A transient femtosecond population inversion in graphene was recently reported by Li et al., Phys. Rev. Lett. 108, 167401 (2012). Based on a microscopic theory we clarify the underlying microscopic mechanism: Transient gain and population inversion in graphene occurs due to a complex 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.
@article{arxiv.1209.4833,
title = {Microscopic mechanism for transient population inversion and optical gain in graphene},
author = {Torben Winzer and Ermin Malic and Andreas Knorr},
journal= {arXiv preprint arXiv:1209.4833},
year = {2015}
}