Phonon-assisted inter-valley scattering determines ultrafast exciton dynamics in MoSe${}_2$ bilayers

Abstract

While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster time scale in MoSe${}_2$ bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the $K$ valley to other lower energy $\Gamma$ and $\Lambda$ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to manipulation of spin/valley degrees of freedom in TMDC bilayers.

arXiv comment6 pages, 4 figures

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• Highly nonlinear dipolar exciton-polaritons in bilayer MoS${}_2$
• Time-linear scaling NEGF methods for real-time simulations of interacting electrons and bosons. I. Formalism
• Nonlinear optics in 2D materials: from classical to quantum
• First Principle Study for Optical Properties of TMDC/Graphene Heterostructures
• Magnetically tunable exciton valley coherence in monolayer WS${}_2$ mediated by the electron-hole exchange and exciton-phonon interactions
• On the importance of electron-electron and electron-phonon scatterings and energy renormalizations during carrier relaxation in monolayer transition-metal dichalcogenides
• Two-dimensional polaron spectroscopy of Fermi superfluids
• Laser-field detuning assisted optimization of exciton valley dynamics in monolayer WSe${}_2$: Geometric quantum speed limit
• Fundamentals of heterodyne wave mixing spectroscopy: a tutorial
• Fast Interlayer Energy Transfer from the Lower Bandgap MoS2 to the Higher Bandgap WS2