Threshold of terahertz population inversion and negative dynamic conductivity in graphene under pulse photoexcitation

We present a theoretical study of population inversion and negative dynamic conductivity in intrinsic graphene in the terahertz (THz) frequency range upon pulse photoexcitation at near-/mid-infrared wavelengths. The threshold pulse fluence required for population inversion and negative dynamic conductivity can be orders of magnitude lower when the pulse photon energy is lower, because of the inverse proportionality of the photoexcited carrier concentration to the pulse photon energy and because of the weaker carrier heating. We also investigate the dependence of dynamic conductivity on momentum relaxation time. Negative dynamic conductivity takes place either in high- or low-quality graphene, where Drude absorption by carriers in the THz frequency is weak.