Piezoelectric‐Metal Phononic Crystal Enabling GHz Tunable Ultrahigh Q Quasi‐BIC Mode

Abstract 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 LiNbO ₃ 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 × 10 ⁴ at ≈1 GHz in ambient air is achieved, corresponding to an f × Q product of ≈6.4 × 10 ¹³ 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.