Low-angle Brillouin scattering under a temperature gradient

The asymmetry appearing in the dynamic structure factor of a medium under a non-equilibrium state of temperature gradient was experimentally verified. Brillouin scattering was carried out in water with the optical beating spectroscopy technique at frequencies lower than 60 MHz where a large asymmetry was expected. The finite-cell-size effect to which the disagreement between the theory and experiment had been attributed was excluded by a specially designed tall cell which suffers from the least effect of the finite size. The observed Stokes line was shown to have intensity 2% to 8% higher than the anti-Stokes line depending on the phonon frequency for a temperature gradient applied into the same direction as the scattering vector. Based on a concept of anisotropic phonon propagation under a temperature gradient, a simple theory was proposed which succeeded in estimating the asymmetry for the temperature distribution of actual experimental condition including the effect of cell boundaries. The observed asymmetry was quantitatively in good agreement with the theoretical prediction.