Edge and Surface Plasmons in Graphene Nanoribbons

Abstract

We report on nano-infrared (IR) imaging studies of confined plasmon modes inside patterned graphene nanoribbons (GNRs) fabricated with high-quality chemical-vapordeposited (CVD) graphene on Al2O3 substrates. The confined geometry of these ribbons leads to distinct mode patterns and strong field enhancement, both of which evolve systematically with the ribbon width. In addition, spectroscopic nano-imaging in midinfrared 850-1450 cm -1 allowed us to evaluate the effect of the substrate phonons on the plasmon damping. Furthermore, we observed edge plasmons: peculiar one-dimensional modes propagating strictly along the edges of our patterned graphene nanostructures. KeywordsGraphene nanoribbons, CVD graphene, nano-infrared imaging, plasmon-phonon coupling, edge plasmons Main textSurface plasmon polaritons, collective oscillation of charges on the surface of metals or semiconductors, have been harnessed to confine and manipulate electromagnetic energy at the nanometer length scale. 1 In particular, surface plasmons in graphene are collective oscillations of Dirac quasiparticles that reveal high confinement, electrostatic tunability and long lifetimes. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Plasmons in graphene are promising for optoelectronic and nanophotonic applications in a wide frequency range from the terahertz to the infrared (IR) regime. 16,17 One common approach to investigate plasmons is based on nano-structuring of plasmonic media. 12,18 Large area structures comprised of graphene nanoribbons (GNRs) and graphene nano-disks have been extensively investigated by means of various spectroscopies. [12][13][14][15][16][17] These types of structures are of interest in light of practical applications including: surface enhanced IR vibrational spectroscopy 19,20 , modulators 21 , photodetectors 22 and tunable metamaterials 23,24 . Whereas the collective, area-averaged responses of graphene nanotructures are well characterized, the real-space characteristics of confined plasmon modes within these nanostructures remain completely unexplored.In this work, we performed nano-IR imaging on patterned GNRs utilizing an antennabased nanoscope that is connected to both continuous-wave and broadband lasers 25 (Supporting Information). As shown in Figure 1a, the metalized tip of an atomic force microscope (AFM) is illuminated by IR light thus generating strong near fields underneath the tip apex. These fields have a wide range of in-plane momenta q thus facilitating energy transfer and momentum bridging from photons to plasmons. [3][4][5][6][7][8][9][10][11][12] Our GNR samples were fabricated by lithography patterning of high quality CVD-grown graphene single crystals 26 on aluminum oxide (Al2O3) substrates (Supporting Information). As discussed in detail below, the optical phonon of Al2O3 is below  = 1000 cm -1 ( Figure S2), allowing for a wide mid-IR frequency region free from phonons.In Figure 1b, we show the AFM phase image displaying arrays of GNRs with various widths (darker parts correspond to gr...

Quantum description of the optical response of charged monolayer–thick metallic patch nanoantennas

“…This mode is axially symmetric and it is characterized by an antisymmetric charge density distribution induced at the top and bottom surfaces of the nanodisk. Different from the propagating edge plasmons discussed classically 20,[23][24][25]73,95 , the edge mode of the nanodisk found here stems from lateral coordinate dependence of the confining potential and it is similar to the transverse edge mode reported in quantum calculations of the monoatomic wires [91][92][93] For nanodisks negatively charged by electron doping several quantum effects are important:…”

“…Nowadays, plasmonic devices exploit propagating plasmons confined to (nanostructured) metal surfaces, or localised plasmons in single nanoparticles and nanoparticle assemblies. Along with these three-dimensional structures, plasmonics in lower dimensional structures such as nanowires [14][15][16][17][18] or edges [19][20][21][22][23][24][25] or two-dimensional materials [23][24][25][26][27][28][29] offers further perspectives in miniaturization of optoelectronic devices, light confinement, active control of response, and directional plasmon propagation. Albeit the ongoing discussion on the applicability of the term "plasmonic" to describe optical resonances in molecular structures, these structures represent the smallest devices where the optical response reflects the quantum nature of collective electronic excitations and thus requires quantum description [30][31][32] .…”

Real-space mapping of tailored sheet and edge plasmons in graphene nanoresonators

“…A comprehensive experimental characterization of graphene plasmonic nanoresonators and their sheet and edge modes has thus been elusive so far. On the other hand, plasmonic edge modes have been shown to propagate along sharp edges of gold films, graphene and 2D electron gases 11,[23][24][25][26][27][28] and provide stronger confinement of the electromagnetic fields compared to the sheet plasmons.Here we image and analyze the near-field structure of both plasmonic sheet and edge modes in graphene disks and rectangular nanoresonators. We employ scattering-type scanning near-field optical microscopy (s-SNOM) 29 , which to date is the only available tool for real-space imaging of the propagation and confinement characteristics of graphene plasmons 8,9,11 .…”

“…A comprehensive experimental characterization of graphene plasmonic nanoresonators and their sheet and edge modes has thus been elusive so far. On the other hand, plasmonic edge modes have been shown to propagate along sharp edges of gold films, graphene and 2D electron gases 11,[23][24][25][26][27][28] and provide stronger confinement of the electromagnetic fields compared to the sheet plasmons.…”

“…We employ scattering-type scanning near-field optical microscopy (s-SNOM) 29 , which to date is the only available tool for real-space imaging of the propagation and confinement characteristics of graphene plasmons 8,9,11 . The lack of a detailed understanding of graphene-plasmonic s-SNOM contrasts, however, has not allowed yet for a comprehensive analysis of plasmon modes in graphene nanostructures.…”