Efficiency of poly(vinylidene fluoride) thin films for excitation of surface acoustic waves

The applicability of poly(vinylidene fluoride) (PVDF) thin stretched films to the excitation of surface acoustic waves (SAWs) is discussed. Previous results with grating transducers reveal high losses, which are attributed to large attenuation in the polymer. An experimental study was carried out on a LiNbO3 YZ delay line with the input transducer in the form of an interdigital PVDF pair. Untuned insertion losses of 80 dB were obtained at room temperature and frequency of 43 MHz. We demonstrate that these losses are largely caused by the high shear compliance of the film, which leads to degradation of the transducer electromechanical coupling. Employing the piezoelectrically strong thickness-longitudinal mode of the film, a 40 dB loss reduction was achieved. To take full advantage of this mode, a split-electrode grating transducer model with alternating polarity of electrodes is discussed. This is expected to allow efficient SAW excitation at frequencies up to tens of MHz.