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Ultraconfined Plasmonic Hotspots Inside Graphene Nanobubbles
We report on a nanoinfrared
(IR) imaging study of ultraconfined plasmonic hotspots inside graphene
nanobubbles formed in graphene/hexagonal boron nitride (hBN) heterostructures.
The volume of these plasmonic hotspots is more than one-million-times
smaller than what could be achieved by free-space IR photons, and
their real-space distributions are controlled by the sizes and shapes
of the nanobubbles. Theoretical analysis indicates that the observed
plasmonic hotspots are formed due to a significant increase of the
local plasmon wavelength in the nanobubble regions. Such an increase
is attributed to the high sensitivity of graphene plasmons to its
dielectric environment. Our work presents a novel scheme for plasmonic
hotspot formation and sheds light on future applications of graphene
nanobubbles for plasmon-enhanced IR spectroscopy
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