▎ 摘　　要

A valley degree of freedom in graphene has been playing significant roles in novel properties of the material. For graphene whose energy spectrum has a gap, valley-contrastive phenomena in nonlinear optics are theoretically studied. With the use of the Floquet method and the wave vector point group theory, we show that N-photon resonant excitation by circularly polarized light obeys a selection rule that is complementary with respect to the number of the photons involved, handedness of the circularly polarized laser field, and the valley index. In particular, a hallmark of inversion symmetry breaking is clarified in the selection rule. Besides, the band gap renormalization as an influence of the applied laser field is found to be also valley dependent.