  Van der Waals materials such as thin films of transition-metal
dichalcogenides (TMDCs) manifest strongly bound exciton states in the visible
spectrum at ambient conditions that provide an ideal platform for
exciton-photon couplings. Utilizing semiconducting TMDCs in the form of
multilayer structure combined with metals can increase significantly the
light-matter interaction. In this way, the interaction between excitons and
surface-plasmon polaritons emerge as a platform for transferring the
electromagnetic energy at confined modal volumes. Here, we theoretically
investigate how moving electrons can be used as a probe of hybrid
exciton-plasmon polaritons of gold-WSe2 multilayers within the context of
electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL)
spectroscopy. Interestingly, and in contrast to WSe2 slab waveguides where
quasi-propagating photonic modes interact with only exciton A, in gold-WSe2
multilayer, exciton A and exciton B can both strongly interact with surface
plasmon polaritons. Hence, we observe CL emission suppression at excitonic or
plasmonic peaks, which reveals the energy transfer between excitons and
plasmons in the form of nonradiating guided waves. Our work provides a
systematic study for deeper understanding of the effect of the configuration
and the thickness of layers on the photonic and plasmonic modes and hence on
the strength of the coupling between excitons and surface-plasmon polaritons.
Our findings pay the way for designing efficient photodetectors, sensitive
sensors, and light emitting devices based on metal/semiconducting hybrid
materials.
