Nonequilibrium carriers in intrinsic graphene under interband photoexcitation

We study nonequilibrium carriers (electrons and holes) in intrinsic graphene at low temperatures under far-infrared (IR) and midinfrared radiation in a wide range of its intensities. The energy distributions of carriers are calculated using a quasiclassical kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of saturation, the carrier distribution functions can exhibit plateaus around the pumping region at elevated intensities. As shown, at sufficiently strong mid-IR pumping, the population inversion can occur below the pumping energy. The graphene dc conductivity as a function of the pumping intensity exhibits a pronounced nonlinearity with a sublinear region at fairly low intensities and a saturation at a strong pumping. However, an increase in the pumping intensity in very wide range leads only to a modest increase in the carrier concentration and, particularly, the dc conductivity. The graphene conductivity at mid-IR irradiation exhibit strong sensitivity to mechanisms of carrier relaxation.