  The nonlinear response of electrons (holes) in doped graphene on ac pumping
is considered theoretically for the frequency region above the energy
relaxation rate but below the momentum and carrier-carrier scattering rates.
Temporally-dependent heating of electrons by a strong ac field, which is
described within the energy balance approach, leads to an effective generation
of high-order harmonics. The efficiency of up-conversion of the 1 mm radiation
into the third harmonic by a single-layer graphene is about 10^{-7} at pumping
level ~100 kW/cm^2, room temperature, and concentration ~5 x 10^{11} cm^{-2}.
