Direct visualization of a Friedrich-Wintgen quasi-bound state in the continuum

Abstract Bound states in the continuum (BICs) are a well known phenomenon in physics, particularly in quantum physics and wave physics. Since BICs are local states, their existence typically manifests as Fano resonances in the transmission or reflection spectrum. However, to date no direct or visual proof of an acoustic BIC has been reported. We demonstrate the existence of a Friedrich-Wintgen BIC in an open acoustic cavity by theory and experiments. The appearance of the BIC is characterized by the vanished line width of the Fano resonance in the measured transmission spectrum. Our theoretical predictions accurately match the experimental observations. We map the pressure field of the transparent open cavity hosting a Friedrich-Wintgen BIC using laser Doppler vibrometry, which is the first reported visualization of an acoustic BIC. Mapping the pressure field of the BIC is a new technique to extract real sound pressure values of high-Q modes. We can draw conclusions from the pressure values on the confinement of the BIC. Our results facilitate the future development of applications of BICs in the field of acoustics, as acoustic sources, filters, and sensors.