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

TL;DR

This paper demonstrates electrical control of second-harmonic generation in monolayer WSe2, enabling tunable nonlinear optical responses for potential chip-scale photonic devices.

Contribution

It introduces a method to electrically tune second-order nonlinearities in monolayer WSe2 via electrostatic doping, a novel approach for dynamic control of optical nonlinearities.

Findings

Over an order of magnitude tunability at low temperature

Nearly fourfold tunability at room temperature

Counter-circular polarization of the SHG signal

Abstract

Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs as a spectroscopic tool. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of 4 at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser, arising from the combination of the two-photon and one-photon valley selection rules that have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.