Bright and dark singlet excitons via linear and two-photon spectroscopy in monolayer transition-metal dichalcogenides

TL;DR

This paper analyzes the spectroscopic selection rules and exciton symmetries in monolayer transition-metal dichalcogenides, revealing how excitonic effects influence optical properties and comparing theoretical predictions with experiments.

Contribution

It provides a detailed microscopic formalism combining band structure and many-body interactions to clarify exciton symmetries and selection rules in these materials.

Findings

Bright excitons are s-type with one-photon allowed transitions.

Dark p-type excitons can be accessed via two-photon spectroscopy.

The exciton spectrum shows a significant energy splitting due to Coulomb interaction deviations.

Abstract

We discuss the linear and two-photon spectroscopic selection rules for spin-singlet excitons in monolayer transition-metal dichalcogenides. Our microscopic formalism combines a fully $k$-dependent few-orbital band structure with a many-body Bethe-Salpeter equation treatment of the electron-hole interaction, using a model dielectric function. We show analytically and numerically that the single-particle, valley-dependent selection rules are preserved in the presence of excitonic effects. Furthermore, we definitively demonstrate that the bright (one-photon allowed) excitons have $s$-type azimuthal symmetry and that dark $p$-type excitons can be probed via two-photon spectroscopy. The screened Coulomb interaction in these materials substantially deviates from the $1/\varepsilon_0 r$ form; this breaks the "accidental" angular momentum degeneracy in the exciton spectrum, such that the 2$p$ exciton has a lower energy than the 2$s$ exciton by at least 50 meV. We compare our calculated two-photon absorption spectra to recent experimental measurements.