Nonequilibrium plasmon emission and amplification in photo-excited graphene

Graphene plasmons are collective excitations of charges within the 2D sheet of carbon atoms. A strong energy confinement, low group velocities and high tunability, make graphene plasmons exciting candidates for plasmonic applications in the THz to mid-IR range. In contrast to most metallic and semiconductor materials graphene plasmons can strongly couple to the particle/hole plasma due to a unique combination of 2D plasmons dispersion, gapless bandstructure and the extreme confinement of the plasmons to the sheet of carbon atoms. In equilibrium this coupling is observable as strong absorption at frequencies above to the Fermi energy, an effect known as Landau damping. When the carrier system is inverted by photoexcitation this process is reversed and plasmons below the inversion line are spontaneously emitted and amplified [1]. Based on the random-phase approximation we calculate the nonequilibrium polarizability of the Dirac plasma, and determine the exact complex-frequency plasmon dispersion (see Fig. 1) and gain/loss spectra under realistic conditions (temperature, collision loss) [2]. With spontaneous emission rates in the 100-fs range, our results predict that plasmon emission is a dominant channel for carrier recombination [3] explaning the rapid decay of inversion in photo-excited graphene as observed by femtosecond pump-probe and time- and angle-resolved photo-emission spectroscopy.