Coherent and Incoherent Coupling Dynamics between Neutral and Charged Excitons in Monolayer MoSe2

TL;DR

This study uses optical two-dimensional coherent spectroscopy to distinguish between coherent and incoherent exciton-trion interactions in monolayer MoSe2, revealing their timescales and underlying mechanisms relevant for optoelectronic applications.

Contribution

It provides the first direct observation of coherent exciton-trion coupling and characterizes the incoherent energy transfer processes in monolayer MoSe2.

Findings

Coherent exciton-trion coupling shows quantum beating lasting a few hundred femtoseconds.

Incoherent energy transfer increases cross-coupling peak intensity up to 10 ps.

Phonon-assisted processes facilitate energy transfer even at cryogenic temperatures.

Abstract

The optical properties of semiconducting transition metal dichalcogenides are dominated by both neutral excitons (electron-hole pairs) and charged excitons (trions) that are stable even at room temperature. While trions directly influence charge transport properties in optoelectronic devices, excitons may be relevant through exciton-trion coupling and conversion phenomena. In this work, we reveal the coherent and incoherent nature of exciton-trion coupling and the relevant timescales in monolayer MoSe2 using optical two-dimensional coherent spectroscopy. Coherent interaction between excitons and trions is definitively identified as quantum beating of cross-coupling peaks that persists for a few hundred femtoseconds. For longer times up to 10 ps, surprisingly, the relative intensity of the cross-coupling peaks increases, which is attributed to incoherent energy transfer likely due to phonon-assisted up-conversion and down-conversion processes that are efficient even at cryogenic temperature.