We theoretically demonstrate the effects of interferences and asymmetries between $K$ and $K^{\prime }$ valleys for pristine graphene in high-order harmonic generation (HHG) by a single-color laser pulse. It is shown that the allowed orders of the observed overall HHG spectra are attributed to the consequences of quantum interferences between $K$ and $K^{\prime }$ valleys. The cancellations of forbidden harmonics are caused by the destructive interferences between two different valleys due to the dynamical symmetries of the laser-solid systems. Apparent valley-contrasting and helicity-resolved harmonic signals can be observed for the $3k$-order ($k\in \mathbb{N}$) harmonics when the polarization state of the driving laser field slightly deviates from the ideal circular polarization. The valley discriminations for these emerging $3k$-order ($k\in \mathbb{N}$) harmonics originate from the unequal responses of the HHG from two different valleys within the perturbation regime. In addition, the valley asymmetries of the electronic excitations are also observed by using an elliptically polarized laser pulse. Our work clearly illustrates the essential intervalley interplay in the HHG process for pristine graphene, facilitating the understanding of ultrafast electron dynamics in zero-band-gap materials.

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• Light-matter interaction
• Strong-field-induced spectra
• Ultrafast optics
• Fermionic condensates
• Dipole approximation
• Nonperturbative methods
• Schroedinger equation
• Two-level models