Probing Dark Excitons in Monolayer MoS$_2$ by NonLinear Two-Photon Spectroscopy

TL;DR

This paper discovers a new dark exciton in monolayer MoS$_2$ using second harmonic generation spectroscopy, revealing quantum interference effects and potential for nonlinear quantum device applications.

Contribution

It reports the first observation of a dark exciton in monolayer MoS$_2$ via nonlinear two-photon spectroscopy and explores its quantum interference and relaxation mechanisms.

Findings

Observation of a dark exciton via second harmonic generation

Anticrossing behavior indicating quantum interference

Potential for controlling relaxation pathways with nanostructuring

Abstract

We report a new dark exciton in monolayer MoS$_2$ using second harmonic generation spectroscopy. Hereby, the spectrally dependent second harmonic generation intensity splits into two branches, and an anticrossing is observed at $\sim$ 25 meV blue detuned from the bright neutral exciton. These observations are indicative of coherent quantum interference arising from strong two-photon light-matter interaction with an excitonic state that is dark for single photon interaction. The existence of the dark state is supported by engineering its relaxation to bright localized excitons, mediated by vibrational modes of a proximal nanobeam cavity. We show that two-photon light-matter interaction involving dark states has the potential to control relaxation pathways induced by nanostructuring the local environment. Moreover, our results indicate that dark excitons have significant potential for nonlinear quantum devices based on their nontrivial excitonic photophysics.