Thickness dependence of conversion efficiency of ZnS film transducers for elastic surface waves

The conversion efficiency of ZnS transducers evaporated on a glass substrate for elastic surface waves has been measured as a function of the ZnS film thickness and analyzed by using a simple model. It is found that the thickness dependence of the conversion efficiency is affected by elliptic particle motion at the position of electrodes located at the interface between the ZnS film and the substrate (type I) or at the surface of the ZnS film (type II). Conversion efficiency calculated from the potential induced at the electrodes is in good agreement with the experiments. The analysis shows that the internal impedance of the thin-film transducer is not simply proportional to Δ vs/vs, where Δ vs is the perturbation in surface-wave velocity when the electrodes are replaced by a thin-film perfect conductor. Theory suggests that type I is most efficient when the thickness is about 0.4 times the surface wavelength.