Electrically Tunable Optical Nonlinearities in Graphene-Covered SiN Waveguides Characterized by Four-Wave Mixing

Third order optical nonlinearities in graphene have been demonstrated to be large and have been predicted to be highly dependent on the Fermi energy of the graphene. This prediction suggests that graphene can be used to make systems with large and electrically tunable optical nonlinearities. In this work, we present what is to our knowledge the first experimental observation of this Fermi energy dependence of the optical nonlinearity. We have performed a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with graphene which was gated using a polymer electrolyte. We observe strong dependencies of the four-wave mixing conversion efficiency on the signal-pump detuning and Fermi energy, that is, the optical nonlinearity is indeed demonstrated to be electrically tunable. In the vicinity of the interband absorption edge (2|<i>E</i>_F| ≈ ℏω), a peak value of the waveguide nonlinear parameter of ≈6400 m^–1W⁻¹, corresponding to a graphene nonlinear sheet conductivity |σ_<i>s</i>⁽³⁾| ≈ 4.3 × 10^–19 A m²V^–3, is measured. These results were qualitatively linked with theoretical calculations. Apart from providing a better understanding of the nonlinear optical response of graphene, these observations could pave the way toward the use of graphene for tunable nonlinear optics