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

声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频段声表面波器件的发展中发挥重要作用。在这项研究中，我们开发了一种用于声表面波传播的实时二维成像技术。它基于超快光泵浦法和极稳定的共光路干涉仪。在金刚石衬底上覆盖Au薄膜的情况下，用这种方法产生并探测到了高达2 GHz的表面声学声子。我们还发展了一种分析方法，可以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的多模。作为一种特别针对不透明样品的新成像技术，提出了一种使用自动反射镜的方法。对于声表面波的检测，提出了基于反射率变化的检测方法，取代了干涉仪。基于此，我们发展了适用于非均匀光学各向异性多层膜样品的法向和非法向探测光入射的光散射理论。作为应用，我们对微米尺度的多晶铜样品中的表面声波进行了成像。清晰地观察到晶界的衍射和反射。将计算结果与基于FDTD方法的仿真结果进行了比较。综上所述，我们发展了时间分辨率为皮秒、空间分辨率为微米级的通用声表面波测量与分析方法。通过这种方法，我们阐明了声表面波在MHz-GHz频段的传播物理。

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.