Coding metalens with helical-structured units for acoustic focusing and splitting

In this work, a helical structure with three spiral channels is employed to build the coding units of metalenses, which can provide high transmission efficiency with an arbitrary phase shift compared to air. The helical unit with the phase shift of pi is used as logical unit 1, and the hollow tube filled with air is regarded as logical unit 0. By arranging these logical units in specific sequences, acoustic metalenses can achieve wave-field manipulation like acoustic focusing and splitting. The focusing efficiency as high as 41.5% is achieved. Meanwhile, the genetic algorithm is applied to seek the optimal arrangement of the bipartite units for 3D sound focusing. Simulations and experiments are conducted to demonstrate the proposed coding metalenses for molding the acoustic wave field in the desired manners.