Low-Energy Population Inversion in Graphene Evidenced in a Three-Pulse Pump-Probe Experiment

Population inversion and optical gain are often difficult to measure in systems that do not exhibit lasing. For example, two-color pump-probe experiments targeting gain require precise reference measurements in order to distinguish gain from simple absorption bleaching due to Pauli blocking. In the mid-infrared (MIR) and far-infrared (FIR) spectral range such experiments are even more difficult as free-carrier absorption complicates the analysis. Here we utilize a three-pulse technique [1], which has been employed to find evidence for gain and spectral hole burning in the near-infrared (NIR), to study the dynamics of the MIR population inversion in optically pumped intrinsic graphene. Graphene is an interesting material to apply this technique since there are on one hand many reports on ultrafast thermalization, excluding inversion, but on the other hand many suggestions to realize gain, in particular in the THz range.