Gate Tunable Third-Order Nonlinear Optical Response of Massless Dirac Fermions in Graphene

Materials with massless Dirac fermions can possess exceptionally strong optical nonlinearity, which has already been explored by several experiments on graphene monolayer. However, the reported variation of the nonlinear optical coefficient by orders of magnitude is still not yet understood. A large part of the difficulty can be attributed to the lack of information on how doping affects the different nonlinear optical processes. In this talk, we will introduce our experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM (DFM) appeared just the opposite. Moreover, the DFM was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the gate-control of chemical potential, which selectively switches on and off photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.