Phase-coherent Rayleigh scattering method: Application to thermal diffusion mode

Abstract

We develop a method of phase-coherent light scattering, which provides “complex” spectra of Rayleigh scattering from a thermal diffusion mode in isotropic liquids. Different from the conventional light scattering methods, which measure the power spectrum of light scattered by spontaneously excited fluctuations, we generate a coherent temperature grating by using a laser-induced scanning interference pattern and measure complex spectra of a thermal diffusion mode by its phase-sensitive superheterodyne detection. We can control the amplitude of the temperature grating by changing the absorption coefficient of a liquid by adding a small amount of dye molecules. This controllability of the signal intensity and the resulting mode selectivity allow us to pick up only a thermal diffusion mode even under the existence of other incoherent modes with large amplitude. For example, our method enables us to study a thermal diffusion mode of a critical binary mixture usually hidden by large concentration fluctuations near its critical point.