The invisibility cloak, a long-standing fantastic dream for humans, has become more tangible with the development of metamaterials. Recently, metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell. However, the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry. In this work, by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials, we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry. The principle is general and applicable to arbitrary shapes. For experimental demonstration, we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions. The cloaking effect is verified by both full-wave simulations and microwave experimental results. The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry, which can eliminate the need for spatially varying extreme parameters. Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.

Rough surfaces lead to diffused light in both reflection and transmission, thereby blurring the reflected and transmitted images. Here, we merge the traditionally incompatible diffuse reflection and undistorted transmission by introducing the concept of random-flip metasurfaces made of randomly flipped components. These metasurfaces have a globally random phase in reflection that leads to diffuse reflection, while the local space inversion and reciprocity principle ensure distortion-free transmission. Notably, the metasurface reflects like a rough surface yet transmits like a smooth one in a broad spectrum. On the basis of complementary random arrays of gold nanorods, we verified this functionality by both optical spectroscopy and imaging experiments over a broad range of frequencies from the visible to the infrared regime. This feature, which originates from breaking the phase correlation between reflection and transmission by the metasurface, could enable a range of new optical materials and display technology.

Metasurfaces with spatially varying reflection phases have promised great possibilities in realizing diffusion-like backward scattering. However, most studies in the field of metasurface reflectors focus on the diffusion generated by a phase gradient from unit cells in periodic grids. In this paper, we propose a general idea of integrating the concept of a hyperuniform disordered structure to realize a metasurface, where the effect of the spatial distribution is taken into account to realize more diffusion. A V-shaped structure is utilized as the unit cell for the metasurface to realize two kinds of elements with a phase difference of 180°. The metasurface is generated by distributing the unit cells according to a hyperuniform disordered distribution, and we compared its performance with a metasurface comprised of the same unit cells, distributed on a fixed periodic lattice. Both simulation and experimental results demonstrate that the metasurface with the hyperuniform disorder performs better than the periodic metasurface by enlarging the operating bandwidth with a significant reduction in the radar cross section of the surface. When the frequency operating frequency increases, only one reflected beam is observed for the metasurfaces based on hyperuniform disordered distribution, which is in contrast to the periodic metasurface where several multiple reflected beams are observed.

The Brewster’s law predicts zero reflection of p-polarization on a dielectric surface at a particular angle. However, when loss is introduced into the permittivity of the dielectric, the Brewster condition breaks down and reflection unavoidably appears. In this work, we found an exception to this long-standing dilemma by creating a class of nonmagnetic anisotropic metamaterials, where anomalous Brewster effects with independently tunable absorption and refraction emerge. This loss-independent Brewster effect is bestowed by the extra degrees of freedoms introduced by anisotropy and strictly protected by the reciprocity principle. The bandwidth can cover an extremely wide spectrum from dc to optical frequencies. Two examples of reflectionless Brewster absorbers with different Brewster angles are both demonstrated to achieve large absorbance in a wide spectrum via microwave experiments. Our work extends the scope of Brewster effect to the horizon of nonmagnetic absorptive materials, which promises an unprecedented wide bandwidth for reflectionless absorption with high efficiency.