It has been shown that acoustic waves with helical wavefronts can carry angular momentum, which can\nbe transmitted towards a propagating medium. Such a wave field can be achieved by using a planar array\nof electroacoustic transducers, forming a given spatial distribution of phased sound sources which produce\nthe desired helical wavefronts. Here, we introduce a technique to generate acoustic vortices, based on the\npassive acoustic metasurface concept. The proposed metasurface is composed of space-coiled cylindrical unit\ncells transmitting sound pressure with a controllable phase shift, which are arranged in a discretized circular\nconfiguration, and thus passively transforming an incident plane wavefront into the desired helical wavefront.\nThis method presents the advantage of overcoming the restrictions on using many acoustic sources, and it is\nimplemented with a transmitting metasurface which can be easily three-dimensionally printed. The proposed\nstraightforward design principle can be adopted for easy production of acoustic angular momentum with minimum\ncomplexity and using a single source.
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