Femtosecond Population Inversion and Stimulated Emission of Dense Dirac   Fermions in Graphene
  We show that strongly photoexcited graphene monolayers with 35fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative optical conductivity for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation. 
T. Li, L. Luo, M. Hupalo, J. Zhang, M. C. Tringides, J. Schmalian and   J. Wang
cond-mat.mes-hall cond-mat.mtrl-sci