We measure the coherent nonlinear response of excitons in a single-layer of molybdenum disulphide embedded in hexagonal boron nitride, forming a h-BN/MoS2/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.
@article{arxiv.1810.00039,
title = {Coherence and density dynamics of excitons in a single-layer MoS$_2$ reaching the homogeneous limit},
author = {Tomasz Jakubczyk and Goutham Nayak and Lorenzo Scarpelli and Francesco Masia and Wei-Lai Liu and Sudipta Dubey and Nedjma Bendiab and Laëtitia Marty and Takashi Taniguchi and Kenji Watanabe and Gilles Nogues and Johann Coraux and Vincent Bouchiat and Wolfgang Langbein and Julien Renard and Jacek Kasprzak},
journal= {arXiv preprint arXiv:1810.00039},
year = {2018}
}