Fig. 3: At the onset, exciton polaritonic modes were found to exhibit a near flat momentum-energy dispersion relation in ≈12 nm thin films of WSe2.
From: Flat dispersion at large momentum transfer at the onset of exciton polariton formation
a Schematic showing the q space of WSe2 and the high symmetry directions along which the energy-momentum maps were acquired. The central diffraction spot is found at Γ and the high symmetry directions are Γ → M (red) and Γ → K (green). Offset data acquisition along Γ′ → M′′ (yellow), equivalent crystallographic vectors to Γ → M (red), improved signal-to-noise. b The calculated energy-momentum q-EELS for a 10 nm thick sample of WSe2 suggest that the exciton polariton dispersion for A and B excitons are expected to be flat for very small momentum q. c Energy-momentum (ωq) map (in log scale) for momentum transfer from |q| = 0.1 × |Γ−Γ′′| to |q| = 0.9 × |Γ−Γ′′| along Γ → K shows a non-dispersive loss feature at the A exciton polariton energy for finite q across the Brillouin zone (arrows). d q-EEL spectra for zero momentum transfer (q = 0) and (e) summed finite momentum q show a peak at the A exciton energy at 1.75 ± 0.01 eV and B exciton energy at ≈2.3 eV respectively. f 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 polariton for finite q across the Brillouin zone (arrows). The peak at the B exciton energy (denoted with B) is also showing a non-dispersive behaviour. Again, (g) q-EEL spectra for zero momentum transfer (q = 0) and (h) summed finite momentum q show a peak for A excitons at 1.75 ± 0.01 eV and B excitons at ≈2.3 eV respectively.
