Excitation of Soft Acoustic Mode by Stimulated Brillouim Scattering and Phase Transition Dynamics

本研究的目的是利用脉冲受激布里渊散射直接激发材料中的声子，并阐明声子在相变现象中的作用。在本工作中，我们进行了以下实验：1)铁电相变附近TGSe（硒酸三甘氨酸）准纵向声模的测量，2)飞秒受激光散射系统的开发，3)飞秒光学Kerr效应光谱与高分辨率光散射光谱的直接比较，4)Debye模型击穿的实验验证。1) TGSe是一种在22？糖化血红蛋白。在过渡点以下，光散射和超声实验发现了准纵向声子模式C_<33>的速度异常。这种异常来自于声子模式与极化波动的耦合。由于极化的特征频率与温度有关，因此速度异常在不同温度下发生，取决于所测声子的频率。在本工作中，我们测量了中频区域（450 MHz）声子速度的温度依赖性，并通过实验证实了这种异常的机制。我们开发了飞秒版本的受激光散射。该系统提供太赫兹声子的激发。3)光学克尔效应光谱与非共振激发区的自发光散射通过涨落耗散定理联系起来，并利用高分辨率光散射和飞秒光学克尔效应测量实验证实了这一定理。4)在相变研究中，经常使用Debye模型来表达波动行为。然而，该模型本身也存在一些困难，如动量和规则的失效。由于响应函数的有限初始上升在时域中清楚地观察到该模型的击穿，因此我们使用典型液体作为样本进行了实验。利用飞秒光学克尔效应和飞秒偏振选择性四波混频实验得到的响应函数表明，在每个样品中都有明显的初始上升。在玻璃化转变等较宽的频率范围内波动时，Debye模型的击穿会产生严重的影响。少

the purpose of this research is to excite phonon in material directly by impulsive stimulated Brillouin scattering and to clarify the role of the phonon in phase transition phenomena. In the present work, we have done the following exeriment : 1) Measurement of quasi-longitudinal acoustic mode in TGSe (triglycine selenate) near the ferroelectric phase transition, 2) Development of femtosecond stimulated light scattering system, 3) Direct comparison of femtosecond optical Kerr effect spectroscopy with high resolution light scattering spectrum, 4) Experimental verification of the breakdown of Debye model.1) TGSe is a crystal which undergoes ferroelectric phase transition at 22? A1C.Below the transition point, the anomaly in the velocity of the quasi-longitudinal phonon mode C_<33> has been found from the light scattering and ultrasonic experiments. This anomaly comes from the coupling of the phonon modes with the polarization fluctuation. Since the characteristic frequency of the polariz … More ation fluctuation is temperature-dependent, the velocity anomaly occurs at different temperatures dependent on the frequency of the phonon measured. In the present work, we have measured the temperature dependence of the phonon velocity in the intermediate frequency region (450 MHz) and have confirmed the proposed mechanism of this anomaly experimentally.2) We have developed the femtosecond version of the stimulated light scattering. This system provides the excitation of THz phonon.3) Optical Kerr effect spectroscopy and spontaneous light scattering in off-resonance excitation region are connected through fluctuation-dissipation theorem and we have confirmed this theorem experimentally by use of the high-resolution ligh scattering and femtosecond optical Kerr effect measurements.4) Debye model has been often used to express the fluctuational behavior in phase transition studies. However, this model accompanies essential difficulty in itself such as breakdown of the momentum sum rule. Since the breakdown of this model is clearly observed in the time-domain by the finite initial rise in the response function, we have performed the experiment using typical liquids as samples. The response function obtained by the femtosecond optical Kerr effect and femtosecond polarization-selective four wave mixing experiment shows a clear initial rise in every sample. It is suggested that the breakdown of Debye model has serious effect when the fluctuation in a wide frequency range is discussed such as in glass transtions. Less