Study of Phonon Transmission Through Solid-Solid Interfaces and Superlattices by Phonon Imaging.

The work in this thesis explores the low-temperature propagation of heat pulses (phonons) through crystals and across solid-solid interfaces. The phonon imaging technique has been used to examine the angular and temporal dependence of the phonon-flux intensity in several systems, listed below. 1. Phonon focusing in cubic crystals. The nature of the intensity pattern associated with phonon propagation is examined theoretically for cubic crystals in general. Experimental images, Monte Carlo flux-intensity simulations, and the slowness-surface topology are analyzed in detail for a wide range of elastic constants. A classification is developed whereby the pattern of phonon-focusing caustics may be predicted for any cubic material given its elastic constants and density. 2. Heat-pulse propagation across a copper-diamond interface. Phonon imaging experiments in diamond reveal sharp structures in addition to phonon-focusing caustics. The structures correspond to the channeling of transverse phonons close to the critical cone for mode conversion of transverse to longitudinal waves at the diamond surface. The process is mediated by pseudo-surface waves, and provides information about the quality of the diamond surface and the mechanical bonding of the copper/diamond interface. 3. Phonon transmission through superlattices. Phonon imaging experiments on several In_{ rm x}Ga_{1-rm x}As/AlAs and AlAs/GaAs superlattices show the angular distribution of high-frequency acoustic phonon transmission through periodic superlattices. Bragg reflection of phonons is examined in detail, and related to gaps, or stopbands, in the phonon dispersion relation. Attenuation in the phonon transmission due to gaps at the folded Brillouin zone boundary are observed for both longitudinal and fast transverse phonons. A newly predicted stopband within the zone due to coupling between the transverse and longitudinal acoustic modes is detected. The experimental results are compared to calculations of the transmission rate and the angular stopband distribution.