Ultra-Wideband Terahertz Absorption in an All-Graphene Metasurface Enabled by a Mode-Merging Mechanism

Abstract

This work presents the design and numerical investigation of a fully graphene-based, tunable ultra-wideband (UWB) electromagnetic absorber for terahertz (THz) applications. The proposed metal-free device consists of an upper patterned graphene rectangular-ring resonator (GRRR) and a lower continuous graphene layer, separated by a silicon dioxide spacer. Ultra-wideband absorption is achieved through a sequential mode-merging method. In this approach, rotated rectangular resonators are systematically integrated to excite multiple plasmonic resonances. These resonances progressively merge into a continuous absorption spectrum through enhanced electromagnetic coupling and spectral overlap. The proposed absorber achieves an absorption bandwidth of 17.9 THz, corresponding to a fractional bandwidth of 197.79%, with absorptivity exceeding 95% over the frequency band of 0.1 to 18 THz. Owing to its high structural symmetry, the absorber demonstrates polarization-insensitive characteristics under normal incidence, and sustains consistent absorption performance for both transverse-electric and transverse-magnetic polarizations under oblique incidence. Additionally, the absorption response can be dynamically adjusted by modulating the graphene chemical potential, without any structural changes. The combination of an exceptionally wide operating bandwidth, compact configuration, polarization insensitivity, and electrical tunability makes the proposed absorber promising for applications in THz sensing, imaging, stealth, and adaptive optoelectronic devices.