The terahertz regime, which sits between microwaves and infrared light on the electromagnetic spectrum, has long promised transformative applications because it is non-ionizing, interacts distinctively with molecular and biological systems, and offers enormous bandwidth. Yet building compact, efficient, and broadband functional components in this range remains a formidable engineering challenge. Metamaterial absorbers, which use carefully engineered resonant structures to capture electromagnetic energy at subwavelength scales, have emerged as leading candidates. Conventional metallic versions, however, tend to suffer from limited bandwidth, weak tunability, and high sensitivity to geometric variations, particularly when multiple resonant modes are needed to cover a wide spectrum.

Graphene offers a way around these constraints. The atomically thin carbon sheet supports strongly confined surface plasmon polaritons, and its optical response can be actively controlled by adjusting its Fermi level through electrostatic gating or chemical doping. This means the resonance frequency and absorption characteristics of a graphene device can be modulated reversibly without changing its physical geometry. Earlier graphene-based terahertz absorbers relied on multilayer stacks, arrays of ribbons of different widths, or hybrid integration with phase-change materials such as vanadium dioxide. While effective, these approaches often demand complex fabrication, precise alignment, or dual tuning mechanisms, and thermally activated phase-change designs introduce challenges related to switching speed and power consumption.

The new design, developed by Hamzeh Sam Daliri, Hamed Saghaei, Hadi Dehbovid, and Reza Yousefi, consists of a three-layer metal-dielectric-graphene architecture. At the top sits a patterned graphene metasurface: a central multi-tip element inspired by the maple leaf, flanked by two rectangular graphene strips, together evoking the Canadian flag. The central pattern spans approximately 2.4 by 1.0 micrometers, with a patterned graphene area of about 0.84 square micrometers, while the lateral strips measure 1.25 by 2.75 micrometers. Beneath the graphene lies a dielectric spacer 6 micrometers thick with a refractive index of 1.4, and at the bottom a continuous gold film 1 micrometer thick acts as a back-reflector that suppresses transmission entirely.

The novelty lies not in the motif itself but in its functional plasmonic topology. The multi-tip central element and the lateral strips introduce multiple characteristic current-path lengths, sharp edges, and high-curvature regions within a single patterned plane. These features excite several interacting plasmonic modes whose controlled spectral overlap produces broadband absorption without vertically stacked resonators or hybrid material systems. The researchers traced the design’s evolution step by step: a single central resonator produced a narrow response, adding the two lateral strips broadened it by introducing additional current paths, and replacing the simple resonator with the multi-tip maple-leaf pattern further enhanced bandwidth through localized charge accumulation at its edges and tips.

The team simulated the structure using a three-dimensional finite-difference time-domain method, launching a broadband plane wave from 0.5 to 10 terahertz and applying periodic boundary conditions to model an infinite metasurface. Because the gold backplane eliminates transmission, absorptance reduces simply to one minus reflectance. Graphene was modeled as an atomically thin conductive sheet using the Kubo formalism, with an effective conductivity scaling factor representing multiple electronically decoupled sheets. An equivalent-circuit interpretation, in which each current path behaves as a lossy series RLC branch coupled to a grounded dielectric transmission-line section, explains why near-unity absorption occurs when the input impedance of the structure approaches that of free space, allowing incident energy to be fully dissipated in the graphene’s finite resistance.

A central finding of the parametric study is that absorption depends non-monotonically on graphene conductivity. For a single graphene layer, raising the chemical potential from 0.2 to 0.6 electronvolts boosted peak absorption from about 62 percent to 98 percent at 5.2 terahertz, close to a critical-coupling condition where impedance is optimally matched to free space. Pushing further to 0.8 electronvolts caused overcoupling, impedance mismatch, and a drop to roughly 85 percent. Similarly, increasing the effective number of graphene layers helped at low Fermi energies but degraded performance at higher ones: at 0.1 electronvolts, three- and four-layer structures achieved broadband absorption near 97 percent, whereas at 0.4 electronvolts and above, one- or two-layer configurations were clearly superior, with thicker stacks suffering detuning and destructive interference.

The dielectric spacer and lateral geometry proved equally consequential. Spacer thicknesses between 6 and 10 micrometers generally yielded absorption above 90 percent, while thinner cavities could not support well-developed resonant modes and thicker ones entered a regime of mode saturation and phase accumulation. For one optimized configuration with four effective graphene layers and a chemical potential of 0.6 electronvolts, an 8-micrometer spacer produced absorption approaching 100 percent across 4.8 to 8.2 terahertz. Low-index dielectrics such as polymethylpentene, PTFE, TOPAS, and PDMS emerged as suitable spacer candidates, and increasing the refractive index generally redshifted resonances while reducing peak absorption. Strip width sweeps showed that widths near 1.25 to 1.5 micrometers maximize plasmonic confinement at low-to-moderate chemical potentials, while narrower strips fail to sustain strong resonances.

Electric-field maps confirmed the physical picture: the strongest field confinement and energy concentration occurred in the 4 to 6 terahertz range, with pronounced hotspots along the edges, tips, and high-curvature regions of the maple-leaf pattern, precisely where charge accumulates and plasmonic modes localize. The authors emphasize that the high absorption should not be confused with the roughly 2.3 percent single-pass absorption of isolated monolayer graphene; here, plasmonic patterning, cavity enhancement, a reflective ground plane, and impedance matching combine to multiply light-matter interaction dramatically. Compared with prior broadband absorbers, the design meets a stricter 95 percent absorption criterion over a 4-terahertz interval within a single patterned plane, avoiding both stacked resonator architectures and thermally activated phase-change materials.

The researchers position the device as a performance-complexity compromise rather than an absolute bandwidth record, and its practical significance lies in the transferable design guidelines it establishes: electronic conductivity, resonator geometry, and cavity parameters must be jointly optimized, since excessive values of any single parameter transform the absorber into a reflector. With electrical tunability achieved through gate voltages that set the graphene chemical potential, and with compatibility with planar fabrication processes, the flag-inspired metasurface points toward reconfigurable terahertz systems for sensing, imaging, filtering, electromagnetic shielding, and future wireless communication networks.

Subject of Research: Electrically tunable broadband terahertz absorption using a Canada-flag-inspired patterned graphene metasurface

Article Title: Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface

Article References: Sam Daliri, H., Saghaei, H., Dehbovid, H., & Yousefi, R. (2026). Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface. Results in Optics, 25, Article 101157. https://doi.org/10.1016/j.rio.2026.101157

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101157

Keywords: terahertz absorber, graphene metasurface, surface plasmon polaritons, broadband absorption, electrical tunability, impedance matching, FDTD simulation, metamaterial, plasmonic resonance, terahertz sensing, dielectric spacer, Kubo formalism