Optical Heterodyne Spectroscopy of Light Scattering by Laser Induced Phonons

超声光谱学的基本要求是在很宽的频率范围内观察速度和吸收。在过去的几年里，我们开发了各种实验技术，包括平凹超声谐振器和高分辨率布拉格反射（简称HRB）方法。本研究的目的是建立一种新的技术，这是有用的声子速度和衰减在高超音速范围内的测量。为此，我们利用光拍频光谱来探测热声子的光散射。HRB方法的成功表明，光拍是观察布里渊散射的非常弱的光最有效的方法。在这项研究中，我们得到了各种重要的结果，总结如下。(a)我们建立了在2GHz频率范围内测量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。对GeO_2玻璃的高频超声特性进行了测量。在600 MHz附近发现了一个由于结构弛豫引起的吸收峰，由此得到了氧原子在玻璃中的双阱势分布。(b)提出了一种新的光学拍频技术，用于测量液体表面热涨落的布里渊散射，测量范围可达1 MHz，比以往的测量范围提高了10倍。该技术将成为研究液体表面动力学特性的有力手段。(c)我们采用了几种光外差系统对热声子非相干布里渊散射光进行探测，并对它们进行了初步的测试，得到了每种系统的特点和性能。

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.