Control of resonance effects in third-harmonic generation from graphene using the ion-gel gating technique

Single-layer graphene shows various third-order nonlinear optical (NLO) responses including two-photon absorption, four-wave mixing, and optical Kerr effect. Third-harmonic generation (THG) is one of the simplest third-order NLO responses. In this research, we focus on the Fermi level (E_F) dependence of THG from graphene. We prepared electrochemical doping devices for graphene using ion-gel based on an ionic liquid [EMIM][TFSI]. The E_F as a function of applied voltage was determined by the Raman spectroscopy. The experiment of THG was performed using the Maker fringe method. This method enables us to obtain both the intensity and phase of the electric field of THG. As a result of the measurement, we observed the variation of THG due to doping. To discuss the E_F dependence of THG from graphene, we carried out a model calculation for third-order nonlinear optical susceptibility. Based on the experimental and calculated results, we discuss the dominant resonance process and its control in THG from graphene.