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

Exciton-plasmon polaritons (EPPs) are hybrid modes formed due to the coupling of surface plasmons in metals with excitons in semiconductors. Here we present a nano-optical imaging study of EPPs in heterostructures formed by stacking tungsten diselenide (WSe₂) thin flakes on gold films. By employing the scattering-type scanning near-field optical microscopy, we mapped the interference fringes of EPPs at various excitation energies, based on which we constructed the dispersion relations of these polaritonic modes. We found that the polariton dispersion demonstrates sensitive dependence on the thickness of WSe₂ flakes. When the thickness of WSe₂ is below 40 nm, EPPs tend to be plasmon-like surface modes. As the thickness surpasses 40 nm, EPPs behave as waveguide polaritons. The finite-element polaritonic simulations further confirm these observations. Our work uncovers in real-space the transport properties of EPPs and paves the way for future applications of these modes in nanophotonic circuits and devices.