Optical Stark Effect of Enhanced Charged Biexciton Interactions in Monolayer Semiconductors

ABSTRACT The intrinsically weak exciton‐exciton interactions in monolayer transition metal dichalcogenides (TMDs) limit optical nonlinearities crucial for switching. While valley‐selective manipulation offers pathways for enhanced intra‐ or inter‐valley excitonic coupling, however, the weak interaction strength remains a bottleneck, primarily constrained by the small spatial extent of neutral excitons (X, ∼1 nm radius). To overcome this limitation, we exploit the charged biexciton (CB), a five‐body complex with a significantly expanded radius (∼5 nm). This known extent facilitates greater excitonic wavefunction overlap, thereby enhancing Coulomb repulsion and exchange interactions beyond the capabilities of X. For quantification, we employ valley‐resolved pump‐probe spectroscopy combined with optical Stark effect (OSE) modulation in monolayer WS 2 . Results reveal that CB exhibits a transient shift of ∼6 meV under given pump fluence and detuning parameters, representing a value slightly above a quintupling (7.2‐fold) of the nonlinear interaction strength relative to X. This finding highlights CB's large‐scale capacity for nonlinear amplification, positioning it as a candidate for advancing high‐efficiency optical applications.