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

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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.