Lowering diffraction of surface acoustic waves by phononic crystals

Surface acoustic wave (SAW) devices were widely used today and applied as filters, sensors, and wireless RFID tags. In the cases which SAW has to propagate a long distance, the noticeable diffraction of surface waves limited the performance of SAW devices. Phononic crystal (PnC) consisting of periodic media is a new acoustic metamaterial. PnC structures perform anisotropic propagation and acoustic band gaps for SAW were observed. In this paper, the anisotropic propagation of SAW in PnCs was analyzed further and applied to lowering the diffraction of surface waves. PnC structures consisted of cylindrical tungsten films on Lithium niobate substrate were analyzed. The anisotropic propagation was calculated and then be controlled by applying metal films with different radius and thickness. The diffraction of surface waves was suppressed by enhancing the anisotropy. The finite element analysis showed that 45 degrees rotated square lattice PnC enhance the anisotropic propagation. The PnC made of tungsten film supported anisotropic effects, and the reflection of surface wave encountering PnC was acceptable. Then the phononic lens was designed in front of IDT of SAW devices to lower beam diffraction. This result can be applied to raise performance of SAW devices with long delay lines.