Improved pure SH SAW transduction efficiency on LGS using finite thickness gratings

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Pure shear horizontal surface acoustic wave (SH SAW) orientations exist on trigonal class 32 crystals along (0°, θ, 90°). In particular, this mode has been recently considered on single crystal langasite (LGS) for liquid and biological sensor applications due to reduced propagation loss in the presence of liquid-loaded surface. A limitation of the pure SH SAW orientations for device applications has been the significant excitation of the shear horizontal bulk acoustic wave (SH BAW) by the interdigital transducer (IDT), which compromises the SH SAW transduction efficiency and thus increases SH SAW device loss. Previous work by the authors verified that an infinitesimally thin metallic guiding layer used in the propagation path around the IDT typically increases the SH SAW transduction efficiency ηSHSAW , defined as the ratio of SH SAW power to total IDT input power, from less than 1% (no layer) to 60% (infinitesimally thin metallic layer) on LGS, Euler angles (0°, 22°, 90°) for a 240 split finger electrode IDT. In this work, finite thickness periodic grating guiding structures are explored along LGS (0°, 22°, 90°) to further improve the IDT ηSHSAW. The structure studied consists of a finite number of IDT electrodes bordered by gratings on both sides. The model implemented uses orthogonal Chebyshev polynomial basis functions in conjunction with the finite element method and harmonic admittance (HA) technique to study the SH SAW mode excitation efficiency in the referred structure. The IDT ηSHSAW is examined as a function of: electrode material and normalized thickness (h/λ) where h is electrode thickness and λ is the wavelength; metallization ratio (a/p) where a and p are electrode width and center-to-center spacing, respectively; and number of split IDT finger electrodes. The analysis performed shows a significant difference in ηSHSAW if high density electrode gratings, such as gold (Au) or platinum (Pt) are used instead of low-density gratings, such as aluminum (Al). For instance, using 282 split finger electrode IDT and surrounding gratings composed of Au or Pt electrodes, with h/λ = 1%, results in ηSHSAW = 99%. If Al is used, a reduced ηSHSAW = 54% is achieved for the same h/λ = 1%. Moreover, to achieve ηSHSAW = 99% with Al electrodes, the required value of h/λ is about 8.5%, a nearly ten-fold increase in metallization thickness over Au or Pt electrodes. The effects of metallization ratio a/p on ηSHSAW have also been studied and indicate that higher metallization ratios lead to higher ηSHSAW. Numerical and experimental IDT admittance is compared, validating the analysis performed. The work reported shows that finite thickness gratings can significantly improve the SH SAW IDT performance, reducing the amount of SHBAW excited by the IDT, and thus leading to high efficiency LGS SH SAW devices for biosensor applications.