Optical harmonic generation in monolayer group-VI transition metal dichalcogenides

TL;DR

This study measures and compares the second- and third-order nonlinear optical susceptibilities of four monolayer TMDs, providing reliable data and theoretical insights to identify optimal materials for nonlinear photonic applications.

Contribution

It introduces a method to accurately compare NLO susceptibilities of multiple TMDs on a single substrate and provides the first comprehensive comparison of these properties across four common TMD materials.

Findings

Distinct NLO responses observed among the four TMDs.

Experimental susceptibilities agree qualitatively with theoretical simulations.

Identifies promising TMDs for nonlinear photonic devices.

Abstract

Monolayer transition metal dichalcogenides (TMDs) exhibit high nonlinear optical (NLO) susceptibilities. Experiments on MoS$_2$ have indeed revealed very large second-order ($\chi^{(2)}$) and third-order ($\chi^{(3)}$) optical susceptibilities. However, third harmonic generation results of other layered TMDs has not been reported. Furthermore, the reported $\chi^{(2)}$ and $\chi^{(3)}$ of MoS$_2$ vary by several orders of magnitude, and a reliable quantitative comparison of optical nonlinearities across different TMDs has remained elusive. Here, we investigate second- and third-harmonic generation, and three-photon photoluminescence in TMDs. Specifically, we present an experimental study of $\chi^{(2)}$, and $\chi^{(3)}$ of four common TMD materials (\ce{MoS2}, \ce{MoSe2}, \ce{WS2} and \ce{WSe2}) by placing different TMD flakes in close proximity to each other on a common substrate, allowing their NLO properties to be accurately obtained from a single measurement. $\chi^{(2)}$ and $\chi^{(3)}$ of the four monolayer TMDs have been compared, indicating that they exhibit distinct NLO responses. We further present theoretical simulations of these susceptibilities in qualitative agreement with the measurements. Our comparative studies of the NLO responses of different two-dimensional layered materials allow us to select the best candidates for atomic-scale nonlinear photonic applications, such as frequency conversion and all-optical signal processing.