  Graphene plasmonics has become a highlighted research area due to the
outstanding properties of deep-subwavelength plasmon excitation, long
relaxation time, and electro-optical tunability. Although the giant
conductivity of a graphene layer enables the low-dimensional confinement of
light, the atomic scale of the layer thickness is severely mismatched with
optical mode sizes, which impedes the efficient tuning of graphene plasmon
modes from the degraded light-graphene overlap. Inspired by gap plasmon modes
in noble metals, here we propose low-dimensional graphene gap plasmon waves for
large light-graphene overlap factor. We show that gap plasmon waves exhibit
superior in-plane and out-of-plane field concentrations on graphene compared to
those of edge or wire-like graphene plasmons. By adjusting the chemical
property of the graphene layer, efficient and linear modulation of graphene gap
plasmon modes is also achieved. Our results provide potential opportunities to
low-dimensional graphene plasmonic devices with strong tunability.
