Fig. 1: Time- and angle-resolved photoemission measurement of interlayer charge and energy transfer in a ML-WSe₂/graphene heterostructure.

a Following the near-infrared pump, electrons are photoionized by the delayed XUV-probe pulses and collected by a three-dimensional (3D) (E_kin, k_x, k_y) detector as a function of pump-probe delay Δt. b The 3D snapshot of the 4D data, I(E_kin, k_x, k_y, Δt = 0 fs) presents the valence band structures from the Γ point to the Brillouin zone boundary of WSe₂, as well as the linearly dispersing graphene bands. The excited state population can be clearly mapped at the \({{{{{{{{\rm{K}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) and \({{{{{{{{\rm{Q}}}}}}}}}_{{{{{{{{{\rm{WSe}}}}}}}}}_{{{{{{{{\rm{2}}}}}}}}}}\) valleys, and the π^* band of graphene (K_Gr). c By changing the pump wavelength, we can selectively prepare different initial excited states: quasi-free carriers in graphene with below-bandgap excitation (red arrow) or excitons in WSe₂ using excitation on the excitonic resonance (blue arrow).