Steering of Acoustic Reflection from Metasurfaces through Numerical Optimization

Space-coiling metamaterials that exploit the so-called generalized Snell's law are suitable for the subwavelength manipulation of acoustic waves, leading to extraordinary refracting and reflecting metasurfaces. Typically, space-coiling metasurfaces are characterized by a narrow operating range in frequency due to the intrinsic connection between the design wavelength and the characteristic dimensions of the metasurface. The main goal of the present work is to design a broadband and modular space-coiling based metasurface, extending the range of frequency where the effective metabehaviour is achieved. The metasurface building set is composed by eight different unit cells, each introducing a tailored phase shift in the field, that can be combined side by side to produce the desired acoustic effect. The design parameters of each cell are selected as the result of an optimization process that minimizes the dependency on the operating frequency of the metabehaviour, keeping the overall thickness below a quarter of the design wavelength. Significant results are obtained for benchmark problems.