Electrical access to critical coupling of circularly polarized waves in graphene chiral metamaterials

Abstract

Active control of polarization states of electromagnetic waves is highly desirable because of its diverse applications in information processing, telecommunications, and spectroscopy. However, despite the recent advances using artificial materials, most active polarization control schemes require optical stimuli necessitating complex optical setups. We experimentally demonstrate an alternative-direct electrical tuning of the polarization state of terahertz waves. Combining a chiral metamaterial with a gated single-layer sheet of graphene, we show that transmission of a terahertz wave with one circular polarization can be electrically controlled without affecting that of the other circular polarization, leading to large-intensity modulation depths (>99%) with a low gate voltage. This effective control of polarization is made possible by the full accessibility of three coupling regimes, that is, underdamped, critically damped, and overdamped regimes by electrical control of the graphene properties.

Fig. 1. Schematic views and device image of gate-controlled active graphene CDZM.

(A) CD and OA in graphene CDZM. CD: Transmissions for RCP and LCP waves are different to each other because of the different absorption between RCP and LCP waves (left). OA: The electric field vector of linearly polarized light rotates around the axis parallel to its propagation direction while passing the graphene CDZM (right). (B) Schematic rendering of a gate-controlled active graphene CDZM composed of a single-layer graphene deposited on the top layer of CDZM and subsequently covered by a layer of ion gel [thickness (t) = 20 μm]. The geometry parameters are given as l = 100 μm, w = 7 μm, and s = 10 μm. (C) Top-view microscopy image of the fabricated gate-controlled active graphene CDZM. The gap width between chiral metamolecules is given as g = 2 μm. (D) Schematic rendering of the fabricated graphene CDZM. B is a base connected to the ion gel, and G is a gate connected to the graphene layer.

Fig. 2. Gate-controlled circular transmission and CD.

(A) Measured and simulated intensity of transmission spectra for RCP (T_RCP; solid line) and LCP (T_LCP; dashed line) waves are plotted for different gate voltages ΔV. (B) T_RCP (orange) and T_LCP (green) at the resonance frequency of 1.1 THz as a function of ΔV. Whereas T_LCP is almost unchanged, T_RCP can be markedly modified by the applied voltage. (C) Measured (scatters) and simulated (line) intensity modulation depth for ΔT_RCP/T_RCP,CNP plotted as a function of ΔV at the resonance frequency of 1.1 THz. The maximum modulation depth for T_RCP is measured to be 99%.

Fig. 3. Gate-controllable CD.

(A) Measured and simulated CD defined by the difference in transmission for RCP and LCP waves Δ = |T_RCP – T_LCP|. (B) CD Δ at the resonance frequency of 1.1 THz as a function of gate voltages ΔV.

Fig. 4. Gate controllable OA.

(A) Comparison between measured and simulated ellipticity η with different gate voltages ΔV. The dashed purple line represents the frequency of pure OA (η ≈ 0). (B) Measured and simulated azimuthal rotation angle θ with different ΔV. (C) Azimuthal rotation angle θ at the frequency for which f_η≈0 as a function of ΔV.

Fig. 5. Smith curves of T_RCP with different gate voltages.

(A) Temporal coupled mode theory for two resonances, f₁ and f₂, with a two-port model. (B) Simulated (solid line) and fitted (dashed line) phases of RCP waves are plotted for three different gate voltages ΔV. Smith curves of the T_RCP for coupled mode theory of the two-port model representing (C) underdamping (ΔV < 1.8 V), (D) critical damping (ΔV = 1.8 V), and (E) overdamping (ΔV > 1.8 V) behavior. The radiation and intrinsic losses Γ_1r, Γ_1i, Γ_2r, and Γ_2i for (F) the RCP excitation and (G) the LCP excitation as a function of ΔV.