Coherence and density dynamics of excitons in a single-layer MoS$_2$ reaching the homogeneous limit

TL;DR

This study investigates the exciton coherence and density dynamics in a single-layer MoS₂ heterostructure, revealing the dominance of homogeneous broadening in certain regions and the effects of encapsulation on exciton behavior.

Contribution

It provides the first detailed correlation of exciton homogeneous and inhomogeneous broadening in encapsulated MoS₂ using four-wave mixing microscopy.

Findings

Encapsulation reduces surface contamination and exciton broadening.

Localized regions exhibit purely homogeneous broadening.

Weak inhomogeneous broadening observed across the sample.

Abstract

We measure the coherent nonlinear response of excitons in a single-layer of molybdenum disulphide embedded in hexagonal boron nitride, forming a $h$-BN/MoS$_2$/$h$-BN heterostructure. Using four-wave mixing microscopy and imaging, we correlate the exciton homogeneous and inhomogeneous broadenings. We find that the exciton dynamics is governed by microscopic disorder on top of the ideal crystal properties. Analyzing the exciton ultra-fast density dynamics using amplitude and phase of the response, we investigate the relaxation pathways of the resonantly driven exciton population. The surface protection via encapsulation provides stable monolayer samples with low disorder, avoiding surface contaminations and the resulting exciton broadening and modifications of the dynamics. We identify areas localized to a few microns where the optical response is totally dominated by homogeneous broadening. Across the sample of tens of micrometers, weak inhomogeneous broadening and strain effects are observed, attributed to the remaining interaction with the $h$-BN and imperfections in the encapsulation process.