  Semiconducting transition-metal dichalcogenides (TMDCs) provide a fascinating
discovery platform for strong light-matter interaction effects in the visible
spectrum at ambient conditions. While most of the work has focused on
hybridizing excitons with resonant photonic modes of external mirrors,
cavities, or nanostructures, intriguingly, TMDC flakes of sub-wavelength
thickness can themselves act as nanocavities. Here, we determine the optical
response of such freestanding planar waveguides of WSe$_2$, by means of
cathodoluminescence spectroscopy. We reveal strong exciton-photon interaction
effects that foster long-range propagating exciton-polaritons and enable direct
imaging of the energy transfer dynamics originating from cavity-like
Fabry-Perot resonances. Furthermore, confinement effects due to discontinuities
in the flakes are demonstrated as an efficient means to tailor the
exciton-photon coupling strength, along the edges of natural flakes. Our
combined experimental and theoretical results provide a deeper understanding of
exciton-photon self-hybridization in semiconducting TMDCs and may pave the way
to optoelectronic nanocircuits exploiting exciton-photon interaction.
