Abstract
The ballistic heat flux radiating from a point source of heat in a cold crystal displays a complex pattern of caustics due to phonon focusing. The caustics correspond to folds in the elastic-wave surface of the crystal. The pattern of caustics is independent of phonon frequency unless the phonon wavelength is comparable to the lattice spacing; i.e., for values of wavevectork approaching π/a. We have measured both the shift in the caustic pattern (angular dispersion) and the increasing time-of-flight (velocity dispersion) for ballistic phonons in InSb with wavevectors up to 40% of the Brillouin-zone boundary. Comparison with existing lattice-dynamics models favors the Bond Charge Model (BCM). The phonon-imaging method gives information about theshapes of the wave surfaces which is complementary to the dispersion curves measured only along symmetry directions by neutron scattering.
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