tensor (Pockels coefficients). Brillouin and Raman scattering differ in that the first is due to acoustic vibration modes while the latter is due to optical vibration modes. The frequency shift caused by Brillouin scattering is in the order of 1 cm-1 (= 29.98 GHz). We use Brillouin techniques to study the elastic constants of crystals as a function of pressure. From such studies, we obtain information on sound velocities which can be directly compared to seismic velocity profiles for the Earth. Brillouin scattering can also be used to study high-pressure phase transformations.
Plan
view of major optics
Nd:Vanadate
Laser source with optics
Schematic
diagram