Microscopic mechanism for transient population inversion and optical gain in graphene

Based on microscopic calculations, we predict a transient femtosecond population inversion in graphene suggesting graphene as a new active gain material covering a broad frequency range. In this paper, we microscopically shed light on the underlying elementary many-particle processes: Transient gain and population inversion occur due to an interplay of strong optical pumping and carrier cooling that fills states close to the Dirac point giving rise to a relaxation bottleneck. The subsequent femtosecond decay of the optical gain is mainly driven by Coulomb-induced Auger recombination. Our findings are in excellent agreement with recent experimental data.