Ultrasonic braided ring beams generated by phase modulation metasurfaces

Coaxial coupling between two quasi-perfect ultrasonic vortices (QPUVs) with distinct ring radii and topological charges (TCs) has been suggested and implemented to create ultrasonic braided ring beams (UBRBs). It is possible to switch between a double-ring pattern, a braided ring pattern, and a petal-like pattern in the linked ultrasonic field distribution by varying the coupling strength of two QPUVs. We focus on the braided ring pattern and investigate the influence of TCs on the acoustic intensity and phase distributions. It is found that the UBRB contains multiple individual phase singularities, and the number of singularities is determined by the TCs of the two QPUVs. Furthermore, a phase modulation metasurface (PMM) composed of photosensitive resin cubes is well designed to produce the UBRB in water. The effectiveness of the single-layer PMM in producing the UBRB is confirmed by both simulations and experimental results. The proposed UBRBs based on metasurfaces could potentially have uses in multi-particle manipulation and acoustic communication.