Exploring high-Q ultrasound resonators in liquid-based metasurfaces through bound states in the continuum

The trapping of waves in open systems is a fundamental concept extensively explored across fields like optics, acoustics, and quantum physics. Recently, there has been increasing interest in bound states in the continuum (BIC), a unique type of wave mode that remains confined without dissipating energy, even in environments where waves would typically spread out. BICs have shown great potential for applications in areas such as sensors, filters, and lasers. In this work, we explore both the theory and experimental realization of quasi-BIC (QBIC) for ultrasonic waves. This is achieved using a metasurface resonator based on liquid-elastic Fabry-Pérot structures, which efficiently trap ultrasonic waves. Our findings reveal that these QBICs exhibit remarkable stability, maintaining their unique properties even when key parameters are varied, highlighting their robustness. We also provide experimental evidence of an exceptionally high quality factor (Q-factor) of 350 at approximately 1 MHz, significantly outperforming previous fully acoustic underwater systems. This work deepens our understanding of BIC in the realm of acoustics and paves the way for developing highly efficient, ultra-high Q-factor ultrasound devices.