Strong coupling of monolayer WS2 excitons and surface plasmon polaritons in a planar Ag/WS2 hybrid structure

TL;DR

This study demonstrates strong light-matter coupling between monolayer WS2 excitons and surface plasmon polaritons in a simple planar Ag/WS2 structure, validated by ellipsometry, simulations, and analytical models, with implications for tunable nanophotonic devices.

Contribution

It provides experimental and theoretical evidence of strong coupling in a minimal planar Ag/WS2 system, advancing understanding of 2D material-plasmon interactions.

Findings

Strong coupling regime achieved between WS2 excitons and SPPs.

Double point topology change indicates transition from weak to strong coupling.

Validated results with analytical and numerical methods.

Abstract

Monolayer (1L) transition metal dichalcogenides (TMDC) are of strong interest in nanophotonics due to their narrow-band intense excitonic transitions persisting up to room temperature. When brought into resonance with surface plasmon polariton (SPP) excitations of a conductive medium opportunities for studying and engineering strong light-matter coupling arise. Here, we consider a most simple geometry, namely a planar stack composed of a thin silver film, an Al2O3 spacer and a monolayer of WS2. We perform total internal reflection ellipsometry which combines spectroscopic ellipsometry with the Kretschmann-Raether-type surface plasmon resonance configuration. The combined amplitude and phase response of the reflected light at varied angle of incidence proves that despite the atomic thinness of 1L-WS2, the strong coupling (SC) regime between A excitons and SPPs propagating in the thin Ag film is reached. The phasor representation of rho corroborates SC as rho undergoes a topology change indicated by the occurrence of a double point at the cross over from the weak to the strong coupling regime. Our findings are validated by both analytical transfer matrix method calculations and numerical Maxwell simulations. The findings open up new perspectives for applications in plasmonic modulators and sensors benefitting from the tunability of the optical properties of 1L-TMDCs by electric fields, electrostatic doping, light and the chemical environment.