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 absorption. 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.