Fig. 2: Layer- and valley-resolved ultrafast dynamics upon below-bandgap pumping.
a–d Energy-momentum cuts of the photoemission signal along the \({{{{{{{{\rm{{K}}}}}}}^{{\prime} }}}}_{{{{{{{{\rm{Gr}}}}}}}}}\)- \({{{{{{{{\rm{{K}}}}}}}^{{\prime} }}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{2}}\)-Γ-\({{{{{{{{\rm{K}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{2}}\)-KGr high symmetry direction, at selected pump-probe time delays. a The 2D spectrum at negative time delay reveals the equilibrium band structure of ML-WSe2 as well as the linearly dispersing π band of graphene. The gray lines represent the DFT-calculated band structures (details in methods). Snapshots of the energy-momentum cuts at time delays of b Δt = 0 fs, c Δt = 200 fs, and d Δt = 400 fs, respectively. e Momentum map of the excited states (energy integrated for E > EF and time integrated for the first 400 fs), showing the KGr points of graphene (black box) as well as the \({{{{{{{{\rm{K}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) and \({{{{{{{{\rm{Q}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) valleys (red and green boxes, respectively). The dashed gray lines represent the hexagonal Brillouin zones of both layers. f Normalized population dynamics within the three ROIs defined in e: \({{{{{{{{\rm{K}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) (red) and \({{{{{{{{\rm{Q}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) (green) are populated with a ~50 fs delay with respect to KGr (black). g Schematic of the early-time carrier dynamics upon below-bandgap excitation: photo-generated hot carriers within the graphene layer are transferred to the conduction bands of WSe2 via hot electron injection after the thermalization.
