Complementary-polarity double-layer LiTaO3 resonators for symmetry-selective SH2 excitation with ultrahigh electromechanical coupling (kt^2 = 25.7%)
We report a structurally simple double-layer lithium tantalate (LiTaO3) bulk acoustic resonator that enables symmetry-selective excitation of the second-order thickness-shear (SH2) mode with ultrahigh electromechanical coupling. Two 31 deg Y-oriented single-crystal LiTaO3 films are rotation-bonded with complementary polarization (+X/-X) and driven by a longitudinal electric field. Matching between the effective piezoelectric symmetry and the SH2 mode yields an effective electromechanical coupling coefficient of kt^2 = 25.7% at 5.24 MHz. To our knowledge, this is the highest kt^2 reported for a LiTaO3 resonator architecture to date. The measured response is dominated by the target SH2 mode, with only weak parasitic features in the operating band. The structure is also tunable: the resonance frequency and coupling can be adjusted through geometric parameters while maintaining stable modal behavior, indicating good process tolerance. Finite-element analysis further suggests straightforward frequency scaling beyond 5 GHz by reducing the film and electrode thickness while preserving approximately 25% kt^2. In addition, introducing a SiO2 compensation layer is predicted to improve the temperature coefficient of frequency to approximately -25 ppm/deg C. These results establish complementary-polarity double-layer LiTaO3 as a practical platform for high-coupling, spurious-suppressed acoustic resonators and provide a scalable route toward wideband ultrasonic resonators, filters, and related transducers.