ABSTRACT
This chapter discusses the dynamic ac conductivity of a nonequilibrium two-dimensional electron-hole system in optically pumped graphene. Considering the contribution of both interband and intraband transitions, the chapter demonstrates that at sufficiently strong pumping the population inversion in graphene can lead to the negative net ac conductivity in the terahertz range of frequencies. The main effect determining the low end of the frequency range where the ac conductivity is negative is associated with the intraband transitions electrons and holes due to their scattering on impurities and acoustic phonons. The effect of population inversion and negative ac conductivity might be used in graphene-based coherent sources of terahertz electro-magnetic radiation utilizing stimulated photon emission. A similar effect can occur in gated graphene structures, for example, in the structures with graphene separated from a highly conducting substrate playing the role of the gate.
