Physical Review Letters
We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, 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 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.
Li, T., Luo, L., Hupalo, M., Zhang, J., Tringides, M. C., Schmalian, J., & Wang, J. (2012). Femtosecond population inversion and stimulated emission of dense Dirac fermions in graphene. Physical Review Letters, 108(16), 167401.