Study on Acoustic Soft-Mode Dispersion by Impulsive Stimulated Brillouin Scattering

我们发展了一种新的利用脉冲受激布里渊散射的声子谱。这种方法是基于两个交叉的光脉冲产生声子和声子衍射的探测光。与最近的测量不同，我们采用了连续激光作为探针。这使我们能够测量相变点附近的一个非常慢的响应，表明在频率范围内的高分辨率声子谱。此外，所产生的声子的相干性质提供了在极低的声子波数的测量。结合球面法布里-珀罗干涉仪，我们可以直接对产生的非平衡声子进行频域测量。为了证实这种方法的优点，我们研究了声子产生的机制，所产生的声子的传播，共振效应对声子产生的脉冲序列。已经进行了几个应用，其中包括在液体，金属表面和液晶中产生声波。为了测量声学软模，我们采用了具有C_软声子模的铁弹晶体KD_3（SeO_3）_2作为样品<66>。我们已经完成了空间强度分布的激光通过晶体，这是由以下的转变点的磁畴形成的影响。特别地，畴壁处的折射光强度给出了一种观察相变点附近弹性涨落的新方法。事实上，在畴壁处的折射光甚至在转变点以上也是可观察到的，这表明畴形成的波动发生在那些温度下。结合ISBS实验和上述透射实验，我们得到了转变点附近的软模行为。目前正在对结果进行详细分析。

We have developed a new phonon spectroscopy using impulsive stimulated Brillouin scattering. This method is based on the phonon generation by two crossing light pulses and the diffraction of a probe light by the phonon. Unlike recent measurements, we have employed a cw laser as a probe. This enables us to measure an extremely slow response near the phase transition point, indicating a high-resolution phonon spectroscopy in frequency regime. Furthermore, coherent nature of the generated phonons offers a measurement at extremely low phonon wave number. Combining with a spherical Fabry-Perot interferometer, we can perform directly the frequency-domain measurement for the generated non-equilibrium phonons. To confirm the advantage of this method, we have investigated the mechanisms of the phonon generation, the propagation of the generated phonon, the resonance effect on the phonon generation by the pulse train. Several applications have been performed, which include the generation of acoustic waves in liquids, metal surfaces, and liquid crystals. For the measurement of the acoustic soft-mode, we have employed a ferroelastic crystal KD_3(SeO_3)_2 as a sample, which is known to show a C_<66> soft phonon mode. We have performed the spatial intensity profile of the laser light transmitted through the crystal, which is influenced by the domain formation below the transition point. Particularly, the intesity of the refracted light at the domain wall gives a new observation method of the elastic fluctuation near the transition point. Indeed the refracted light at the domain wall is observable even above the transition point, suggesting the fluctuation of the domain formation takes place at those temperatures. Combining the ISBS experiment with the above transmission experiment, we have obtained the soft-mode behavior near the transition point. The detailed analysis of the result is now in progress.