Optical fingerprint of dark 2p-states in transition metal dichalcogenides

TL;DR

This paper investigates how disorder influences higher excitonic states in transition metal dichalcogenides, revealing a way to detect dark 2p states through optical spectra and showing disorder can modify optical selection rules.

Contribution

It demonstrates that disorder-induced coupling can reveal dark 2p excitonic states in optical spectra, challenging traditional selection rules in 2D semiconductors.

Findings

Dark 2p states are detectable via absorption spectra.

Disorder cancels degeneracy of 2s and 2p states.

Optical selection rules can be softened by exciton-disorder coupling.

Abstract

We present a microscopic study on higher excitonic states in transition metal dichalcogenides in the presence of disorder. We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of bright and dark states offers a strategy to circumvent optical selection rules. As a prove, we show a direct fingerprint of dark 2p states in absorption spectra of WS$_2$. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors.