Fig. 4: A near flat dispersion of the A excitonic feature and below gap signal intensity at large momentum transfer suggest contributions from exciton polaritons as well as optically ’dark’ excitons.

a Energy-momentum (ωq) map (log scale) for momentum transfer from |q| = 0.1 × |Γ−Γ^′′| to |q| = 0.9 × |Γ−Γ^′′| along Γ → K shows a non-dispersive feature at the A exciton energy for finite q across the Brillouin zone (arrows) including at the M’ point. b EEL spectra for zero momentum transfer (q = 0) and (c) summed finite q show a peak corresponding to the A exciton energy. Higher energy peaks at energies associated with the B and C exciton transitions are also present in the summed finite q spectrum. d ωq map (log scale) for momentum transfer from |q| = 0.1 × |Γ−Γ^′| to |q| = 0.9 × |Γ−Γ^′| along Γ → M shows a non-dispersive feature at the energy corresponding to the A exciton for finite q across the Brillouin zone (arrows) including at the M point. e q-EELS for q = 0 as well as (f) for summed finite q exhibit peaks at energies associated with the A exciton energy as well as higher energy excitonic transitions. When comparing spectra shown in (c) and (f) there is also added intensity below the A excitonic energy (blue arrows) along Γ → K, suggesting the presence of dark excitons at finite q contributing to the overall intensity.