Phonon Prism utilizing phonon focusing in thin films

为了探索半导体薄膜作为“声子棱镜”用于固体声子谱分析的可能性，利用共振超声光谱学中的xyz算法研究了GaAs薄膜中的声子聚焦效应。由于薄膜的弹性各向异性，声波的正常模式的色散关系被称为Rambwave，取决于传播方向。波的坡印亭矢量的频率改变其方向，特别是，当波长成为相同数量级的膜厚度。因此，声子聚焦效应似乎被发现是有用的声学声子的频率分析。然而，由于色散关系的子带结构，使得声子谱的分辨存在困难。子带结构源于薄膜法线方向上的波矢量的量化。简正模具有各自的色散关系，即使在相同的频率下，各模的群速度也不相同，从而导致声波的混合，基于波的偏振性来解决这一问题的方法被研究，但发现除了在薄膜的法线方向上的波数之外，其他波具有相同的偏振性，因此该方法是失败的。另一种可能的应用的声子聚焦在薄膜和方法来解决的声子正在考虑中。

In order to search for possible use of a semiconductor film as 'phonon prism' for spectral analysis of phonons in solids, phonon focusing effects in GaAs films were investigated by means of xyz algorithm developed in the resonant ultrasound spectroscopy. Dispersion relations of the normal modes of the acoustic waves are known as Ramb waves depend on the propagating directions due to the elastic anisotropy of the films. The Poynting vector of the Ramb waves change its direction on frequency, in particular, when the wavelength becomes the same order of the film thickness. Hence it seems that the phonon focusing effect is found to be useful for the frequency analysis of the acoustic phonons. However, there is a difficulty in resolution of the phonon spectrum due to subband structure of dispersion relation. The subband structure is originated from quantization of wave vector in the normal direction of the films. The normal modes have their own dispersion relations, and then the group velocities of the modes are different even at the same frequency, mixing of the acoustic waves.A possible method to resolve them based on the polarization of the waves was examined, but it was found to fail since the waves have the same polarization except for the wavenumber in the normal direction of the film. Another possible application of the phonon focusing in the thin films and a method to resolve the phonons are under consideration.