Acoustic helical dichroism enhanced by chiral quasibound states in the continuum

Acoustic helical dichroism (HD) arises from the interaction between vortex beams carrying orbital angular momentum (OAM) and chiral media, yet such chiral sound-matter interactions are typically weak. Here, we employ quasibound states in the continuum (QBICs) in acoustic metacavities composed of coupled Helmholtz resonators to enhance acoustic HD. We design both achiral and chiral metacavities that support QBICs in the form of vortex states with high 𝑄 factors. Using full-wave numerical simulations, we show that the QBICs in the achiral metacavities cannot enhance acoustic HD due to the absence of a chiral wave front. In contrast, the chiral metacavity exhibits a pronounced HD enhancement through the QBICs with a three-dimensional (3D) helical wave front, which can be excited by incident waves either with or without OAM. Our work identifies two essential requirements for enhancing acoustic HD effect via QBICs: a high 𝑄 factor of the states and 3D chirality of the state fields, which usually compromise each other in conventional acoustic resonators. The findings open avenues for achieving strong chiral sound-matter interactions, with potential applications in acoustic chiral sensing and acoustic OAM manipulation. © 2026 American Physical Society.