  Graphene plasmons are rapidly emerging as a viable tool for fast electrical
manipulation of light. The prospects for applications to electro-optical
modulation, optical sensing, quantum plasmonics, light harvesting, spectral
photometry, and tunable lighting at the nanoscale are further stimulated by the
relatively low level of losses and high degree of spatial confinement that
characterize these excitations compared with conventional plasmonic materials.
We start with a general description of the plasmons in extended graphene,
followed by analytical methods that lead to reasonably accurate estimates of
both the plasmon energies and the strengths of coupling to external light in
graphene nanostructures, including graphene ribbons. We discuss several
possible strategies to extend these plasmons towards the visible and near
infrared, including a reduction in the size of the graphene structures and an
increase in the level of doping. Specifically, we discuss plasmons in narrow
ribbons and molecular-size graphene structures. We further formulate
prescriptions based on geometry to increase the level of electrostatic doping
without causing electrical breakdown. Results are also presented for plasmons
in highly-doped single-wall carbon nanotubes, which exhibit similar
characteristics as narrow ribbons and show a relatively small dependence on the
chirality of the tubes. We further discuss perfect light absorption by a
single-atom carbon layer, which we illustrate by investigating arrays of
ribbons using fully analytical expressions. Finally, we explore the possibility
of exploiting optically pumped transient plasmons in graphene, whereby the
optically heated graphene valence band can sustain collective plasmon
oscillations similar to those of highly doped graphene, and well-defined during
the picosecond time window over which the electron is at an elevated
temperature.
