| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 102 === A phononic crystal (PC) is a structure whose mechanical properties are periodical-ly arranged. 2-D PCs started to attract great attentions two decades ago. A lot of concern has been focused on their acoustic reflecting phenomenon due to the band gap. However the PC can also be an acoustic conductor, which its conductance is determined by the band structure. Novel properties such as negative refraction or acoustic lensing effect can be achieved with PCs.
Among the many applications, in the area of ultra-high frequency (UHF) acoustic wave devices, complete or partial band gap of air/silicon PC was utilized as the reflec-tive gratings to reduce the devices’ size. In those devices, a piezoelectric thin film has to be deposited on the silicon substrate to generate acoustic waves for silicon is non-piezoelectric. Although the fabrication process of the silicon based phononic device has the CMOS compatible advantage, the insertion loss of such a device is high rela-tively. The other more straightforward way of making a phononic acoustic wave device is to construct periodically micro-holes directly on a piezoelectric substrate. Although the electro-acoustic conversion is higher, it suffered from the anisotropic nature of lith-ium niobate that lead to a complicated fabrication process.
On the other hand, SH-type SAWs have several advantages compared to conven-tional Rayleigh-type SAWs. For example SH-type SAWs possess larger piezoelectricity than Rayleigh-type SAWs on the same substrate material. Also they are faster than Rayleigh-type SAWs therefore desirable for high frequency applications. And in liquids, SH-type SAWs lose less energy than Rayleigh-type SAWs do due to their polarization, also they are sensitive to surface loadings, so they are desirable for (bio-) sensing appli-cations. Love wave, being one of the SH-type SAWs, shares similar physical character-istics with other SH-type SAWs. Investigating the propagating of Love waves in PCs will expand the PC applications to SH-type SAWs that have bright outlook.
In this study, we investigate Love waves propagating in a piezoelectric substrate coated with a phononic guiding layer. The phononic layer is consisted of a thin layer with periodic machined holes. It is worth noting that the thin phononic layer can be non-piezoelectric which makes the fabrication process relatively simple. And since most energy is trapped in the guiding layer it may serve as efficient reflective gratings in Love wave devices. The method proposed in this study is suitable for other SH-type SAWs also.
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