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

Abstract

Plasmons in graphene nanoresonators have large application potential in photonics and optoelectronics, including room-temperature infrared and terahertz photodetectors, sensors, reflect-arrays or modulators [1][2][3][4][5][6][7] . Their efficient design will critically depend on the precise knowledge and control of the plasmonic modes. Here, we use near-field microscopy 8-11 between λ = 10 to 12 m wavelength to excite and image plasmons in tailored disk and rectangular graphene nanoresonators, and observe a rich variety of coexisting Fabry-Perot modes. Disentangling them by a theoretical analysis allows for identifying sheet and edge plasmons, the later exhibiting mode volumes as small as 10 λ . By measuring the dispersion of the edge plasmons we corroborate their superior confinement compared to sheet plasmons, which among others could be applied for efficient 1D coupling of quantum emitters 12 . Our understanding of graphene plasmon images is a key to unprecedented in-depth analysis and verification of plasmonic functionalities in future flatland technologies.2 At infrared and terahertz frequencies, doped graphene can support electrically tunable graphene plasmons (GPs) -electromagnetic fields coupled to charge carrier oscillations -with extremely short wavelengths and large confinement [13][14][15][16][17] . For that reason, graphene has a great potential for controlling radiation on the nanometer scale 18 , which largely benefits the development of highly sensitive spectroscopy 3 and detection [19][20][21] applications. The electromagnetic field concentration achieved by GPs can be further enhanced by fabricating nanostructures acting as Fabry-Perot resonators for GPs (for example disks or ribbons) 1,2, 6, 7,22 , favoring strong absorption in arrays of the resonators (up to 40%) 7 . Until now, localized plasmonic modes in graphene ribbons and disks have been analyzed experimentally essentially by far-field spectroscopy 1,2, 6, 7,22 . With this technique, however, neither the mode structure, nor the unique plasmonic edge modes are accessible. 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 . 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. We tackled this problem by three-dimensi...

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] .…”

Optically Unraveling the Edge Chirality‐Dependent Band Structure and Plasmon Damping in Graphene Edges

“…[15,16] However, it is challenging to distinguish the edge-specific phenomena from bulk response. Recently, the observation of edge plasmons in graphene nanoribbon [18][19][20][21] and superlattice plasmons in graphene-hBN moiré structures [22] opens doors for exploring topological properties and electronic structure via polaritonic probes. The tunability of edge states, prerequisite for future nano-optoelectronic devices, is not accomplished, either.…”

Tunable Terahertz Deep Subwavelength Imaging Based on a Graphene Monolayer

“…Such one-dimensional (1D) excitations are generic for thin lossy metal films with finite width, although their dispersion depends on the thickness and the dielectric environment of the edge35. The 1D edge mode is also theoretically and experimentally observed on a graphene ribbon or a ribbon array3637. Actually, a graphene ribbon can be either a real graphene strip or something that is “virtually” created by spatially varying external gates acting on a graphene sheet, as proposed in ref.…”