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Ultrafast optical pump-probe spectroscopy measurement on monolayer grapheneobserves significant optical nonlinearities. We show that strongly photoexcitedgraphene monolayers with 35 fs pulses quasi-instantaneously build up abroadband, inverted Dirac fermion population. Optical gain emerges and directlymanifests itself via a negative conductivity at the near-infrared region forthe first 200fs, where stimulated emission completely compensates absorptionloss in the graphene layer. To quantitatively investigate this transient,extremely dense photoexcited Dirac-fermion state, we construct atwo-chemical-potential model, in addition to a time-dependent transient carriertemperature above lattice temperature, to describe the population invertedelectronic state metastable on the time scale of tens of femtoseconds generatedby a strong exciting pulse. The calculated transient optical conductivityreveals a complete bleaching of absorption, which sets the saturation densityduring the pulse propagation. Particularly, the model calculation reproducesthe negative optical conductivity at lower frequencies in the states close tosaturation, corroborating the observed femtosecond stimulated emission andoptical gain in the wide near-infrared window.

Transient Charge and Energy Balance in Graphene Induced by Ultrafast Photoexcitation