  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 nano-imaging 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.
