  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.
