  We develop a density matrix formalism in the length gauge to calculate the
nonlinear response of intrinsic monolayer graphene at terahertz frequencies.
Employing a tight-binding model, we find that the interplay of the interband
and intraband dynamics leads to strong harmonic generation at moderate field
amplitudes. In particular, we find that at low temperature, the reflected field
of undoped suspended graphene exhibits a third harmonic amplitude that is 32%
of the fundamental for an incident field of 100 V/cm. Moreover, we find that up
to the seventh harmonic and beyond are generated.
