Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons (1701.05938v1)

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.

• The paper introduces a near-field SPP coupling technique that directly detects dark excitons in TMD monolayers.
• It shows that out-of-plane dipole transitions can exhibit up to 30 times higher emission rates compared to in-plane dipoles.
• The study highlights the role of spin-orbit coupling in exciton dynamics and its potential for advancing nano-optoelectronic applications.

Probing Dark Excitons in Atomically Thin Semiconductors via Near-Field Coupling to Surface Plasmon Polaritons

This paper addresses the investigation of dark excitons in transition metal dichalcogenide (TMD) monolayers through a novel approach involving near-field coupling to surface plasmon polaritons (SPPs). TMD monolayers are direct bandgap semiconductors with unique excitonic properties, including strong spin-orbit coupling and spin-valley interactions, which give rise to both optically bright and dark excitons. Detecting dark excitons is particularly challenging due to their spin-forbidden transitions and zero in-plane transition dipole moment. This paper introduces an advanced methodology for probing these excitonic states using SPPs, which preferentially enhance optical transitions with out-of-plane dipole orientations, overcoming the limitations of conventional far-field optical techniques.

The researchers utilized exfoliated monolayers of WSe₂ and MoSe₂, encapsulated by hexagonal boron nitride (hBN) layers, placed on a single-crystal silver film. Their technique relies on harnessing the properties of the silver film, such as its ability to support SPPs and act as a gate electrode. This configuration allows for high-resolution photoluminescence (PL) spectroscopy, effectively distinguishing between in-plane and out-of-plane dipole moments by comparing far-field photoluminescence (FF-PL) and SPP-coupled photoluminescence (SPP-PL) spectra.

One of the key findings is the direct detection of a dark exciton in WSe₂, represented by an out-of-plane dipole-coupled spectral feature, which indicates an enhanced emission rate into SPPs. The measured emission rate for an out-of-plane dipole can be 30 times larger than that of an in-plane dipole, showing a significant increase in the detection capabilities for these excitonic states. Additionally, the paper highlights the presence of charged dark excitons, evident in the emergence of features like the L1 peak outside the bandgap region, which brighten significantly at specific carrier densities manipulated via a gate voltage.

The paper further emphasizes the implications of spin-orbit coupling in transition metal dichalcogenides on exciton dynamics. The method developed allows for the detailed probing of nominally spin-forbidden transitions utilizing band structure characterization and group-theoretic analysis. The observations confirm theoretical predictions on transitions involving out-of-plane dipole moments delineated by SOC physics.

On a practical level, this research opens avenues for advancing studies of exciton dynamics and interaction with phononic and dielectric environments in atomically thin materials. The near-field coupling method holds potential utility in exploring indirect excitons, exciton-phonon interactions, and light-matter coupling in van der Waals heterostructures. Additionally, by enabling strong light-matter interactions involving dark excitons, this approach could influence the development of active metasurfaces and quantum photonic applications. The high specificity of dark excitonic states for SPP interactions may further the exploration of spatial modulation techniques and contribute significantly to the understanding of excitonic matter transport and manipulation within two-dimensional systems.

In conclusion, the presented SPP-based near-field spectroscopy technique provides a highly effective tool for the detection and analysis of dark excitons, representing a notable advancement in the paper of two-dimensional semiconductor materials and their complex excitonic properties. Future work could potentially expand on these findings, leveraging the long lifetimes and unique properties of dark excitons for further explorations in nanoscale photonics and optoelectronic device applications.