Broadband acoustic vortex beam generator based on coupled resonances

A high-performance acoustic vortex beam generator (VBG) based on artificial micro-structured metamaterials is of great significance in acoustic communication. However, to date, the research on metamaterial VBGs mainly focused on their single frequency properties in the narrow band. Here, we propose a design strategy of broadband VBGs constructed by gradient coupled-resonant meta-atoms, all of which show near-unity transmission amplitudes, while covering 2 π phase shifts linearly varied along with frequency throughout the desired overlapping frequency range. Moreover, the phase differences between adjacent meta-atoms are constant at regular intervals within this entire frequency range, allowing the unique wideband response of the proposed VBG. We demonstrate, both in simulations and in experiments, the efficient mode conversion from plane sound wavefronts into vortex beams with a topological charge of 1. Our study provides a platform to manipulate broadband wavefront conversion based on acoustic coupled-resonant metamaterial, which allows us to envision promising acoustic devices with versatile applications.