Channeling of acoustic phonons in silicon: the polarization dependence in elastic scattering

Ballistic phonon flux in crystals at low temperatures is highly directional due to phonon focusing. In this paper, a phonon-imaging experiment is reported which shows that scattered phonons, too, can retain a highly directed flux. Basically, the combination of phonon focusing and the elastic-scattering selection rule act to channel phonons along the ballistic focusing directions. Together with Monte Carlo simulations, the experiments show that the scattered phonon flux is indeed due to elastic scattering processes, which depend on the polarizations of the scattering phonons. Isotropic scattering is inconsistent with the data. The experimental technique shows promise for quantifying the microscopic scattering processes and revealing the dynamics of a phonon hot spot.