Charting the Exciton–Polariton Landscape of WSe 2 Thin Flakes by Cathodoluminescence Spectroscopy

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 works have focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Herein, the optical response of such freestanding planar waveguides of WSe 2 by means of cathodoluminescence spectroscopy is determined. 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–Pérot resonances are revealed. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor mode energies, spin–momentum couplings, and the exciton–photon coupling strength, as well as to promote photon‐mediated exciton–exciton interactions. The 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 beyond the routinely employed two‐oscillator coupling effects.