Phonon Focusing of Large-kAcoustic Phonons in Germanium

Abstract
Combining heat-pulse scanning and tunnel-junction detection, we have observed ballistic propagation of high frequency (>700 GHz) acoustic phonons over ∼1-mm path lengths in germanium. The ballistic nature of the phonons is determined by sharp variations in phonon flux with propagation angle. The phonon focusing pattern is significantly altered from that obtained with an Al bolometer (which is sensitive to longer-wave-length phonons) revealing a modification of the phonon slowness surface due to dispersion.