Inverse-designed single-phase elastic metasurfaces for underwater acoustic vortex beams

During the past decade, the passive metasurfaces enabling vortex beams carrying orbital angular momentum have drawn great interest in the fields of electromagnetic and acoustic wave. In the underwater environment, however, the elastic metasurface with a certain bandwidth is more difficult to be realized due to the strong fluid-solid interaction and rigorous impedance matching. To overcome this challenge, an inverse-design strategy is proposed to systematically construct a single-phase underwater metasurface for converting the incident plane waves into vortex beams on demand. All topology-optimized metasurface elements, possessing both the positive/negative effective dynamic properties for the full phase control, can support the desired phase modulation and high transmission. We numerically and experimentally demonstrate the acoustic functionality of converting the plane waves in free space to the first-order vortex modes. The present passive single-phase metasurface by inverse design may offer opportunities for underwater acoustic communication.