  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.
