Broadband and flexible generation of divergence-suppressed acoustic vortex beam through subwavelength dipole metamaterials

The acoustic vortex beam carrying orbital angular momentum (OAM) provides a new degree of freedom for sound manipulation, with promising applications in diverse fields ranging from acoustic communication to particle manipulation. However, their propagation in free space is subject to the severe divergence effect, resulting in limited operating distance and reduced signal intensity. Here, we propose a mechanism for generating a divergence-suppressed acoustic vortex beam through subwavelength dipole metamaterials in a broadband, flexible, and compact way. We theoretically reveal that the enhanced directivity of the man-made dipole can effectively reduce the high-spatial-frequency wave-vector component of the generated vortex beam in the radial direction and thereby suppress the beam's divergence. As a practical implementation, a blocklike acoustic metamaterial is systematically designed for converting the monopole emission into a dipole in a broad bandwidth, whose acoustic properties are also analytically deduced. The effectiveness of a dipole metamaterial-empowered phased array is demonstrated both numerically and experimentally, showcasing that the divergence angle and the main lobe width of a particular first-order vortex beam are reduced by 45% and 23% on average, respectively, within the range of 1000--1600 Hz. Thanks to the compatibility with the phased array, both the topological charge and the deflection angle of the emitted vortex beam can be flexibly modulated without altering metamaterials. We anticipate that our proposed mechanism will promote the application of acoustic vortex beams in various fields, such as long-distance OAM communication and beyond.