  We study nonequilibrium carriers (electrons and holes) in an intrinsic
graphene at low temperatures under far- and mid-infrared (IR) radiation in a
wide range of its intensities. The energy distributions of carriers are
calculated using a quasiclassic 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 sub-linear
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 momentum relaxation.
