Strong Coupling of Excitons in Monolayer WS₂ and Surface Plasmon Polaritons of a Silver Film

Monolayer (ML) transition metal dichalcogenides (TMDC) are currently of strong interest in nanophotonics due to their narrow-band intense excitonic transition which persist even up to room temperature. When brought into resonance with electromagnetic fields of a plasmonic nanostructure unique opportunities for studying and engineering strong light-matter coupling arise. This has motivated quite intense research efforts in the past years and indeed indications of strong coupling of TMDC excitons of ML flakes with localized surface plasmons hosted in a variety of metallic nanostructures have been found in scattering spectra. Here, we propose an alternative geometry as well as experimental method to study the exciton-plasmon coupling. We consider a most simple planar geometry comprised of a thin Ag film, an Al₂O₃ spacer layer and a ML of WS₂ and we employ 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 proves that despite the ML thinness of WS₂, the strong coupling regime between the A-excitonic resonance and the surface plasmon polariton propagating in the Ag film is achieved. We show that the presentation of the phasor <i>r</i>_p/<i>r</i>_s in the complex plane (<i>r</i>_p and <i>r</i>_s are the reflection coefficients in p- and s-polarization, respectively) provides a convenient means to verify the strong coupling regime. Furthermore, ellipsometry measurements at varying angle of incidence yield the dispersion plot. The observed anticrossing of the two polariton branches confirms that the strong coupling regime is achieved. Transfer matrix method simulations were performed to support the experimental findings. The present planar geometry can be regarded as a building block for the realization of all-optical or electro-optical plasmonic modulators. The tunability of the TMDC optical properties via external stimuli like electric fields and light, as well as the dielectric/chemical environment provides means of active control over the light matter interaction which paves the way for the construction of novel plasmonic devices.