  We report on nano-optical imaging study of WSe2 thin flakes with the scanning
near-field optical microscopy (NSOM). The NSOM technique allows us to visualize
in real space various waveguide photon modes inside WSe2. By tuning the
excitation laser energy, we are able to map the entire dispersion of these
waveguide modes both above and below the A exciton energy of WSe2. We found
that all the modes interact strongly with WSe2 excitons. The outcome of the
interaction is that the observed waveguide modes shift to higher momenta right
below the A exciton energy. At higher energies, on the other hand, these modes
are strongly damped due to adjacent B excitons or band edge absorptions. The
mode-shifting phenomena are consistent with polariton formation in WSe2.
