Research ArticleAPPLIED OPTICS

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

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Science Advances  29 Sep 2017:
Vol. 3, no. 9, e1701377
DOI: 10.1126/sciadv.1701377
  • 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 (TRCP; solid line) and LCP (TLCP; dashed line) waves are plotted for different gate voltages ΔV. (B) TRCP (orange) and TLCP (green) at the resonance frequency of 1.1 THz as a function of ΔV. Whereas TLCP is almost unchanged, TRCP can be markedly modified by the applied voltage. (C) Measured (scatters) and simulated (line) intensity modulation depth for ΔTRCP/TRCP,CNP plotted as a function of ΔV at the resonance frequency of 1.1 THz. The maximum modulation depth for TRCP 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 = |TRCPTLCP|. (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 fη≈0 as a function of ΔV.

  • Fig. 5 Smith curves of TRCP 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 TRCP 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 Γ1r, Γ1i, Γ2r, and Γ2i for (F) the RCP excitation and (G) the LCP excitation as a function of ΔV.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1701377/DC1

    fig. S1. Characterization of single-layer graphene by Raman spectroscopy.

    fig. S2. Electric field distribution.

    fig. S3. Transmission phase spectra.

    note S1. Temporal CMT for two ports and two resonances.

    table S1. Fitting parameters of temporal CMT.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Characterization of single-layer graphene by Raman spectroscopy.
    • fig. S2. Electric field distribution.
    • fig. S3. Transmission phase spectra.
    • note S1. Temporal CMT for two ports and two resonances.
    • table S1. Fitting parameters of temporal CMT.

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