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Nonlinear optical frequency conversion, in which optical fields interact witha nonlinear medium to produce new field frequencies, is ubiquitous in modernphotonic systems. However, the nonlinear electric susceptibilities that giverise to such phenomena are often challenging to tune in a given material, andso far, dynamical control of optical nonlinearities remains confined toresearch labs as a spectroscopic tool. Here, we report a mechanism toelectrically control second-order optical nonlinearities in monolayer WSe2, anatomically thin semiconductor. We show that the intensity of second-harmonicgeneration at the A-exciton resonance is tunable by over an order of magnitudeat low temperature and nearly a factor of 4 at room temperature throughelectrostatic doping in a field-effect transistor. Such tunability arises fromthe strong exciton charging effects in monolayer semiconductors, which allowfor exceptional control over the oscillator strengths at the exciton and trionresonances. The exciton-enhanced second-harmonic generation iscounter-circularly polarized to the excitation laser, arising from thecombination of the two-photon and one-photon valley selection rules that haveopposite helicity in the monolayer. Our study paves the way towards a newplatform for chip-scale, electrically tunable nonlinear optical devices basedon two-dimensional semiconductors.

Electrical Control of Second-Harmonic Generation in a WSe2 Monolayer Transistor