Nonlinear spectroscopy of excitonic states in transition metal dichalcogenides

TL;DR

This paper explores how second-harmonic generation (SHG) spectroscopy can be used to analyze excitonic states in transition metal dichalcogenides (TMDCs), revealing how excitons influence nonlinear optical responses and their dependence on environment and exciton type.

Contribution

It demonstrates the impact of excitons on nonlinear dynamics in TMDC monolayers and highlights SHG spectroscopy as a sensitive tool for excitonic fine structure analysis.

Findings

Excitons cause a significant enhancement of the nonlinear signal.

SHG polarization diagrams are highly sensitive to exciton type.

Nonlinear dynamics are qualitatively altered by strong excitation and environment.

Abstract

Second-harmonic generation (SHG) is a well-known nonlinear spectroscopy method to probe electronic structure, specifically, in transition metal dichalcogenide (TMDC) monolayers. This work investigates the nonlinear dynamics of a strongly excited TMDC monolayer by solving the time evolution equations for the density matrix. It is shown that the presence of excitons qualitatively changes the nonlinear dynamics leading, in particular, to a huge enhancement of the nonlinear signal as a function of the dielectric environment. It is also shown that the SHG polarization angular diagram and its dependence on the driving strength are very sensitive to the type of exciton state. This sensitivity suggests that SHG spectroscopy is a convenient tool for analyzing the fine structure of excitonic states.