Unidirectional Hot-Electron Injection Induced Out-Of-Plane Second-Harmonic Generation in Monolayer Transition Metal Dichalcogenides

Nonlinear optics is one of the most important topics in modern optical physics. It has a wide range of applications in the frequency control of lasers, super-resolution optical microscopy, nanoscale surface optics and biosensing. Second-harmonic generation (SHG) is the first discovered nonlinear optical effect that only occurs in physical systems possessing broken inversion symmetry. SHG in transition metal dichalcogenides (TMDCs) has become an extensively studied subject since the atomic thickness of monolayer TMDCs could scale the nonlinear optical devices down to the deep subwavelength scale, useful for on-chip all-optical signal processing and switching. Several notable studies have demonstrated the capability of enhancing and tuning the in-plane polarized SHG in monolayer TMDCs through applying electrical fields, charges and currents.While existing reports provide us some new perspectives on the generation and control of nonlinear optical effects in general TMDCs, more explicit theoretical understanding of those intriguing nonlinear phenomena is lacked. Moreover, it still remains greatly unexplored to induce and tune the out-of-plane polarized SHG in monolayer TMDCs. The present proposal aims to develop a comprehensive theoretical scenario depicting hot-electron injection induced out-of-plane polarized SHG in monolayer WS2, and also provides experimental verification. The theoretical study of this proposal will be carried out in collaboration with Prof. Peter Nordlander, Wiess Chair and Professor of Physics and Astronomy at Rice University. Specifically, we will use comprehensive lattice symmetry analysis to develop an analytical relation between plasmon mode strength, hot-electron generation and injection efficiency, and induced second-order nonlinear susceptibility in a WS2 monolayer sandwiched in a metallic particle-on-film nanocavity. To verify the theoretical prediction, we will carry out a static “pump-probe” experiment where a continuous wave laser at the plasmon resonance wavelength will be used to pump the hybrid nanocavity system for unidirectional injection of hot electrons from the metallic nanostructures to the WS2 layer while a femtosecond pulsed laser will simultaneously excite the system for SHG. Our recent study has corroborated the efficient generation of non-radiative plasmon-decay induced hot electrons and transport into a nearby semiconductor in a similar nanocavity system. Excitation and detection polarization-resolved SHG mapping and spectroscopy measurements as well as pumpprobe power dependence will be carried out to elaborate the origin of the induced SHG. This project is expected to open up a new avenue for manipulating the SHG response of atomically thin TMDCs in an all-optical manner, with great applications potentially in TMDC-based nonlinear optical devices.