INTERFACES AND BOUNDARIESTHE SOLID-SOLID INTERFACE IN THERMAL PHONON RADIATION

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Abstract
The problem of thermal phonon transmission across a solid-solid interface between two dissimilar materials is discussed especially in connexion with the generation of THz-phonon beams by thermal phonon radiators. The thermal phonon radiators are current heated thin metal films deposited on cooled dielectric monocrystals which serve as transmission media. Measurements of the radiation temperature as function of the transmitted phonon power are reported and show that the acoustic-mismatch model describes the thermal phonon transmission quite well in a frequency range where dispersion effects are still not important. In order to ensure that the formulas of acoustic-mismatch between two isotropic solids are applicable without too large error, these experiments have been performed by using crystals – like sapphire and especially diamond – showing only a small mechanical anisotropy. To avoid such limitations in future, an extension is given for the general case of an interface between an isotropic radiator and an anisotropic substrate. No large differencies are to be expected in most crystals for the emissivities in comparison to the values calculated from the isotropic approximation, since one has to integrate over the half space. By far more important seems to be this extension of theory for the evaluation of the radiant characteristics, respectively for the evaluation of the time dependent phonon power falling onto phonon detectors or other devices

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