Tunneling Plasmonics in Bilayer Graphene

Abstract

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At sub-nanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single-and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene: yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers. KeywordsInfrared nano-imaging, bilayer graphene, plasmons, tunneling, plasmon-off region Main TextQuantum plasmonics is a rapidly growing field of research that involves the study of light-matter interactions in the quantum regime 1,2 . In particular, tunneling plasmonics explores the role of electron tunneling on the optical responses of coupled plasmonic nanostructures. Previous studies about tunneling plasmonics were focused on coupled metal nanoparticles within sub-nanometer separation [3][4][5][6] . The plasmonic resonance of the coupled system deviates from classical predictions where only Coulomb coupling is considered. In order to fully describe the plasmonic response, quantum tunneling of electrons between the nanoparticles has to be taken into account. Here we report experimental observation of novel plasmonic phenomena due to electron quantum tunneling between adjacent layers of graphene -a novel plasmonic material that carries infrared plasmons with high confinement, low loss and gate tunability [7][8][9][10][11][12][13] . Specifically, we observed a high plasmon confinement and effective plasmon-off state when two graphene layers are placed close to each other in a Bernal-stacking order. These effects are attributed to interlayer electron tunneling that plays a prominent role in graphene due to the reduced dimensionality and relatively low carrier density. In order to study the plasmonic properties of coupled graphene layers, we performed infrared (IR) nano-imaging experiments using an antenna-based nanoscope (Supporting Information). As shown in Figure 1a, the metalized tip of the 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 therefore facilitating energy transfer and momentum bridging from photons to plasmons [7][8][9][10][11][12][13] . Our samples were fabricated by mechanical exfoliation of bulk graphite and then transferred to ...

Edge and Surface Plasmons in Graphene Nanoribbons

“…In nano-IR experiments, fringes originate from constructive interference between tip-launched and boundary-reflected plasmons (Figure 1a). [3][4][5][6][7][8][9][10][11][12] Apart from these familiar patterns, we also observed plasmonic characteristics due to ribbon confinement as the ribbon width (W) shrinks. First, the two principal fringes move closer to each other with decreasing W and consequently the total number of distinct fringes observable within GNRs decreases.…”

“…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.…”

“…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.…”

Real-Space Imaging of the Tailored Plasmons in Twisted Bilayer Graphene

“…Indeed, the 2D nature of graphene makes it extremely sensitive to interlayer coupling that could dramatically modify the properties of Dirac fermions and their plasmonic excitations. For example, earlier studies about Bernal-stacked bilayer graphene (BLG) [20,22] and graphene/hBN heterostructures [23,24] have demonstrated many unique plasmonic characteristics compared to those of SLG.…”

“…1(a), the infrared light (solid arrow) from a continuous-wave infrared laser is focused at the apex of a metalized AFM tip. The laser-illuminated tip acts as both a launcher and a detector of surface plasmons [13][14][15][16][17][18][19][20][21][22][23]. The backscattered light (dashed arrow) off the tip-sample system contains essential information about plasmons underneath the tip.…”