Metasurface-enabled airborne fractional acoustic vortex emitter

An annulus acoustic metasurface (AAM) composed of composite labyrinthine structure (CLS) subunits has been well designed to generate fractional acoustic vortices (FAVs) in air. The FAVs with different topological charges (TCs) are realized by modulating the transmitted phase shifts through the CLS subunits. The evolution of the pressure field and phase distributions of the FAV is investigated numerically using the finite element method and demonstrated theoretically. As TC increases from 1 to 2, the central phase singularity first splits into two phase singularities and then gradually merges into a higher-order phase singularity. Meanwhile, the corresponding pressure field distribution first evolves from the annular intensity pattern to two discontinuous parts and then gradually recovers to the annular ring distribution with larger radius. We further find that the FAV generated by the AAM could extend to a relatively long distance. Finally, experiments are performed to verify the FAV by the AAM and demonstrate its long-distance propagation. The airborne FAVs by the AAMs may find potential applications in micro-particle manipulation, acoustic communication, and edge-detection imaging. Published by AIP Publishing.