Stimulated Brillouin Scattering
Brillouin scattering is the scattering of light from acoustic disturbances in a medium. Classically, the source of the scattered wave can be linked to a fluctuation in the dielectric constant of the medium. The change in the dielectric constant is related to a change in the mass density of the medium  by
Δ ε =
p Δp (1)
where Ae and Ap are the small changes in dielectric constant and density, respectively. The density fluctuation is further related to the thermodynamic variables p (pressure) and S (entropy):
Δp = ρ p S
Entropy fluctuations ΔS lead to Rayleigh scattering (no frequency shift). Brillouin scattering can thus be described as scattering of light from acoustic density or pressure waves, or from acoustic phonons.