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Sandwich-like space-coiling metasurfaces for weak-dispersion high-efficiency transmission

Space-coiling metamaterials have attracted significant attention due to their large number of potential applications. However, their practical application faces various challenges because of the unavoidable wavefront distortions resulting from the strong dispersion or the low transmission efficiency resulting from the impedance mismatching. Here, we show that it is feasible to circumvent the problem via a sandwich-like unit, which contains one functional layer and four auxiliary layers and both of them can be constructed from the space-coiling structure. Therein, the functional layer is used to adjust the refractive index of the structure while the auxiliary layers ensure a gradual change of the impedance. We further utilize these units to construct acoustic metasurfaces for beam redirection and focusing. The numerical results demonstrated that a highly efficient and stable wavefront manipulation can be achieved over multiple frequency bands (2.5–4.0 kHz and 8.0–10.0 kHz).Space-coiling metamaterials have attracted significant attention due to their large number of potential applications. However, their practical application faces various challenges because of the unavoidable wavefront distortions resulting from the strong dispersion or the low transmission efficiency resulting from the impedance mismatching. Here, we show that it is feasible to circumvent the problem via a sandwich-like unit, which contains one functional layer and four auxiliary layers and both of them can be constructed from the space-coiling structure. Therein, the functional layer is used to adjust the refractive index of the structure while the auxiliary layers ensure a gradual change of the impedance. We further utilize these units to construct acoustic metasurfaces for beam redirection and focusing. The numerical results demonstrated that a highly efficient and stable wavefront manipulation can be achieved over multiple frequency bands (2.5–4.0 kHz and 8.0–10.0 kHz).