Conversion of electromagnetic into acoustic energy using metal films in the temperature range of 4.2-100 K

Transverse-acoustic phonons were generated electromagnetically at 9 GHz in the temperature range of 4.2-100 K using thin films of indium, tin, and gold on silicon substrates. As the temperature increases the conversion efficiency of electromagnetic into acoustic energy increases at low temperatures while at higher temperatures it strongly decreases. The temperature dependence of the conversion efficiency is interpreted as due to the decrease of the electronic mean free path upon raising the temperature. A simplified theory is presented for electromagnetic phonon generation which assumes diffuse surface scattering of the conduction electrons but takes into account bulk scattering as well. The theory predicts a decreasing conversion efficiency as the mean free path becomes smaller than the film thickness and an even stronger decrease when the mean free path becomes small compared to the phonon wavelength. A good agreement is found between theory and experiment.