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Imaging the Localized Plasmon Resonance Modes in Graphene Nanoribbons
We report a nanoinfrared
(IR) imaging study of the localized plasmon
resonance modes of graphene nanoribbons (GNRs) using a scattering-type
scanning near-field optical microscope (s-SNOM). By comparing the
imaging data of GNRs that are aligned parallel and perpendicular to
the in-plane component of the excitation laser field, we observed
symmetric and asymmetric plasmonic interference fringes, respectively.
Theoretical analysis indicates that the asymmetric fringes are formed
due to the interplay between the localized surface plasmon resonance
(SPR) mode excited by the GNRs and the propagative surface plasmon
polariton (SPP) mode launched by the s-SNOM tip. With rigorous simulations,
we reproduce the observed fringe patterns and address quantitatively
the role of the s-SNOM tip on both the SPR and SPP modes. Furthermore,
we have seen real-space signatures of both the dipole and higher-order
SPR modes by varying the ribbon width
Tunneling Plasmonics in Bilayer Graphene
We report experimental signatures
of plasmonic effects due to electron tunneling between adjacent graphene
layers. At subnanometer 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