Ultrathin acoustic metasurfaces for reflective wave focusing

An advanced concept of reflective acoustic focusing based on an ultrathin metasurface is numerically and analytically investigated. We propose a designed reflective metasurface with a thickness of λ/15, with λ being wavelength, composed of locally resonant Helmholtz-like elements which discretely realize the 2π phase shift. The theoretical design based on the generalized Snell's law is numerically achieved by the proposed ultrathin metasurface. Numerical simulations and theoretical analysis have converged to a good consensus and validated the ultrathin reflective metasurface concept for acoustic focusing. Furthermore, another reflective metasurface with sub-wavelength thickness (λ/8) and based on the coiling-up-space concept constructed by three-dimensional (3D) labyrinthine elements is investigated and compared to the ultrathin one. Despite both metasurfaces illustrating equivalent good performances for acoustic focusing, the ultrathin one demonstrates its superiority with thinner thickness, simpler design, and easier fabrication, which would greatly facilitate its real implementation in relevant applications.