Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons

TL;DR

This paper presents a novel near-field coupling technique to detect dark excitons in monolayer TMDs by enhancing optical transitions with dipoles normal to the 2D plane, overcoming limitations of conventional methods.

Contribution

The authors introduce a new near-field coupling method to directly detect and analyze dark excitons in 2D materials, enabling better understanding of their optical properties.

Findings

Dark excitons detected via near-field coupling to SPPs

Spectral features match dark neutral and charged excitons

Enhanced probing of exciton dynamics in 2D materials

Abstract

Transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons, strong spin-orbit coupling, and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a new method for probing the optical properties of two-dimensional (2D) materials via near-field coupling to surface plasmon polaritons (SPPs), which selectively enhances optical transitions with dipole moments normal to the 2D plane. We utilize this method to directly detect dark excitons in monolayer TMDs. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly enhances experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials.