Nano-optical imaging of exciton-plasmon polaritons in WSe 2 /Au heterostructures.

We report a nano-optical imaging study of exciton-plasmon polaritons (EPPs) in WSe 2 /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 WSe 2 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 WSe 2 . Our work uncovers the transport properties of EPPs and paves the way for future applications of these highly confined polaritons in nanophotonics and optoelectronics.