Piezoelectric-Metal Phononic Crystal Enabling GHz Tunable Ultrahigh $Q$ Quasi-BIC mode

The integration of GHz-frequency, high quality factor ($Q$), and electrically tunable acoustic resonators holds significant potential for advancing applications in quantum information technologies, microwave photonics, and reconfigurable RF systems. However, simultaneously achieving these three characteristics within a single, scalable platform remains a fundamental challenge. Here, we report the experimental demonstration of a GHz quasi-BIC resonator in a piezoelectric thin-film shear horizontal (SH) wave system, achieved through a structurally simple piezoelectric-metal phononic crystal (PnC) architecture on a LiNbO$_3$ thin film. This approach enables leaky Fabry-Perot coupling mode and localized trapping quasi-BIC mode. Without the need for deep etching or intricate patterning, we achieve a room-temperature quality factor of $6\times 10^4$ at ~1 GHz in ambient air, corresponding to an $f\times Q$ product of $6\times 10^{13}$ Hz at quasi-BIC mode. Furthermore, we demonstrate efficient electrical tunability via low-voltage (0.6 V) electrothermal modulation of the PnC structure, enabling a reversible transition between trapped and transmission states and yielding a high-contrast amplitude modulation of 47.75 dB. Our results establish a lithography-friendly, fabrication-tolerant platform for realizing tunable, high-$Q$ acoustic resonators at GHz frequencies, overcoming longstanding barriers in phononic device engineering. This work opens new directions for scalable on-chip phononic circuits in quantum acoustics, reconfigurable RF systems, and signal processing applications.