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, 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 LiNbO₃ thin film, is reported. This approach enables leaky Fabry-Perot coupling mode and localized trapping quasi-BIC mode. Without the need for deep etching or intricate patterning, a high room-temperature quality factor of ≈6.5 × 10⁴ at ≈1 GHz in ambient air is achieved, corresponding to an f × Q product of ≈6.4 × 10¹³ Hz at quasi-BIC mode. Furthermore, efficient electrical tunability is demonstrated 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. This work opens new directions for scalable on-chip phononic circuits in quantum acoustics, reconfigurable RF systems, and signal processing applications.