Non-Planckian mid-infrared radiation originating from hot plasmons in graphene

The decay dynamics of excited carriers in graphene have attracted wide scientific attention, as its gapless Dirac electronic band structure opens up relaxation channels that are not allowed in conventional bulk materials. We report bright mid-infrared emission from laser-pumped graphene, originating from a previously unobserved decay channel: hot plasmon emission. The observed Fermi-level dependence of radiation rules out all possible mid-infrared Planckian light emission mechanisms. Calculations for our experimental conditions indicate that conditions for plasmon gain exist on the sub-100 fs timescale due to the net positive emission processes. As the population inversion of excited carriers depletes at longer times, spontaneous plasmon emission dominates stimulated plasmon emission, and the cumulative observed emission per excitation pulse is dominated by spontaneous plasmon emission. The calculated time-integrated plasmon-assisted spontaneous emission spectra corroborate with the experimentally measured Fermi-level-dependent emission spectra from a planar graphene. The plasmon emission processes produce non-Planckian behavior. The spectral flux of spontaneously generated plasmons is found to be several orders of magnitude higher than that of spontaneous photon emission. Evidence for bright hot plasmon emission is further supported by a large emission enhancement detected from graphene decorated with gold NDs, which promote localized plasmon excitation and serve as outcoupling scatterers. These observations pave a new avenue for the exploration of ultrafast and ultrabright mid-infrared stimulated and spontaneous emission processes and bright infrared light sources.