Energy ratio controlled gate-tuning four-wave mixing in monolayer graphene.

Monolayer graphene, with a gapless conical electronic band structure, demonstrates scale invariance, showing universal linear optical responses. The impacts of this feature on nonlinear optical responses remain unclear. Our work reveals that the gate-tunable difference-frequency four-wave mixing (DFM) responses in monolayer graphene are significantly influenced by the energy ratios between excitation photons.