▎ 摘　　要

We have calculated the fifth-order nonlinear optical response at experimentally relevant field strengths within the model of massless Dirac fermions by coupling the massless Dirac fermions to the time-dependent electric field quantum mechanically. It demonstrates that the fifth-order nonlinear optical response plays an important role in the contribution to the optical conductivity of pristine single-layer graphene in the low frequency part of the terahertz regime. The nonlinear effect can enhance the optical activity of single-layer graphene in the terahertz regime and significantly decreases the transmittance of graphene in the regime of frequencies from 0.1 to 0.5 THz. These properties of graphene may be used for photonic and optoelectronic device in the terahertz regime.