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Phonon frequency down-conversion observed in thin metallic films

The frequency of phonons launched into a sapphire substrate from an electrically heated metal film has been investigated using a heat pulse technique. It was found that for an emitted phonon spectrum calculated using the Perrin and Budd model, the predicted dispersive broadening of the phonon pulse in the substrate was much greater than the authors observed. They have shown that scattering by subsurface damage is unlikely to account for this observations. By correlating the phonon pulse broadening with the energy dependence and the polarisation magnitudes of the heat pulse signals, they have concluded that phonon frequency down-conversion occurs within the heater film.