Phonon Imaging: Theory and Applications

The subject of this chapter is the propagation of thermal energy through crystalline solids at low temperatures. A remarkable observation is that thermal energy emanating from a point source of heat is strongly channelled into various directions in the crystal. This effect is simply caused by the elastic anisotropy of the crystal and is known as phonon focusing. Over the past several years phonon imaging techniques have been developed which graphically demonstrate phonon focusing and use it to study the scattering of high frequency phonons in crystals. We will review here the experimental methods and the physics learned from such experiments. We begin with a general discussion of ballistic heat propagation in real crystals.