Nano-optical imaging of exciton-plasmon polaritons in WSe₂/Au heterostructures

We report a nano-optical imaging study of exciton-plasmon polaritons (EPPs) in WSe₂/Au heterostructures with scattering-type scanning near-field optical microscopy (s-SNOM). By mapping the interference fringes of EPPs at various excitation energies, we constructed the dispersion diagram of the EPPs, which shows strong exciton-plasmon coupling with a sizable Rabi splitting energy (∼0.19 eV). Furthermore, we found a sensitive dependence of the polariton wavelength (λ_p) on WSe₂ thickness (d). When d is below 40 nm, λ_p decreases rapidly with increasing d. As d reaches 50 nm and above, λ_p drops to 210 nm, which is over 4 times smaller than that of the free-space photons. Our simulations indicate that the high spatial confinement of EPPs is due to the strong localization of the polariton field inside WSe₂. Our work uncovers the transport properties of EPPs and paves the way for future applications of these highly confined polaritons in nanophotonics and optoelectronics.