Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons

Yunyun Dai,Kaihui Liu,Dong Sun,Xiaoxia Yang,Tawfique Hasan,Susobhan Das,Yadong Wang,Zhipei Sun,Harri Lipsanen,Shiwei Wu,F Javier García De Abajo,Qing Dai,Shisheng Li,Fadıl İyikanat ,Masood Ghotbi,Jinluo Cheng,Xiaojie Guo ,Fangwei Ye,Xuerong Hu,Xuetao Gan

doi:10.1021/acsphotonics.1c00466

Abstract

All-optical control of nonlinear photonic processes in nanomaterials is of significant interest from a fundamental viewpoint and with regard to applications ranging from ultrafast data processing to spectroscopy and quantum technology. However, these applications rely on a high degree of control over the nonlinear response, which still remains elusive. Here, we demonstrate giant and broadband all-optical ultrafast modulation of second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides mediated by the modified excitonic oscillation strength produced upon optical pumping. We reveal a dominant role of dark excitons to enhance SHG by up to a factor of ∼386 at room temperature, 2 orders of magnitude larger than the current state-of-the-art all-optical modulation results. The amplitude and sign of the observed SHG modulation can be adjusted over a broad spectral range spanning a few electronvolts with ultrafast response down to the sub-picosecond scale via different carrier dynamics. Our results not only introduce an efficient method to study intriguing exciton dynamics, but also reveal a new mechanism involving dark excitons to regulate all-optical nonlinear photonics.

Highlights

Second-harmonic generation (SHG), a nonlinear optical process originating in the second-order response of noncentrosymmetric materials, is arguably the most commonly used nonlinear optical effect.[1]
Our results confirm that SHG modulation is strongly related to dark excitonic states, with the SHG modulation being enhanced by the creation of dark
We have demonstrated giant all-optical modulation of SHG mediated by excitons in monolayer transition metal dichalcogenides (TMDs)

Summary

Introduction

Second-harmonic generation (SHG), a nonlinear optical process originating in the second-order response of noncentrosymmetric materials, is arguably the most commonly used nonlinear optical effect.[1]. The reported modulation of SHG in noncentrosymmetric materials is generally very weak (typically with enhancement factors ≲ 4). This lack of efficient all-optical modulation strategies represents a major bottleneck toward the development of emerging and future applications, such as quantum photonics and on-chip nonlinear devices. Excitonic Rydberg states exhibit general characteristics of hydrogen-like atoms, possessing a series of discrete optically accessible (bright; 1s, 2s, ...) and optically forbidden (dark; 2p, 3p, ...) states, as determined by optical selection rules.[22] Through strong resonant enhancement of bright excitons, SHG can be actively tuned using several methods, such as electrical and chemical doping.[19−21,23−27] Received: March 28, 2021 Published: July 13, 2021

Methods

Results

Conclusion