Optical nano-imaging of waveguide exciton polaritons in transition-metal dichalcogenides

Exciton polaritons, which are collective oscillations of photons and excitons in semiconductors, trigger tremendous research interests in both fundamental physics and technological applications. Previous studies retain to spectroscopic studies of exciton polaritons confined in microcavities. Here, we report on optical nano-imaging study of waveguide exciton polaritons of thin flakes of transition-metal dichalcogenides (TMDCs) using the near-field scanning optical microscopy. The observed polaritons are formed by strong coupling between waveguide photons and A excitons in TMDCs. The wavelength of these exciton polaritons can reach as low as 300 nm. By tuning the laser frequency, we are able to map the entire polariton dispersion both above and below the A exciton energy. Further analysis indicates that polaritons in the lower-energy branch have a propagation length over many microns while the modes in the upper-energy branch are strongly damped due to the Landau damping.