Grating-Induced Waveguiding of Surface-Skimming Bulk Waves

There is ever growing interest in faster surface waves for applications in surface acoustic wave (SAW) devices. Faster SAW means higher frequency operation for the same technological difficulty of making metal strips of interdigital transducers (IDTs) of micrometer width and nanometer resolution. The fastest acoustic waves are the bulk waves which cannot propagate at the substrate surface nor yield strong interaction with IDTs over longer propagation distances required in the SAW filters for narrow-band devices; the surface signal of bulk waves quickly vanish due to the wave propagation (and diffraction) into depth of the body. There is however, a remedy for it: the surface grating like periodic grooves or periodic metal strips (being these of the applied IDTs in the device), can help to trap the bulk wave at the surface making it a complex surface wave-guided mode. This phenomena is analyzed in this paper using the earlier developed spectral theory method in contrast to the couple-of-modes method presented afterwards. Here, only mechanical fields are discussed.