Effect of many-body interactions on the valley-selective optical Stark in WS₂

Breaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect can be exploited for quantum valleytronic operations such as coherent manipulation of valley superposition states. The strong light-matter interactions that give rise to the optical Stark effect have historically been described by a two-level dressed-atom model, which assumes non-interacting particles. While this model works well far from resonance where the Rabi frequency is larger than the exciton formation rate, here we show experimentally that it does not apply for excitation near resonance in monolayer WS₂. Instead, our observations are well described by an excitonic model of the optical Stark effect that includes many-body Coulomb interactions. We observe a blue-shift due to the valley-selective optical Stark effect for excitation both below and above resonance, confirming the prediction from this theory that repulsion between virtual excitons dominates the light-matter interactions for photoexcitation detuned from resonance by less than the exciton binding energy. We expect our findings to be general to low-dimensional semiconductors that support bound excitons and other many-body Coulomb interactions.