Surface acoustic wave trapping in a periodic array of high aspect ratio electrodes

Surface acoustic wave transduction by the use of an interdigital transducer (IDT) is a well-known and widespread technique. However, a usual IDT makes use of electrodes with limited heights, so that surface mode properties do not differ appreciably from those of a free or a fully metalized surface. We proposed previously [V. Laude et al.,J. Appl. Phys.90, 2492 (2001)] a theoretical analysis of the transduction of surface acoustic waves (SAW) under a metallic array of electrodes with a large aspect ratio on a piezoelectric substrate, whereby allowing the electrode height to become larger than one wavelength. We here report on the experimental observation of the multimode character of SAW propagation under a periodic array of electrodes. We also obtain experimentally the explicit dependence of the SAW velocities as a function of the electrode height. In that purpose, we fabricated interdigital transducers on the Y+128 cut of lithium niobate using electroplating of nickel. The slightly pyramidal electrodes are up to five times higher than wide. The variation of the resonances frequencies of the various surface modes was obtained experimentally. Up to a 10-fold slowing of surface waves is observed, with the phase velocity dropping from 4000 m/s down to 450 m/s. The comparison with a theory based on a coupled finite-element boundary-element-method is excellent.