Radiatively limited dephasing and exciton dynamics in MoSe$_2$ monolayers

TL;DR

This study uses four-wave mixing micro-spectroscopy to investigate exciton coherence, dephasing, and dynamics in MoSe₂ monolayers at cryogenic temperatures, revealing radiatively limited dephasing and inter-valley exciton behavior.

Contribution

It provides the first detailed measurement of exciton dephasing times approaching the radiative limit in MoSe₂ monolayers at 6 K, and explores exciton-trion coupling and local disorder effects.

Findings

Dephasing times T₂ approach twice the lifetime T₁, indicating radiative limit.

Observation of exciton and trion coherence and their coupling.

Detection of polarization dynamics and exciton relaxation pathways.

Abstract

By implementing four-wave mixing (FWM) micro-spectroscopy we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe$_2$. We reveal their dephasing times T$_2$ and radiative lifetime T$_1$ in a sub-picosecond (ps) range, approaching T$_2$=2T$_1$, and thus indicating radiatively limited dephasing at a temperature of 6$\,$K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At a nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse towards the dark states, but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer inter-valley exciton dynamics, showing an initial polarization degree of around 30$\,\%$, constant during the initial sub-picosecond decay, followed by the depolarization on a picosecond timescale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing to investigate the neutral exciton, the charged one or both transitions at the same time. In the latter case, we observe the exciton-trion beating in the coherence evolution indicating their coherent coupling.