Broadband asymmetric acoustic vortex generator based on integrative meta-atoms

Abstract

The significance of asymmetric acoustic vortices primarily derives from their distinctive influence on acoustic vortex applications, such as acoustic communication receiving and sending isolation. However, the existing asymmetric acoustic vortices suffer from strong narrow-band confinement. In this paper, we eliminate this constraint by proposing a novel asymmetric scheme based on integrative meta-atoms (IMs). The IM is assembled by a phase gradient metasurface (PGM) and a mode selection structure (MSS), which serve the mode conversion and selection functions respectively. The PGM is constructed by a coiling-up space, converting the incident plane wave mode into a first-order vortex mode. We introduce three mirror scatterers in a cylindrical waveguide to form the MSS, allowing the first-order vortex mode to pass but blocking the plane wave mode in the mode band gap. The mode selection mechanism of MSS is revealed by employing the dispersion relation of the unit cell and the mode transmission performance of the MSS. By coupling the acoustic modulation effects of PGM and MSS, the asymmetric generation for the first-order vortex mode is realized, and further demonstrated numerically and experimentally. Significantly, the designed IMs exhibit commendable performance in both broadband and mode purity. In detail, a relative bandwidth of 25.3 % and a purity metric of 0.88 can be achieved. Our study may offer an alternative approach to producing asymmetric acoustic vortex devices, paving the way for a myriad of advanced applications.