Phase-engineered 3D-printed metalenses for spatial and spectral control of underwater acoustic vortex beams

Acoustic vortex beams, which inherently carry orbital angular momentum, are emerging as powerful tools for applications ranging from particle manipulation to biomedical imaging and underwater communication. However, generating such beams with precise spatial control remains challenging and often requires complex transducer arrays. In this study, we introduce a unified design framework for tailoring underwater acoustic vortex beams using 3D-printed metalenses. By engineering the local phase profile of the metalens, this approach enables distinct functionalities, including the generation of tilted vortex beams, precise control of off-axis singularities, and the creation of dual-focus vortex beams via coaxial spiral zones. Furthermore, a frequency multiplexing strategy is implemented in the dual-focus configuration, enabling independent control of vortex foci at distinct frequencies using a single compact lens. The ability of these metalenses to shape acoustic pressure fields with high precision is validated through both numerical simulations and underwater measurements. This work demonstrates a versatile, fabrication-ready platform for the tunable spatial and spectral control of acoustic vortex beams, offering new possibilities for applications in acoustofluidics, therapeutic ultrasound, and underwater sensing and communication.