Phonon focusing in tellurium dioxide

We have observed the propagation of heat pulses in ${\mathrm{TeO}}_2$ at low temperatures (T=1.6 K) using the technique of phonon imaging. Well-defined heat pulses are observed, indicating the ballistic propagation of several-hundred-GHz phonons. Phonon images display highly anisotropic heat flux, showing well-defined caustics associated with the phonon-focusing effect. The correlation between time-of-flight signals and phonon images is examined. Theoretical calculation of the three-dimensional slowness surface gives insights into the mixing of longitudinal and transverse modes and shows the origin of the phonon caustics. Monte Carlo calculations of phonon flux are in substantial agreement with the data.