Phonon Optics, or, How to Control Non-equilibrium Heat Flow in Solids

In the non-diffusive regime, heat flow is greatly affected by crystalline anisotropy. In particular, low frequency (~100 GHz) phonons transport energy along preferred crystalline directions because of the anisotropy of group velocity. This phonon focusing effect is an intrinsic property of crystals and has been exploited to study a wide variety of material properties including elasticity, thermal transport, defect interactions and phonon-carrier interactions. It should, however, be noted that this type of phonon focusing is not a true focusing: it is low-divergence propagation in an anisotropic media. True phonon focusing is only possible when flux is redirected at extremely clean, defect-free interfaces. Such interfaces may be engineered for applications in which phonons need to be dissipated or channeled in particular directions. The degree of refraction at an interface will depend upon the acoustic properties of the material on either side. My talk will survey a number of imaging studies of phonon propagation in low temperature semiconductors and discuss possibilities for phonon optics based upon these studies.

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