Two-dimensional real time imaging of the propagation of surface waves on materials in picosecond temporal regime

表面声子聚焦现象的成像揭示了声能在各向异性材料中传播的复杂模式。该成像技术有望在GHz频段表面声波器件的发展中发挥重要作用。在这项研究中，我们开发了一种表面声波传播的实时二维成像技术。它是基于超快光泵浦探测方法和极稳定的共路光干涉仪。结果表明，在覆盖有金膜的金刚石衬底上，该方法可产生并检测到高达2 GHz的表面声子。我们还开发了直接从透明和不透明、各向同性和各向异性样品的实验结果中获得表面声子多模态的分析方法。作为一种针对不透明样品的新型成像技术，本文提出了一种利用自动反射镜成像的方法。对于表面声波的探测，提出了一种基于反射率变化的方法来代替干涉仪。为此，我们建立了适用于具有非均匀光学性质的各向异性多层样品的正向和非正向探针光入射的光散射理论。作为该技术的一个应用，表面声波在由微米级晶粒组成的多晶铜样品中成像。在晶界处可以清楚地观察到衍射和反射。并与基于时域有限差分法的仿真结果进行了比较。它提供了评估晶界周围的地下结构的机会。综上所述，我们开发了具有皮秒时间分辨率和微米尺度空间分辨率的通用表面声波测量和分析方法。通过这种方法，我们明确了表面声波在MHz-GHz频段传播的物理特性。

Imaging of the surface acoustic phonon focusing phenomena reveals complicated patterns of the acoustic energy propagation in anisotropic materials. The imaging technique is expected to play an important role in the development of surface acoustic wave devices in GHz frequency region. In this study, we develop a real-time two-dimensional imaging technique for the surface acoustic wave propagation. It is based on the ultrafast optical pump-probe method and the extremely stable common-path optical interferometer. In case of the diamond substrate covered with Au film, it is shown that the surface acoustic phonon up to 2 GHz is generated and detected by this method. We also develop the analysis method to obtain the multiple mode of the surface acoustic phonon directly from the experimental results on transparent and opaque, isotropic and anisotropic samples.As a new imaging technique especially for opaque samples, a method using automated mirror is developed. As for the detection of the surface acoustic wave, instead of the interferometer, the method based on the reflectivity change is developed. In relation to this, we develop the light scattering theory that is applicable to the normal and non-normal probe light incidence to the anistropic multilayer sample with inhomogeneous optical properties.As an application of the technique, the surface acoustic wave is imaged in the poly-crystalline Cu sample consists of micron size grains. The diffraction and reflection at the grain boundary is clearly observed. The result is compared with the simulation based on the FDTD method. It provides the opportunity to evaluate the subsurface structure around the grain boundaries.In summary, we have developed the general purpose surface acoustic wave measurement and analysis method with pico-second time resolution and micron scale spatial resolutions. By this method, we have clarified the physics of surface acoustic wave propagation in MHz-GHz frequency region.