# Resonantly excited exciton dynamics in two-dimensional MoSe$_2$ monolayers

## TL;DR

This study investigates exciton and trion dynamics in monolayer MoSe$_2$ using resonant three-pulse FWM, revealing ultrafast radiative decay, dark state scattering, and trion ionization, with implications for understanding excitonic processes in 2D materials.

## Contribution

The paper provides a detailed model of exciton and trion dynamics in MoSe$_2$ monolayers using resonant FWM, highlighting intrinsic band structure effects without extrinsic influences.

## Key findings

- Exciton radiative lifetime is sub-picosecond.
- Dark state scattering occurs over 10 ps to 10 ns.
- Trion decay occurs in about 1 ps, with ionization happening within tens of picoseconds.

## Abstract

We report on the exciton and trion density dynamics in a single layer of MoSe$_2$, resonantly excited and probed using three-pulse four-wave mixing (FWM), at temperatures from 300K to 77K . A multi-exponential third-order response function for amplitude and phase of the heterodyne-detected FWM signal including four decay processes is used to model the data. We provide a consistent interpretation within the intrinsic band structure, not requiring the inclusion of extrinsic effects. We find an exciton radiative lifetime in the sub-picosecond range consistent to what has been recently reported. After the dominating radiative decay, the remaining exciton density, which has been scattered from the initially excited bright radiative state into dark states of different nature by exciton-phonon scattering or disorder scattering, shows a slower dynamics, covering 10ps to 10ns timescales. This includes direct bright transitions with larger in-plane momentum, as well as indirect dark transitions to indirect dark states. We find that exciton-exciton annihilation is not relevant in the observed dynamics, in variance from previous finding under non-resonant excitation. The trion density at 77K reveals a decay of the order of 1ps, similar to what is observed for the exciton. After few tens of picoseconds, the trion dynamics resembles the one of the exciton, indicating that trion ionization occurs on this timescale.