▎ 摘　　要

We investigate the harmonic generation of graphene irradiated by linearly polarized lasers with intensities in a wide range from 108 W/cm2 to 1013 W/cm2. We find a striking knee structure in the laser intensity dependence of the harmonic yield, which consists of a perturbative region, followed by a plateau of saturated harmonic yield, and then a transition to nonperturbative growth. The knee structure is rather universal for the various harmonic orders and has been certified by two-band density-matrix equation calculations, as well as ab initio time-dependent density-functional theory calculations. Taking the third harmonic as an example and based on the two-band model, we reveal the underlying mechanisms: the perturbative region can be analytically depicted by perturbation theory, while the plateau of saturated harmonic yield and the transition to nonperturbative growth are caused by destructive and constructive quantum interference of harmonics generated by the electrons corresponding to the lattice momenta around Dirac points and M points in the Brillouin zone, respectively. In particular, we find that tuning the Fermi energy can effectively alter the knee structure, while the profile of the knee structure is not sensitive to temperature. Our calculations of the third-order harmonic versus tuned Fermi energy are compared with recent experiments, showing good agreement. Our predicted knee structure and its associated properties are observable with the current experimental techniques.