  Infrared plasmons in doped graphene nanostructures produce large optical
absorption that can be used for narrow-band thermal light emission at tunable
frequencies that strongly depend on the doping charge. By virtue of Kirchhoff's
law, thermal light emission is proportional to the absorption, thus resulting
in narrow emission lines associated with the electrically controlled plasmons
of heated graphene. Here we show that realistic designs of graphene plasmonic
structures can release over 90% of the emission through individual infrared
lines with 1% bandwidth. We examine anisotropic graphene structures in which
efficient heating can be produced upon optical pumping tuned to a plasmonic
absorption resonance situated in the blue region relative to the thermal
emission. An incoherent thermal light converter is thus achieved. Our results
open a radically different approach for designing tunable nanoscale infrared
light sources.
