Fig. 4: Ultrafast changes of the hidden spin polarization of the WSe₂ bulk band structure.

a Time- and spin-resolved photoemission yield (out-of-plane spin component) of the valence band structure (see white dashed line) at three characteristic time delays. The data were recorded at the same electron momentum as the static data in Fig. 1c. The red and blue curves represent the fit to the spin-up and spin-down spectrum, respectively. The contributions of the first and second-layer valence bands to the spectral yield are fitted and illustrated as green and blue Gaussian curves underneath the spectra. The vertical solid lines indicate the significantly larger shift of the valence band of the first WSe₂ layer compared to the second layer. b Energy level diagram illustrating the ultrafast changes of the layer- and spin-dependent WSe₂ valence band structure after optical excitation with 3.2 eV photons. The gray horizontal dashed lines indicate the energy level alignment in the ground state, i.e., before the optical excitation (t < t₀), while the black dashed lines highlight the layer-dependent shift of the first and second layer WSe₂ valence band after optical excitation (t > t_CT).