Development of phase-coherent light scattering method using mode-selective light excitation

点击翻译按钮获取中文摘要

We developed a new method of dynamic light scattering, which enables us to make 'mode-selective' measurement of Rayleigh-Brillouin scattering in liquids. Different from the conventional light scattering methods, which measure the power spectrum of light scattered by spontaneously excited fluctuations, we generate a coherent grating by using a laser-induced scanning interference pattern and measure complex spectra of a light-excited mode by its phase-sensitive superheterodyne detection. The mode-selectivity of our phase-coherent light scattering method is a unique feature, which cannot be realized by the conventional light scattering methods measuring the power spectrum of light scattered by spontaneously excited fluctuations. This is particularly useful to study the mode usually hidden behind large fluctuations of another mode. For example, 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. To demonstrate the capability of our method, we study a thermal diffusion of liquid crystals in isotropic phase ; in conventional methods in frequency domain, the spectra of thermal diffusion mode are usually hidden behind ones of orientational relaxation mode near its phase-transition point. Using our method, we found sudden decreases in both spectral amplitudes and linewidths near a transition point, which we regard as the reflection of the singular behavior of the specific heat and the thermal diffusivity, respectively. This is the first measurement of the thermal singularity near the phase-transition point of liquid crystalline sample in isotropic phase by using dynamic light scattering methods.