Progression from ballistic phonon focusing to internal diffraction of ultrasound in crystals

The aim of this paper is to bridge the divide between phonon focusing and internal diffraction patterns of crystals. We show how phonon caustics,at finite frequency, unfold into characteristic diffraction patterns. Then, as the frequency is lowered further, the fringes progressively broaden and merge, until ultimately only the broadest vestiges of the original focusing pattern can be discerned. Time gating and limiting the wave vector domain over which the requisite Fourier integrals are done, is used to isolate the diffraction patterns of elementary caustic structures such as cusps and folds. Calculations are reported for GaAs and Si, which are in good agreement with experiment. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)