Optical Heterodyne Spectroscopy of Light Scattering by Laser Induced Phonons

超声光谱学的基本要求是在较宽的频率范围内观察声速和吸收。在过去的几年里，我们发展了各种实验技术，包括平凹超声谐振器和高分辨率布拉格反射(简称HRB)方法。这项研究的目的是建立一种新的技术，用于测量高超声速范围内的声子速度和衰减。为此，我们利用拍光光谱来检测热声子的光散射。HRB方法的成功表明，光拍频是观测布里渊散射中非常微弱的光的最有效方法。在本中心的研究中，我们取得了如下重要成果：(A)建立了测量2 GHz以下透明固体中声子速度和吸收的方法。根据高超声速测量结果估算了蓝宝石的Gruneisen常数，它代表了声子-声子相互作用的强度。测量了GeO_2玻璃的高频超声性能。在600 MHz附近发现了一个结构弛豫吸收峰，由此得到了玻璃中氧原子的双势分布。(B)发展了一种新的光学拍频技术来观测液体表面热起伏引起的布里渊散射，测量范围扩展到了1 MHz，比以前的工作提高了10年。这项技术将成为研究液体表面动力学性质的有力手段。(C)我们搭建了几个探测热声子非相干布里渊散射光的光学外差系统，通过对它们的初步测试，得到了每个系统的特性和性能。

The fundamental requirement in the ultrasonic spectroscopy is to observe the velocity and absorption over extensive frequency range. In the last few years, we have developed various experimental techniques, including planoconcave ultrasonic resonator and high resolution Bragg reflection (to be referred as HRB) method. The purpose of this research project is to establish a new technique which is useful for the measurement of phonon velocity and decay at hypersonic range. For this purpose, we took advantage of the light beating spectroscopy to detect the light scattering by the thermal phonons. The success of the HRB method has shown that the light beating is most effective to observe the very weak light of Brillouin scattering. In the investigation by this Grantin-Aid,we obtained various important results which are summarized as follows.(a) We established the means to measure the phonon velocity and absorption in transparent solids at the frequency range up to 2GHz. The Gruneisen constant which represents the strength of phonon-phonon interaction, was estimated for sapphire from the results of hypersonic measurement. High frequency ultrasonic property was measured in GeO_2 glass. An absorption peak due to the structural relaxation was found around 600 MHz, which yields the distribution of the double well potential of oxygen atom in glass.(b) A new optical beating technique was developed to observe Brillouin scattering by thermal fluctuation of liquid surface.The range of the measurement was extended up to 1MHz, which is one decade higher than the previous works. This technique will be a powerful means to investigate the dynamic property of liquid surface.(c) We constracted several optical heterodyne systems to detect incoherent Brillouin scattering light by thermal phonons and obtained the characteristics and performance of each system through the preliminary test with them.