In this work, we show that negative refraction with ultrahigh transmission can be obtained via gradient space-coiling metamaterials. By using a retrieval method for our design, we find that both the effective density and bulk modulus of metamaterials are simultaneously negative in the sense of the effective medium, accompanied by nearly perfect impedance matching. The negative refraction effects with the impedance matched prism and plate are numerically demonstrated, and the transmission coefficients are higher than 90% with the scattered-wave energy less than 4%. The numerical analyses of the subwavelength imaging effect of the superlens based on the proposed metamaterials are also presented, and the super-resolution beyond the diffraction limit and ultrahigh efficiency are evidenced.

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).

In this paper, we show that the enhanced directional acoustic emission can be obtained via a class of anisotropic metamaterial based on a cavity-channel network and producing a near-zero group index of refraction ng. This material consists of two types of air cavities which are alternately arranged in layers, and the dispersion relation is studied theoretically and numerically. Additionally, a retrieval approach for the anisotropic effective constitutive parameters is presented for this material, and it is shown that one component of the density tensor in the vertical direction of the expected propagation axis is close to zero. Numerical simulations are used to verify the effectiveness of our proposal and the results show that a highly directional radiation field with a half-power angular width of less than 10° can be obtained

We demonstrate an extraordinary acoustic transmission through a class of hornlike space-coiling metamaterials, in which the gradual change in channel width ensures such a transmission property in a wide frequency band. This phenomenon is explained in terms of the constitutive parameters based on an effective medium approach, and the underlying mechanism stems from the improved impedance matching and effective near-zero density. Numerical simulations based on finite element analysis are used to verify the effectiveness of our designs, and the results show that this class of metamaterials provides an efficient design methodology for broadband extraordinary acoustic transmission.

The acoustic transmission enhancement in metamaterials may have many potential uses in various areas; however, the narrow bandwidth limits their device applications. Here, we show that a two-dimensional space-coiling acoustic metamaterial having a gradual-type channel with a continuous cross section can be designed to exhibit an anomalous transmission enhancement over a wide frequency range. An effective medium approach finds that this phenomenon stems from the combined action of an effective nearly-1 impedance and an effective nearly-0 refractive index within three proposed schemes, in which the continuously tapered channel design can obtain the best broadband acoustic transparency, and numerical simulations demonstrate that a nearly unitary transmission of sound amplitude (above 97%) can be achieved in the frequency range from 3760 to 4830 Hz through this metamaterial.