Broadband terahertz absorber based on dispersion-engineered catenary coupling in dual metasurface

Terahertz (THz) absorbers have attracted considerable attention due to their potential applications in high-resolution imaging systems, sensing, and imaging. However, the limited bandwidth of THz absorbers limits their further applications. Recently, the dispersion management of metasurfaces has become a simple strategy for the bandwidth extension of THz devices. In this paper, we used the capability of dispersion management to extend the bandwidth of THz absorbers. As a proof-of-concept, a dual metasurface-based reflective device was proposed for broadband near-unity THz absorber, which was composed of two polarization-independent metasurfaces separated from a metallic ground by dielectric layers with different thickness. Benefiting from the fully released dispersion management ability in adjusting the dimensions of the metasurfaces, we obtained an absorbance above 90% in the frequency range from 0.52 to 4.4 THz and the total thickness for the bandwidth approaching the theoretical Rozanov limit. The experimental results verified the ability of dispersion management in designing broadband absorbers and the performance of the designed absorber. The underlying physical mechanism of dispersion management was interpreted in the general equivalent circuit theory and transmission line model. In addition, the catenary optical model was used to further interpret the physics behind this dual metasurface. Moreover, we found that the alignment deviations between the dual metasurface had little impact on the performance of the designed absorber, which indicates that the dual-metasurface does not require center alignment and is easy to be fabricated. The results of this work could broaden the application areas of THz absorbers.