In this work, a wide-angle, polarization-independent, and broadband superstrate-assisted water-based metamaterial absorber (SWMA) covering the whole X-band is theoretically and experimentally demonstrated. Our SWMA design is a copper-backed structure comprising a thin substrate of distilled water, an FR-4 lossy layer, and a magneto-electric anisotropic metamaterial to boost achieving broadband and wide-angle features. The absorptivity of the proposed SWMA has been elaborately assessed in a full analytical framework involving oblique illuminations and both transverse electric (TE) and transverse magnetic (TM) polarizations. Numerical results demonstrate that exploiting magneto-electric anisotropy, the impedance matching between air and SWMA has been remarkably improved for both major polarizations, especially at near grazing angles. Owing to the end-to-end analytical design, the designed SWMA does not suffer from the drawbacks associated with the traditional designs including intricate particle geometries and brute-force optimizations. As a proof of concept, the proposed SWMA is fabricated and its absorptivity is measured in an anechoic microwave chamber between 8 and 12 GHz. The experimental results depict good conformity with the numerical simulations and the theoretical predictions, elucidating that our design retains its strong absorptivity over the whole X-band and for a wide angular range up to near grazing angles for both TE- and TM-polarizations.

In this paper, a theoretical framework relying on the reciprocity theorem is proposed to accurately design a spectrally-selective tHz superstrate-loaded metamaterial absorber (SLMA) exhibiting wide-angle feature. By leveraging high-order Floquet harmonics in a generalized transmission line model characterizing the conventional metamaterial absorbers (MAs), it is demonstrated that MAs suffer from impedance mismatch, especially at near grazing angles. From an impedance matching viewpoint, this major challenge is tackled in this paper via two different designs, exploiting a magneto-electric anisotropic Huygens' metamaterial and a multilayer dielectric structure at a certain distance over the MA plane. The numerical results corroborate well the theoretical predictions, elucidating that the proposed SLMA significantly broadens the angular performance of the MA up to near grazing angles (about 80°), where high absorptivity is still achieved in both principal planes. The deteriorating effect of diffraction modes has been comprehensively analyzed. In comparison to the previous wide-angle MA reports based on intricate particle geometries and brute-force optimizations, the proposed design features a straightforward semi-analytical algorithm, which can also be redeveloped for microwave, mid-infrared, and optical frequency bands and for any type of MA element. The proposed SLMA would be very promising for various wavelength-selective applications such as sensors and imaging.