Bimorph Lithium Niobate Thickness-Shear Overtone Film Bulk Acoustic Resonator

High quality factor ($Q$) and overtone operation enable narrow-linewidth acoustic devices with multiple discrete frequencies in a single cavity. Maintaining both high $Q$ and sufficient electromechanical coupling at higher mode orders remains challenging. Here, we demonstrate a bimorph periodically poled piezoelectric film (P3F) lithium niobate (LN) platform for high-order thickness-shear (TS) overtone excitation. The device comprises a bonded 80-$\mu$m-thick single-crystal X-cut LN bimorph with opposite polarizations, patterned top and floating bottom electrodes, and a suspended air cavity. The P3F configuration mitigates charge cancellation from the alternating stress distribution of higher-order TS modes, enabling measurable coupling across a broad sequence of overtones. The thick LN acoustic cavity and increasingly confined high-order mode profiles support low-loss operation. Measured TS overtones extend to 1.75 GHz. At room temperature, representative overtones at 0.77 and 0.89 GHz exhibit 3-dB $Q$ values of 11,338 and 11,917, corresponding to $fQ$ products of $8.74\times {10}^{12}$ and $1.06\times {10}^{13}$ Hz, respectively. Cooling from 297 to 12 K systematically enhances $Q$, yielding a peak 3-dB $Q$ of 20,507 at 779 MHz and a maximum $fQ$ product of $1.98\times {10}^{13}$ Hz at 1.379 GHz. These results establish bimorph P3F LN as a promising platform for high-$Q$, frequency-scalable micro-acoustic resonators in the sub-GHz and low-GHz regimes.