Abstract
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 re-developed 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.
Highlights
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
To evaluate the sensitivity of the SLMA absorption upon deviations in the anisotropic parameters of the employed superstrate, we present the results in Fig. 7a, b for the performance of the optimized SLMA of Fig. 5b when the magnetic or/and electric tensor parameters deviate by 10% from the optimized solutions
A wide-angle and spectrally-selective THz SLMA based on a fully analytical framework involving the near-field interactions between the Floquet harmonics and the superstrate layer has been presented in this paper for the first time
Summary
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. The sensitivity to the polarization and angle of incidences is reduced to some extent, the vast majority of reports in all spectra deal with sophisticated, optimization-based, and costly designs, as they require the deformation of all array elements[30,31,32,33,34,35,36,37] These approaches only support certain types of array elements, probably creating features which make them difficult for applications over a large area[38]. In most of them, the design of absorbing metamaterial unit cells relied on trial and error steps, no rigorous theoretical formulation supported the study, and the design was based on a heuristic approach Another possible solution to realize artificial magnetic response is to use a high-impedance surface (like mushroom structure)[45,46]. The proposed superstrate-loaded MA (SLMA) does not require a complex design, and more importantly, can be applied to any type of MA and sensor element
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