Dual-band Air-like Transparent Slab by Full Polarization Compensation

Three-dimensional (3D) air-like metamaterials (ALMs) appear omnidirectionally invisible in free space, which enables the possibility of material existence without involving any scattering to arbitrary incident electromagnetic (EM) waves. Due to their peculiar property, ALMs are quite interesting in microwave and optical engineering. However, the existing ALMs used to work with some predefined conditions, e.g., either incident polarization or operation bandwidth is limited, which prevents their implementation in the wide practical applications. In this paper, we present the design and measurement of a slab-type ALM, which is polarization-free and works in a couple of radar bands. This ALM is made by utilizing a full polarization compensation in 3D, i.e., a multi-layered structure with triangular constituents. The designed ALM possesses constitutive parameters identical to air in both X and Ku bands, with adopting double Lorentz resonances. In the full-wave simulations, the ALM shows air-like scatteringless at around 8 GHz and 13.5 GHz. In the experimental measurements, the ALM is nearly scatteringless in the same bands, with incident angles varying from 0° to 60° for both the vertical and horizontal polarizations. In-depth analysis shows that zero phase delay is introduced to the propagating waves, with the ALM being present. To the best of our knowledge, it is the first attempt to simultaneously break the polarization and bandwidth limitations of ALMs. The designed ALM would be a good candidate in facilitating superior antenna radomes, EM windows, as well as through-wall detections and communications.