A broadband terahertz absorber with gradient ring resonators based on a discrete spiral topological distribution

In this work, we propose a novel broadband terahertz (THz) metamaterial absorber (MMA) based on double-spiral structures, including discrete and continuous ones. The gradient ring resonators are in a discrete spiral topological distribution to expand the absorption band. By stacking two layers with a certain proportion, the proposed THz MMA can further enhance the energy loss to achieve absorption over 0.9 for TE mode at 2.57–5.19 THz and TM mode at 2.43–5.6 THz, with relative absorption bandwidths (RABWs) of 67.5% and 81.7%, respectively. Meanwhile, the polarization-insensitive bandwidth is 2.57–5.19 THz, with RABW of 67.5%. In addition, the normalized impedance for both modes is investigated. Furthermore, according to the analysis of the distributions of the surface current and electric field, it is clear that the electromagnetic energy loss mainly originates from the electric and magnetic resonances induced by the interaction between the metal resonators. To a certain extent, the Fabry–Perot resonance existing between the dielectric layers also contributes to the improvement of the absorption. This THz MMA, based on a unique double-spiral arrangement, can be used in many fields owing to its ultra-broad absorption band and polarization-insensitive characteristic in a broad band.