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

Monolayer (1L) transition-metal dichalcogenides (TMDCs) 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 arise for studying and engineering strong light-matter coupling. Here, we consider a very 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 angles 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 = r(p)/r(s), where r(p) and r(s) are the Fresnel refection coefficients in p- and s-polarization, respectively, corroborates SC as rho undergoes a topology change indicated by the occurrence of a double point at the crossover 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.