High Efficient Reflective Gratings for Surface Acoustic Waves Using Phononic Crystals

Phononic crystals (PCs) composed of periodically arranged media have an important property of acoustic band gaps. The band gaps are some particular frequency ranges and waves inside the ranges cannot pass the PC structure. Band gaps were observed for acoustic waves in various PC structures, including bulk acoustic wave in infinite structures, surface acoustic wave (SAW) in a half-space, and Lamb waves in plates. Based on band gap property, tens of lattice periods can stop wave propagation efficiently and many applications, including filters, waveguides, and resonant cavities, were designed accordingly. Because PCs can stop surface acoustic wave propagation efficiently with tens of lattice periods, PC was further designed wave reflectors in SAW devices. SAW devices were used as commercial products, such as filters and resonators, for several decades. A SAW device basically consists of interdigital transducers (IDTs) and reflective gratings on piezoelectric material substrates. To enhance the performance of SAW devices, reflective gratings were usually designed as hundreds of periodically-spaced metal strips. Thus using PCs as reflective gratings has the advantage over producing space-sparing SAWdevices. In this project, using PCs as SAW reflective grating will be further investigated to raise the reflection efficiency. Firstly, finite element (FE) method will be developed and modified to analyze the dispersion of SAWs in PC structures. Using the developed FE method, band gaps for SAWs in a PC can be discussed. The geometric designs of PCs will be varied gradually to investigate the variation of band structure. Then SAW encountering various PC structure can be calculated and the reflection of SAW will be observed. Thus a high efficiency PC grating for SAWs can be designed accordingly. In the experiment, millimeter-scale PCs will be designed and fabricated on a metal substrate. Ultrasonic transducers and laser interferometer will be used to measure SAWs reflected from the PC gratings. The experimental results will be compared with the numerical results. The study is expected to be an important reference for the realization of the PC grating in SAW devices operating in MHz range.

Project ID:PB10011-0181
External Project ID:NSC100-2218-E182-007