Ballistic Phonon Imaging in Solids—A New Look at Phonon Focusing

Abstract

A technique is described which graphically displays the anisotropic propagation of non-equilibrium phonons in crystals at low temperatures. Ballistic phonons emanating from a localized heat pulse are channeled preferentially along various crystal directions due to elastic anisotropies. A spatial map of phonon intensities in Ge is collected by continuous scanning over the propagation directions. The data have precipitated a new approach to analyzing the theory of phonon focusing in solids.