Conversion of electromagnetic into acoustic energy using Sn, Au, and Cu films at low temperatures

Transverse acoustic waves were generated electromagnetically at 9 GHz using thin films of tin, gold, and copper on silicon substrates. We report measurements of the conversion efficiency $\alpha$ of electromagnetic into acoustic energy as a function of temperature and film thickness at and below 4.2 K and preliminary results at liquid-${\mathrm{N}}_2$ temperature. The highest conversion efficiencies found at 4.2 K for Sn and Au were 1 × ${10}^{-5\mathrm{}}$ and 2.5 × ${10}^{-5\mathrm{}}$, respectively. In the temperature range of 1.4-4.2 K, the values of $\alpha$ for normal metals were found to be practically temperature as well as microwave-power independent. The value of $\alpha$ for superconducting tin decreases strongly below the superconducting transition temperature and becomes magnetic field and microwave-power dependent. This is in agreement with our previous results on indium. Using gold films we measured the thickness dependence of $\alpha$ in the range of $\pi <qd<\mathrm{}4\mathrm{}\pi$, where $q$ is the acoustic wave number in the metal in $d$ is the film thickness. We find a pronounced maximum at $qd=2\mathrm{}\pi$ and indications for another maximum at $qd=4\mathrm{}\pi$. The values of α at 4.2 K as well as the temperature and thickness dependence of $\alpha$ are found to be in good agreement with the theory based on diffuse surface scattering of the conduction electrons.