Generation of diverse acoustic vortices by superimposed multipole emissions

Abstract

Acoustic vortices (AVs) based on artificial metamaterials have attracted intense research efforts for their potential use in subdiffraction imaging, particle manipulation, and acoustic communication. However, to date, major strategies to generate AVs have an intrinsic limitation of fundamentally depending on complex phase modulations, which require a higher number of multiple elements as the desired topological charge increases, let alone the elaborate dephasing between the individual microstructured elements in the switching process of topological charges. In this paper, we propose an annulus anisotropic structure coupled with a pair of eccentric asynchronous monopole sources that can realize multiple-order AV modes at different frequencies. The generated AVs originate from the interactions of two superimposed multipoles characterized with orthogonal emission patterns and $\ensuremath{\pi}/2$-phase differences. The AVs with topological charges from 1 to 5 are achieved simultaneously at dipole, quadrupole, hexapole, octupole, and decapole resonances, respectively. Numerical simulations and experimental results clearly confirm the feasibility and practical applicability of the proposed design strategy in realizing topologically diverse AVs. Our design, exempted from complex phase modulations, possesses flexible topological charge modes at the expense of frequency, which may foresee potential capabilities in multiplexing and demultiplexing of AVs.