Observation of acoustic Friedrich-Wintgen bound state in the continuum with bridging near-field coupling

Bound states in the continuum (BICs) have attracted increasing interest in recent years owing to their intriguing physical characteristics such as the infinitely high quality factor (Q factor) and the enhanced wave-matter interaction. In this study, we report the theoretical and experimental observation of a Friedrich-Wintgen BIC with bridging near-field coupling in an asymmetric two-state acoustic system. Through the temporal coupled-mode theory, we comprehensively elucidate the role of bridging near-field coupling in constructing Friedrich-Wintgen BICs. Then, we present a two-state system consisting of two asymmetric cavities with a bridging tube. By tuning the diameter and position of the bridging tube, we can effectively modulate the near-field coupling effect of the presented system and achieve a Friedrich-Wintgen BIC. Furthermore, the presented system is modulated to deviate from the BIC, which provides a quasi-BIC and allows for a high -Q perfect absorption when the radiation and dissipation of the quasi-BIC-supporting system achieve the critical coupling condition. The experimental results validate the theoretical and simulation results, demonstrating both the existence of a BIC and a quasi-BIC-based perfect absorp-tion. Our work opens up an avenue to investigate acoustic Friedrich-Wintgen BICs with bridging near-field coupling in asymmetric systems, which enriches the field of acoustic BICs and offers opportunities for the development of acoustic devices with high Q factor and asymmetric wave control.