The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising…
Figures at a glance
Figure 1: Imaging propagating and localized graphene plasmons by scattering-type SNOM. 
a, Diagram of the experimental configuration used to launch and detect propagating surface waves in graphene (represented as blue rings). The metallized AFM tip (shown in yellow) is illuminated by an infrared laser beam with wavelength λ0. b, Near-field amplitude image acquired for a tapered graphene ribbon on top of 6H-SiC. The imaging wavelength is λ0 = 9.7 μm. The tapered ribbon is 12 μm long and up to 1 μm wide. c, Colour-scale image of the calculated local density of optical states (LDOS) at a distance of 60 nm from the graphene surface, and assuming substrate εr = 1. Simulation fitting parameters: graphene mobility μ = 1,000 cm2 V−1 s−1 and Fermi energy EF = 0.4 eV.
Figure 2: Controlling the plasmon wavelength over a wide range. 
a, b, Coloured plots show near-field optical images taken with imaging wavelengths (λ0) of 9,200 nm (left), 9,681 nm (middle) and 10,152 nm (right), corresponding respectively to SiC dielectric constants of 2.9, 2.0 and 0.7. a, Images of a graphene ribbon ~1 μm wide, revealing a strong dependence of the fringe spacing, and thus plasmon wavelength, on the excitation wavelength; b, images of a tapered graphene ribbon; both ribbons are on the same 6H-SiC substrate. The topography (obtained by AFM) is shown in greyscale in the leftmost and rightmost panels, and outlined by dashed lines in the central, coloured panels. The line traces in the leftmost and rightmost panels are extracted from the near-field images for λ0 = 9,200 nm and λ0 = 10,152 nm. Red and white arrows indicate the resonant localized modes.
Figure 3: Comparison of theoretical model with experimental results. 
a, Experimentally extracted plasmon wavelength λp as a function of incident wavelength λ0. Values for λp are obtained from interference fringes (blue crosses) and localized modes (red cross), compared to the calculated plasmon dispersion (blue curves, see Supplementary Information) for graphene assuming intrinsic doping of 0.2 and 0.4 eV on a SiC-6H substrate. Green dashed line, SiC substrate permittivity. b, Experimentally obtained resonance conditions W/λp extracted from localized-mode measurements. Red crosses and black circles correspond to the modes indicated by red and white arrows in Fig. 2, respectively. c, Spatial distribution of the LDOS calculated for homogeneous ribbons of increasing width (from bottom to top), supported on a dielectric with εr = 3 (left) or εr = 0.5 (right). The ribbon width of the two lowest-order modes is shown in units of the plasmon wavelength of extended graphene, λp.
Figure 4: Plasmonic switching and active control of the plasmon wavelength by electrical gating.
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