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Phonon Focusing Caustics in Crystals and their Diffraction Broadening at Ultrasonic Frequencies

This paper is concerned with the caustics that occur in the phonon focusing patterns of crystals, and their diffraction broadening at finite frequencies as observed in transmission acoustic microscopy. It goes beyond previous investigations on the subject in that the emphasis is on isolating the diffraction patterns of relatively simple caustic structures such as cusps and single and intersecting folds, which are normally observed interfering with each other in complex and incompletely resolved overlapping structures. This resolution is effected by three means, going to higher frequencies than hitherto in calculations, limiting the wave vector domain over which the requisite Fourier integrals are done, and time gating the transmitted signal to capture only the structures of interest. Calculations are reported for GaAs and Si, and measured time-gated diffraction images are presented for GaAs, which are in good agreement with calculation.