Fig. 3. An almost perfect valley polarization.

(A and B) Resident holes in the K valley induce distinctively different absorption changes for the K (A) and K′ (B) valleys; RC of the heterostructure (blue solid line) is also shown for comparison. (A) The absorption change in the K valley features an overall reduction of the absorption oscillator strength and a slight blueshift of the exciton resonance. The inset illustrates the change of absorption spectrum, where the original exciton absorption (black dashed line) is modified in the presence of holes at the K valley (red solid line). This spectral change can be understood by the phase-space filling and Burstein-Moss effects. (B) The absorption change in the K′ valley shows a transfer of oscillator strength from exciton to trion absorption due to the formation of intervalley trions. However, the total oscillator strength is unaffected because there is no Pauli blocking effect. The inset illustrates the reduction of exciton absorption and emergence of intervalley trion absorption (red solid line) compared to the original spectrum (black dashed line). (C) The density of resident holes in the K and K′ valleys obtained by the integrated oscillator strength change of LCP and RCP light, which suggests near-perfect valley polarization (within 10% experimental uncertainty). The total hole density is normalized to 1.