Fig. 2: Valley polarization and helicity of the high-lying trion in UPL from monolayer WSe₂.

a, b Gate-voltage dependence of co-polarized (a) and cross-polarized (b) UPL under left-hand circularly polarized excitation. c Helicity of the UPL as a function of gate voltage. d–f Polarization resolved UPL spectra at gate voltages of 0.2 V (d), −0.15 V (e), and −0.5 V (f). g Illustration of resonant pumping of the A exciton in the +K valley with a σ⁺ circularly polarized CW laser. This excitation selectively polarizes resident spin-down electrons in the conduction band, CB. The inset illustrates the Auger-like exciton-exciton annihilation process that promotes the electron to high-lying conduction bands. h Schematic of negatively charged high-lying trion in the −K valley with σ⁺ polarized emission and a resident spin-down electron in the −K valley. i Schematic of the positively charged high-lying trion in the +K valley with σ⁻ polarized emission.