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

我们发展了一种新的动态光散射方法，使我们能够对液体中的瑞利-布里渊散射进行模式选择测量。不同于传统的光散射方法测量自发涨落散射光的功率谱，我们利用激光诱导的扫描干涉图样产生相干光栅，并通过其相敏超外差检测来测量光激励模的复光谱。我们的相位相干光散射方法的模式选择性是一个独特的特征，这是传统的光散射方法无法实现的，该方法测量自发激发涨落散射的光的功率谱。这对于研究通常隐藏在另一模的大波动背后的模特别有用。例如，我们可以通过添加少量的染料分子来改变液体的吸收系数来控制温度光栅的幅度。这种信号强度的可控性和由此产生的模式选择性使我们即使在存在其他大幅度的非相干模的情况下也只能拾取一个热扩散模。为了验证该方法的有效性，我们研究了液晶在各向同性相中的热扩散；在传统的频域方法中，热扩散模谱通常隐藏在其相变点附近的取向弛豫模谱之后。利用我们的方法，我们发现在过渡点附近的光谱幅度和线宽都突然减小，我们认为这分别反映了比热和热扩散率的奇异行为。这是首次用动态光散射法测量各向同性相液晶样品相变点附近的热奇异性。

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.