Theory of Stimulated Concentration Scattering

From the general formulation of the coupling of a laser wave and a scattered light wave with density, temperature, and concentration fluctuations in a binary fluid mixture, an expression for the gain $g_c$ of stimulated concentration scattering is derived. It is shown that this gain is usually much smaller than the gain for stimulated Brillouin scattering $g_B$. However, in gaseous mixtures at relatively low pressures with a large difference in polarizability as well as in mass between the two components, $g_c$ can become larger than $g_B$. A large difference in polarizabilities increases the coupling of light to the concentration fluctuations, while a large difference in mass produces a pronounced increase in damping of a sound wave. The calculated values for $\frac{g_c}{g_B}$ are compared with experimental results.