Extreme Sound Confinement from Quasibound States in the Continuum

Extreme confinement of incident acoustic waves remains challenging due to the conflict between reflection elimination and weak dissipation. In this study, by realizing a Friedrich-Wintgen quasibound state in the continuum (quasi-BIC), we demonstrate that sound confinement with an arbitrarily high quality factor becomes possible. The proposed proof-of-concept system consists of two slightly detuned resonators sharing a single-port radiating channel and supports a quasi-BIC. When operating with balanced low radiative and dissipative decay rates, it allows frequency-selective trapping of the incoming sound waves. The effect is experimentally and numerically validated as evidenced by the observation of an ultranarrow reflection dip (zero reflection at 420.8 Hz) along with intensive field enhancement (24.5 dB). We also show that the quality factor can be further improved by simultaneously reducing the detuning and the intrinsic loss. Our work breaks through the barrier in obtaining extreme sound confinement and may offer opportunities for the development of acoustic sensors, filters, and harvesters.