Momentum-resolved observation of exciton formation dynamics in monolayer WS$_2$

TL;DR

This study uses advanced momentum-resolved spectroscopy to observe ultrafast exciton formation and dark state dynamics in monolayer WS$_2$, revealing how excitation energy influences these processes.

Contribution

It introduces a novel experimental approach combining momentum imaging with ultrafast spectroscopy to directly observe dark exciton formation in monolayer WS$_2$.

Findings

Observation of dark KΣ exciton formation within tens of femtoseconds.

Manipulation of excitonic polarization through excitation energy tuning.

Excellent agreement with microscopic theoretical models.

Abstract

The dynamics of exciton formation in transition metal dichalcogenides is difficult to measure experimentally, since many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS$_2$. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast timescale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic timescale and observe the formation of a momentum-forbidden dark K$\Sigma$ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS$_2$.