Ballistic phonon imaging of diamond

Abstract

Phonon imaging has been used to study the transport of ballistic phonons in diamond. The images that have been obtained display a number of sharp features. Some of these correspond to mathematical singularities or near singularities in bulk phonon focusing. Other structures are found to correspond to the channelling of transverse phonons close to the critical cone for mode conversion of transverse to longitudinal waves at the diamond surface. This channelling is mediated by longitudinal pseudo-surface waves at the diamond surface, and is extremely sharp because of the near isotropy of diamond and its exceptionally small effective Poisson's ratio. The presence of these channelling structures signifies that the diamond surface is relatively flat on the scale of the dominant phonon wavelength ( approximately=200 AA) and that the bonding between the metal heater and detector films and the diamond substrate is mechanically weak.