  The optical responses of semiconducting transition metal dichalcogenides are
dominated by excitons. Being able to strongly interact with light and other
materials excitations, excitons in semiconductors are prototypes for
investigating many-particle and strong-field physics, including
exciton-exciton, exciton-photon, and exciton-phonon interactions. Strong
exciton-photon interactions, in particular, can lead to the emergence of
exciton-polariton hybrid quasiparticles with peculiar characteristics, and a
tendency toward macroscopic and spontaneous coherence. Normally, far-field and
near-field optical spectroscopy techniques are used to investigate
exciton-photon interactions. Here, we demonstrate that the radiation generated
by moving electrons in transition metal dichalcogenides, namely Cherenkov
radiation, can strongly interact with excitons. We investigate the coherence
properties and spectral signatures of exciton-photon interactions in TMDC bulk
crystals, using cathodoluminescence spectroscopy. Our findings lay the ground
for cathodoluminescence spectroscopy and in particular electron-beam techniques
as probes of exciton-polariton spontaneous coherence in semiconductors, beyond
the well-known plasmonic investigations.
