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We propose a scheme to directionally couple light into graphene plasmons by placing a graphene sheet on a magneto-optical substrate. When a magnetic field is applied parallel to the surface, the graphene plasmon dispersion relation becomes asymmetric in the forward and backward directions. It is possible to achieve unidirectional excitation of graphene plasmons with normally incident illumination by applying a grating to the substrate. The directionality can be actively controlled by electrically gating the graphene, or by varying the magnetic bias. This scheme may have applications in graphene-based opto-electronics and sensing.
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Here we present an all-optical plasmon coupling scheme, utilising the intrinsic nonlinear optical response of graphene. We demonstrate coupling of free-space, visible light pulses to the surface plasmons in a planar, un-patterned graphene sheet by using nonlinear wave mixing to match both the wavevector and energy of the surface wave. By carefully controlling the phase-matching conditions, we show that one can excite surface plasmons with a defined wavevector and direction across a large frequency range, with an estimated photon efficiency in our experiments approaching $10^{-5}$.
We demonstrate that graphene placed on top of structured substrates offers a novel approach for trapping and guiding surface plasmons. A monolayer graphene with a spatially varying curvature exhibits an effective trapping potential for graphene plasmons near curved areas such as bumps, humps and wells. We derive the governing equation for describing such localized channel plasmons guided by curved graphene and validate our theory by the first-principle numerical simulations. The proposed confinement mechanism enables plasmon guiding by the regions of maximal curvature, and it offers a versatile platform for manipulating light in planar landscapes. In addition, isolated deformations of graphene such as bumps are shown to support localized surface modes and resonances suggesting a new way to engineer plasmonic metasurfaces.
Sub-wavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, sub-wavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a THz pump-THz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by two orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results.
Assuming that the resonant surface plasmons on a spherical nanoparticle is formed by standing waves of two counter-propagating surface plasmon waves along the surface, by using Mie theory simulation, we find that the dispersions of surface plasmon resonant modes supported by silver nanospheres match that of the surface plasmons on a semi-infinite medium-silver interface very well. This suggests that the resonant surface plasmons of a metal nanosphere can be treated as a propagating surface plasmon wave.
We comment on the macroscopic model for surface plasmons of H.-Y. Deng [New J. Phys. 21 (2019) 043055; arXiv:1712.06101] and a claim, based on energy conversion from charges to the electric field, that surface plasmons on metallic surfaces may become unstable [J. Phys.: Cond. Matt. 29 (2017) 455002; arXiv:1606.06239, 1701.01060]. The discussion revolves around the formulation of charge conservation in the bulk and the surface of a metal. We elaborate in particular on the role of a finite electric current normal to the surface. Using a scheme of Cercignani and Lampis and of Zaremba, we point out that the model chosen by Deng for the non-specular scattering of electrons needs to be amended to prevent the disappearance of charges at the surface. Different models and approaches in the literature on surface plasmons are reviewed: the interfacial excess field approach of Bedeaux and Vlieger which contains Dengs macroscopic model, the assumption of specular reflection of Ritchie and Marusak, a hydrodynamic model with a composite charge density (partially localized at the surface), the local dielectric model, and a macroscopic method with (anti)symmetric fictitious stimuli (used, e.g., by Garc{i}a-Moliner and Flores). This puts Dengs results into perspective and illustrates problems with his approach.
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