Phonon Prism utilizing phonon focusing in thin films

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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.