We experimentally demonstrated an alternative approach of invisibility cloaking that can combine technical advantages of all current major cloaking strategies in a unified manner and thus can solve bottlenecks of individual strategies. A broadband cylindrical invisibility cloak in free space is designed based on scattering cancellation (the approach of previous plasmonic cloaking), and implemented with anisotropic metamaterials (a fundamental property of singular-transformation cloaks). Particularly, nonsuperluminal propagation of electromagnetic waves, a superior advantage of non-Euclidian-transformation cloaks constructed with complex branch cuts, is inherited in this design, and thus is the reason of its relatively broad bandwidth. This demonstration provides the possibility for future practical implementation of cloaking devices at large scales in free space.
Previous subwavelength imaging using hyperlens is based on negative constitutive parameters that are realized by strongly dispersive materials and work only in a narrow frequency band. Here, we demonstrated that subwavelength imaging can be achieved in a broad frequency band using non-resonant magnetic metamaterials. The metamaterial shows an elliptical dispersion relation and can be fabricated by metallic closed-rings with a broadband magnetic response. With this elliptically dispersive material, most of the evanescent waves with high-k modes can be converted to propagating modes and the subwavelength information is reconstructed. Both simulation and experiment results show that this kind of metalens can achieve a broadband subwavelength imaging effect.
We propose the concept of a meta-substrate to broaden the bandwidth of left-handed metamaterials. The meta-substrate, which behaves like an inhomogeneous magnetic substrate, is composed of another kind of magnetic metamaterials like metallic closed rings. When conventional metamaterial rings are printed on this kind of meta-substrate in a proper way, the interaction of the metamaterials units can be greatly enhanced, yielding an increased bandwidth of negative permeability. An equivalent circuit analytical model is used to quantitatively characterize this phenomenon. Both numerical and experimental demonstrations are carried out, showing good agreement with theoretical predictions.
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