  The decay dynamics of excited carriers in graphene have attracted wide
scientific attention, as the gapless Dirac electronic band structure opens up
relaxation channels that are not allowed in conventional materials. We report
Fermi-level-dependent mid-infrared emission in graphene originating from a
previously unobserved decay channel: hot plasmons generated from optically
excited carriers. The observed Fermi-level dependence rules out Planckian light
emission mechanisms and is consistent with the calculated plasmon emission
spectra in photoinverted graphene. Evidence for bright hot plasmon emission is
further supported by Fermi-level-dependent and polarization-dependent resonant
emission from graphene plasmonic nanoribbon arrays under pulsed laser
excitation. Spontaneous plasmon emission is a bright emission process as our
calculations for our experimental conditions indicate that the spectral flux of
spontaneously generated plasmons is several orders of magnitude higher than
blackbody emission at a temperature of several thousand Kelvin. In this work,
it is shown that a large enhancement in radiation efficiency of graphene
plasmons can be achieved by decorating graphene surface with gold nanodisks,
which serve as out-coupling scatterers and promote localized plasmon excitation
when they are resonant with the incoming excitation light. These observations
set a framework for exploration of ultrafast and ultrabright mid-infrared
emission processes and light sources.
