Phonon Prism utilizing phonon focusing in thin films

为了寻找半导体薄膜作为声子棱镜用于固体声子光谱分析的可能性，利用共振超声光谱学中开发的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，声波的正模色散关系被称为兰姆波，这取决于传播方向。兰姆波的坡印亭矢量随着频率的变化而改变方向，特别是当波长与薄膜厚度相同时。因此，声子聚焦效应似乎对声子的频率分析是有用的。然而，由于色散关系的子带结构，声子谱的分辨存在困难。子带结构来源于薄膜法向波矢量的量化。正常模态具有各自的色散关系，即使在同一频率下，由于声波的混合，正常模态的群速度也不同。研究了一种根据波的偏振来分辨它们的可能方法，但发现它失败了，因为除了在膜的法向上的波数外，波具有相同的偏振。声子聚焦在薄膜中的另一种可能的应用和声子的解析方法正在研究中。

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.