Topological bound state in the continuum induced unidirectional acoustic perfect absorption

The interaction between cavity field and atoms plays an important role in exploring the abundant non-Hermitian physics and constructing powerful wave function devices. In this work, we propose theoretically and realize experimentally unidirectional perfect absorption in a non-Hermitian acoustic system with the help of the topological bound state in the continuum (BIC), which is established by the hybrid interaction between one trivial BIC and another conventional resonant state. In the 2D parameter space spanned by frequency and distance between the two resonators, the topological scattering singularities appear in pairs and are associated with topological distinguished charges. Meanwhile, we reveal the origin of topological charges and their continuous evolution with the loss factor. At a specific loss factor, two topological charges just annihilate together, and acoustic perfect absorption induced by topological BIC is realized at the left incidence, while there is no phase singularity and near-total reflection is observed at the right incidence, hence the system presents extreme asymmetry. Our work bridges the gap between scattering characteristics of non-Hermitian acoustic systems and topological scattering singularities, which may contribute to the research of novel non-Hermitian physics and the practical applications of advanced absorbers and sensors.