Influence of local fields on the dynamics of four-wave mixing signals from 2D semiconductor systems

TL;DR

This paper investigates how local fields influence four-wave mixing signals in 2D semiconductors, revealing persistent spectral features due to exciton interactions and providing analytical and numerical insights into their dynamics.

Contribution

It introduces a Bloch equation model with local field effects to analyze four-wave mixing in 2D semiconductors, highlighting the impact of exciton interactions on spectral features over various delays.

Findings

Spectral line splittings are short-lived with two pulses.

Line splittings persist for long delays with three-pulse excitation.

Analytical and numerical results agree on exciton dynamics influence.

Abstract

In recent years the physics of two-dimensional semiconductors was revived by the discovery of the class of transition metal dichalcogenides. In these systems excitons dominate the optical response in the visible range and open many perspectives for nonlinear spectroscopy. To describe the coherence and polarization dynamics of excitons after ultrafast excitation in these systems, we employ the Bloch equation model of a two-level system extended by a local field describing the exciton-exciton interaction. We calculate four-wave mixing signals and analyze the dependence of the temporal and spectral signals as a function of the delay between the exciting pulses. Exact analytical results obtained for the case of ultrafast ($\delta$-shaped) pulses are compared to numerical solutions obtained for finite pulse durations. If two pulses are used to generate the nonlinear signal, characteristic spectral line splittings are restricted to short delays. When considering a three-pulse excitation the line splittings, induced by the local field effect, persist for long delays. All of the found features are instructively explained within the Bloch vector picture and we show how the exciton occupation dynamics govern the different four-wave mixing signals.