Gate tunable third-order nonlinear optical response of massless Dirac fermions in graphene

TL;DR

This study experimentally and theoretically investigates how tuning the chemical potential in graphene affects its third-order nonlinear optical responses, revealing significant enhancement and divergence in third harmonic generation and four-wave mixing.

Contribution

First experimental and theoretical analysis of doping-dependent third-order nonlinearities in graphene, demonstrating gate-controlled enhancement and divergence of nonlinear optical processes.

Findings

THG enhanced by ~30 times with heavy doping

Difference-frequency FWM shows divergence in undoped graphene

Chemical potential tuning enables control over nonlinear optical responses

Abstract

Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first 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 seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter 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 possibility to gate-control the chemical potential, which selectively switches on and off one- and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.