Nano imaging of exciton polaritons in van der Waals semiconductor WSe₂

Transition metal dichalcogenides (TMDs), such as WSe₂, are a class of atomically thin, layered semiconductor materials, exhibiting strong many-body effects due to low dielectric permittivities and their high effective carrier masses. By integrated in an optical cavity, TMDs may form exciton polaritons given the Rabi frequency exceeds exciton decay rate. However, most reports on propagation exciton polariton to date in WSe₂ investigate the lower-k part of the exciton polariton–guided waves, where the polaritons have primarily a photon character. We report nano-optical imaging of exciton-polaritons in WSe₂ with scattering-type scanning near-field optical microscopy (s-SNOM) to probe the strongly confined, large-k part of the exciton-polariton dispersion. We observed the response of B-exciton-polariton in WSe₂ with long propagation lengths at room temperature. By using the near field nanoscopy with a ~20 nm spatial resolution, we extracted the polariton wavelength of ~ 500 nm along the crystal axis. Based on the spatial period of these quantum waves, we directly establish the frequency-momentum dispersion profile of high-k exciton polaritons in WSe₂. We further performed chemical intercalation to study the effect of Li-ion intercalation on the optical and structural properties of WSe₂. Using STEM and PL/Absorption measurements before and after intercalation, we characterize the role of interaction in tuning the optical properties of WSe₂. Our work uncovers the properties of B-exciton polaritons in WSe₂ and paves the way to probe the highly confined exciton polariton condensates for nanophononics applications, and their interaction with a 2D electron system of vdW materials.