Switchable Terahertz Metasurface: Dual‑Mode Absorber and Polarization Converter Using Graphene and Gold

Introduction

Terahertz technology relies heavily on devices that can manipulate electromagnetic waves. Absorbers dissipate unwanted energy, while polarization converters reshape wavefronts for imaging, spectroscopy, and communication. Recent advances in metasurfaces—sub‑wavelength patterned structures—have produced near‑perfect absorbers using gold or graphene resonators. However, many of these designs are narrow‑band and functionally limited.

Graphene, with its tunable surface conductivity, supports surface plasmon polaritons (SPPs) that can be modulated via bias voltage, enabling broadband absorption. Likewise, gold‑based L‑shaped and cross‑structured patterns have demonstrated efficient polarization conversion over wide frequency ranges. Yet, no reported system simultaneously offers both high absorption and high PC in a single, reconfigurable platform.

To bridge this gap, we propose a multi‑functional metasurface that switches between absorption and polarization‑conversion modes by simply varying graphene’s chemical potential. The design remains planar, compact, and manufacturable with standard lithographic techniques.

Methods

The MFD comprises three layers: a gold‑based PCM, a graphene‑based AM, and a reflective gold backplane. The PCM is a dual‑L‑shaped gold pattern printed on a TOPAS polymer substrate; the AM consists of a 3 × 3 array of cross‑slot graphene patterns etched into the same polymer. The two metasurfaces are separated by a dielectric gap h1, while the polymer thickness above the PCM is h0. The gold backplane (200 nm) guarantees zero transmission, simplifying analysis to reflection coefficients.

Graphene’s complex surface conductivity is described by the Kubo formula:

σs=σint(ω,μc,Γ,T)+σinter(ω,μc,Γ,T)

where μc is the tunable chemical potential. At μc = 0 eV, graphene behaves as a weak dielectric; at μc = 0.7 eV, its high conductivity excites SPPs, strongly localizing the field within the AM.

Full‑wave simulations (CST Studio Suite, frequency‑domain solver) model a periodic unit cell with Floquet boundaries. Reflection coefficients ryy and rxy for y‑polarized incidence are extracted to compute PCR and absorptivity:

PCR = |rxy|^2 / (|ryy|^2 + |rxy|^2)
Abs = 1 – |ryy|^2 – |rxy|^2

Simulations are performed at 300 K with a momentum relaxation time τ = 0.1 ps. The optimized geometrical parameters are h0 = 17 µm, h1 = 1.5 µm, cell period p = 50 µm, and feature dimensions as listed in the original design.

Typical configuration of a MFD

Results, Physical Mechanisms, and Discussion

Results

The simulated PCR and absorptivity curves confirm the dual‑mode operation. In PC mode (μc = 0 eV), PCR exceeds 0.9 from 2.11 to 3.63 THz, while absorptivity remains below 30 %. In absorption mode (μc = 0.7 eV), absorptivity surpasses 80 % between 1.59 and 4.54 THz, with a peak of 96.4 % at 3.06 THz.

PCR and absorptivity calculation of the proposed MFD. a Simulation model. b Calculated results for PC mode (blue) and absorption mode (cyan). c Results for the structure without AM (red).

Physical Mechanisms

Electric‑field and current maps illustrate the switching behavior. When μc = 0 eV, the field concentrates on the PCM’s L‑shapes, and the induced currents produce orthogonal polarization, confirming efficient PC. Conversely, with μc = 0.7 eV, the field localizes in the cross‑slot graphene, generating strong SPPs that trap and dissipate the energy within the AM, leading to high absorption.

Field distributions for PC mode (μc = 0 eV). a 2.56 THz. b 3.22 THz.

Field distributions for absorption mode (μc = 0.7 eV). a 1.7 THz. b 3.3 THz.

Parametric Studies

Varying μc from 0 to 1 eV tunes the device between PC and absorption regimes. PCR degrades as μc increases, while absorptivity improves, peaking at μc = 0.7 eV. Polarization‑angle and incident‑angle analyses show that the device retains >80 % absorptivity for angles up to 30° and maintains high PCR for s‑polarized incidence below 40°. Adjusting the vertical gap h1 has negligible impact on PC mode but influences absorption by altering SPP coupling.

Characteristics of the MFD for different chemical potentials (μc). a PCR. b Absorption.

Absorption vs. polarization angles. a Angles φ1 and φ2. b φ1. φ2.

PCR for s‑ and p‑polarized incidence vs. angle.

Absorption vs. angle for s‑ and p‑polarized incidence.

Effect of vertical gap h1 on PCR (a) and absorption (b).

Conclusions

We have demonstrated a compact, tunable metasurface that switches seamlessly between >80 % absorption and >90 % polarization conversion in the 1.6–4.6 THz band by controlling graphene’s chemical potential. The design’s low profile, simple fabrication, and robust performance across angles make it a strong candidate for next‑generation terahertz imaging, sensing, photodetection, and modulation systems.

Abbreviations

AM

Absorbing metasurface

CP

Circular polarization

EM

Electromagnetic

LP

Linear polarization

MFD

Multi‑functional device

PC

Polarization conversion

PCM

Polarization conversion metasurface

PCR

Polarization conversion ratio

SPPs

Surface plasmon polaritons