ABSTRACT
This chapter presents 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. It investigates the dependence of dynamic conductivity on momentum relaxation time. The chapter demonstrates that population inversion can occur in optically pumped graphene in the THz/far-infrared range of frequencies; hence lasing in this range is possible, utilizing the gapless linear energy spectrum and relatively high optical phonon energy in grapheme. The carrier dynamics in optically pumped graphene strongly depend on the initial temperature of carriers and the intensity of the optical pumping. The mechanism of population inversion in the THz range is explained as follows. Shortly after pulse excitation, the carrier temperature rapidly increases, except for excitation with low-energy photons, and the quasi-Fermi level can even be negative because of the quasi-equilibration of carriers at very high temperatures induced by the photogenerated carriers.
