Overflow elastic metasurfaces for underwater high-pressure-resistant acoustic wavefront control

Elastic metasurfaces have shown strong ability of controlling the phase, amplitude and polarization of elastic waves on demand. However, limited by the porous-solid, bi-phase-solid and multi-phase-solid microstructural models, the existing elastic metasurfaces have to comprise some thin connecting rods or materials with low elastic modulus so that the transmutative modes can be induced to capture the required phase and amplitude manipulation. More strikingly, these microstructure features may result in the very low strength of reported elastic metasurfaces, let alone the ability to withstand high hydrostatic-pressure. To simultaneously maintain the abundant elastic wave modes of solids and stable complex wavefront control under high pressures, we propose a kind of overflow elastic microstructural model which comprises the internal acoustic-structure coupling and develop an inverse-design methodology of an overflow elastic metasurface for the pressure-insusceptibility underwater acoustic vortex. Firstly, a model regarding topology optimization of microstructures with internal acoustic-structure coupling is built to achieve the prescribed transmissive phases and amplitudes. Specifically, four kinds of topology-optimized overflow microstructures can simultaneously possess the stable phase difference covering 0-2π and high transmission above 0.9 within the frequency range of 15-15.9 kHz. Benefiting from the strong internal acoustic-structure coupling, the microstructures are discovered to support the monopole/dipole resonances in fluid domains and acoustic-induced local vibrations in solid parts, which leads to the customized nonlinear dispersion properties. Finally, the assembled overflow elastic metasurface is numerically and experimentally demonstrated to enable a focused acoustic vortex beam with the high orbital angular momentum (OAM) purity (close to 100%) and high energy-converting efficiency (over 80%) while keeping the high hydrostatic-pressure-resistant capacity. The proposed overflow microstructural model and inverse-design methodology of the overflow elastic metasurface are promising in constructing extreme elastic wave modes, and even developing new-generation flexible acoustic vortex beams for the large-capacity communication in the deep-water environment.