The role of non-equilibrium populations in dark exciton formation

TL;DR

This study investigates how non-equilibrium populations influence dark exciton formation in 2D transition metal dichalcogenides, revealing detailed relaxation dynamics through combined experimental and theoretical approaches.

Contribution

The paper provides the first direct experimental and theoretical analysis of the full exciton relaxation cascade, including dark exciton formation and thermalization timescales in 2H-MoS2.

Findings

Dark excitons show a distinct non-equilibrium occupation distribution.

Formation and thermalization timescales are 85 fs and 150 fs, respectively.

The results agree with microscopic many-particle calculations.

Abstract

In two-dimensional transition metal dichalcogenide structures, the optical excitation of a bright exciton may be followed by the formation of a plethora of lower energy dark states. In these formation and relaxation processes between different exciton species, non-equilibrium exciton and phonon populations play a dominant role, but remain so far largely unexplored as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2H-MoS$_2$, we realize direct access to the full exciton relaxation cascade from experiment and theory. By measuring the energy- and in-plane momentum-resolved photoemission spectral function, we reveal a distinct fingerprint for dark excitons in a non-equilibrium excitonic occupation distribution. In excellent agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a non-equilibrium dark excitonic occupation and its subsequent thermalization to 85~fs and 150~fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of paramount importance for the future characterization of non-equilibrium excitonic phases and the efficient design of optoelectronic devices based on two-dimensional materials.