Anomalous Behavior of Valley-Selective Optical Stark Effect Near Resonance in WS₂

Broken inversion symmetry in 2D transition metal dichalcogenides produces valley-specific optical selection rules that fuel interest in future valleytronic applications. Recently, it was discovered that by breaking time-reversal symmetry with intense circularly polarized light fields the K and K’ valley degeneracy can be lifted and each valley can individually tuned via a valley-selective optical Stark effect. This effect produces a blue-shift of the A-exciton resonance absorption at the K-valley while under the influence of a strong sub-resonant right-hand circularly polarized light field. Based on application of the dressed-atom picture, a red-shift is predicted for above-resonant light fields. Here, we examine the optical stark effect in monolayer WS₂ for exciation near the A-exciton resonance. We show the K-valley absorption is blue-shifted even for above resonant co-circularly polarized light fields. The blue-shift is resonantly enhanced, reaching 40 meV under moderate excitation at 610 nm. No Stark shift is observed for the opposite valley, which rules out the influence of intervalley biexcitons. We discuss our findings in terms of breakdown of the two-level dressed-atom picture.