Acoustic vortex in a waveguide with a chiral gradient sawtooth metasurface

Acoustic vortex states with spiral phase dislocation that can carry orbital angular momentum (OAM) have aroused much research interest in recent years. The mainstream methods of generating acoustic vortexes are based on the Huygens-Fresnel principle to modulate the wave front to create spatial spiral phase dislocation. In this work, we realize acoustic vortexes in a waveguide with a chiral gradient sawtooth metasurface. The physical mechanism of our method is to lift the degenerate dipole eigenmodes through the scattering effect of the chiral surface structure, and then their superposition yields both +1 and \ensuremath{-}1 order vortexes. Compared with the existing methods of acoustic vortex generation, our design has many merits. For instance, it is easy to manufacture and control, the working frequency is relatively broadband, and the sign of vortex order can be readily flipped. Both the full-wave simulations and experimental measurements validate the existence of the acoustic vortexes. The torque effect of the acoustic vortexes is also successfully performed by rotating a foam disk as a practical application. Our work opens a new route for generating acoustic vortex and could have potential significance in areas including microfluidics, acoustic tweezers, and ultrasonic communication.