Dual‑Mode On‑to‑Off Modulation of Plasmon‑Induced Transparency in Graphene Terahertz Metasurfaces

Abstract

We demonstrate a tunable plasmon‑induced transparency (PIT) effect in a patterned graphene metasurface composed of interleaved ribbons and strips. Finite‑difference time‑domain (FDTD) simulations and coupled‑mode theory (CMT) reveal that two independent gate voltages can selectively suppress the left or right transmission dips, achieving a dual‑mode on‑to‑off modulator. The structure simultaneously delivers 50 % absorption and a 0.7 ps group delay, highlighting its promise for THz absorbers and slow‑light devices. Detailed coupling studies elucidate the role of geometry, providing a versatile platform for multifunctional modulators.

Introduction

Surface plasmon polaritons (SPPs) enable sub‑wavelength confinement and strong near‑field enhancement, making them indispensable for integrated photonics. While metal‑based SPPs suffer from intrinsic losses, monolayer graphene offers low‑loss, tunable plasmonic modes in the mid‑infrared and THz regimes. Graphene’s carrier density can be modulated by electrostatic gating, enabling dynamic control of its optical response without the need for structural reconfiguration.

Plasmon‑induced transparency (PIT) arises from destructive interference between a bright (superradiant) and a dark (subradiant) mode. In graphene metasurfaces, PIT has been reported in single‑layer ribbons, multilayer stacks, and hybrid structures. However, most designs allow only single‑mode tuning, typically by shifting the Fermi level, and lack efficient on‑to‑off switching of the transmission peak.

Here, we propose a graphene metasurface comprising periodically arranged ribbons and strips on a silicon substrate. Two independent gate electrodes independently adjust the Fermi levels of ribbons and strips, granting dual‑mode control of PIT. The design is compatible with CVD‑grown graphene and straightforward transfer processes, ensuring high yield and low sheet resistance.

Methods

The metasurface unit cell (Fig. 1c) contains a graphene ribbon (length 6 µm, width 4 µm) and a graphene strip (length 2.9 µm, width 0.8 µm) separated by a distance d = 0.8 µm and laterally displaced by S = 1.55 µm. The silicon substrate (thickness 300 nm) provides the dielectric environment. Gate voltage Vg1 (ribbons) and Vg2 (strips) tune the respective Fermi energies via

where the parameters are the reduced Planck constant, Fermi velocity, and dielectric constants. Carrier concentrations up to 4×10¹⁸ m⁻² (E_f ≈ 1.17 eV) have been achieved experimentally. The optical conductivity is dominated by the intraband term:

with τ = μE_f/(ev_F²) and mobility μ set to 1–3 m² V⁻¹ s⁻¹. The propagation constant β follows from the dispersion relation of graphene‑supported SPPs (Eq. 7). We solve the full‑wave problem using FDTD (CST Microwave Studio) and fit the spectra with CMT (Eq. 13–15) to extract quality factors and coupling coefficients.

Results and Discussion

Simulated transmission (Fig. 2a) shows a single‑mode bright resonance in the strip (dip ≈ 7.9 %) and a dark mode in the ribbon (peak ≈ 1.0). When both elements coexist, the PIT peak reaches 88.6 % due to interference. Electric‑field maps confirm field localization on both ribbon and strip, explaining the enhanced quality factor.

Dual‑mode on‑to‑off switching is achieved by varying Vg1 or Vg2. Fixing the strip Fermi level at 1.0 eV while sweeping the ribbon from 0.6 to 1.2 eV causes the left dip to deepen (on→off) and shift blueward (≈ 0.3 THz). Conversely, sweeping the strip while fixing the ribbon produces a similar effect on the right dip. The switching threshold (transmittance = 0.3) is reached between 0.8 and 1.0 eV for both modes, enabling independent control.

Absorption spectra (Fig. 4) show a 50 % peak even at low Fermi levels, owing to increased intrinsic loss. Blue‑shifts accompany higher carrier densities. CMT fits match FDTD curves, confirming the validity of the two‑oscillator model.

Increasing graphene mobility from 1 to 3 m² V⁻¹ s⁻¹ sharpens the PIT dips, narrows the 3 dB bandwidth, and raises the group delay to 0.7 ps (Fig. 5). The negative group delay at the dips indicates fast‑light behavior. The phase response also scales with mobility.

Geometric tuning (Fig. 6) demonstrates that the coupling distance d mainly affects the left dip, while the strip width and length influence the quality factor and spectral separation. Lateral displacement S has negligible effect under x‑polarized incidence.

Conclusion

We have shown that a graphene metasurface with independently gated ribbons and strips supports a tunable PIT resonance that can be switched on or off in two separate frequency bands. The device simultaneously achieves high absorption (≈ 50 %) and a significant slow‑light delay (0.7 ps). By adjusting geometry and carrier mobility, the spectral position, depth, and quality of the PIT can be tailored, offering a versatile platform for THz modulators, absorbers, and delay lines.