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Internal diffraction of ultrasound in crystals: Phonon focusing at long wavelengths.

We have obtained images of acoustic energy flux in anisotropic media by an ultrasonic analog to ballistic-phonon imaging. The observed flux reveals a phonon-focusing pattern modulated by interference fringes, in accord with prior theoretical predictions. While phonon imaging is largely restricted to insulators at low temperatures, the ultrasonic technique works at ambient temperatures and is also applicable to metals and even noncrystalline anisotropic materials