Mapping of the dark exciton landscape in transition metal dichalcogenides

TL;DR

This study reveals the spectral positions of dark excitons in transition metal dichalcogenides using intra-excitonic transitions, demonstrating the transition from bright to dark states in monolayer WSe2 through combined theoretical and experimental analysis.

Contribution

It introduces a method to experimentally map dark exciton states in TMDs by probing intra-excitonic transitions, clarifying their spectral positions and impact on material properties.

Findings

Dark exciton states can be revealed via intra-excitonic 1s-2p transition measurements.

A blue-shift in optical response indicates transition from bright to dark excitons.

Dark states are lower in energy, affecting the direct-gap nature of TMDs.

Abstract

Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including optically accessible (bright) as well as spin- and momentum-forbidden (dark) excitonic states. So far the dark exciton landscape has not been revealed leaving in particular the spectral position of momentum-forbidden dark states completely unclear. This has a significant impact on the technological application potential of TMDs, since the nature of the energetically lowest state determines, if the material is a direct-gap semiconductor. Here, we show how dark states can be experimentally revealed by probing the intra-excitonic 1s-2p transition. Distinguishing the optical response shortly after the excitation (< 100$\,$fs) and after the exciton thermalization (> 1$\,$ps) allows us to demonstrate the relative position of bright and dark excitons. We find both in theory and experiment a clear blue-shift in the optical response demonstrating for the first time the transition of bright exciton populations into lower lying momentum- and spin-forbidden dark excitonic states in monolayer WSe$_2$.