Making sound vortices by metasurfaces
  Based on the Huygens-Fresnel principle, a metasurface structure is designed to generate a sound vortex beam in airborne environment. The metasurface is constructed by a thin planar plate perforated with a circular array of deep subwavelength resonators with desired phase and amplitude responses. The metasurface approach in making sound vortices is validated well by full-wave simulations and experimental measurements. Potential applications of such artificial spiral beams can be anticipated, as exemplified experimentally by the torque effect exerting on an absorbing disk. 
Liping Ye, Chunyin Qiu, Jiuyang Lu, Kun Tang, Han Jia, Manzhu Ke,   Shasha Peng, and Zhengyou Liu
physics.class-ph