Electrical Tuning of Optical Stark Effect in Monolayer WS ₂

Upon excitation by a strong coherent light field, photon-dressed states form and hybridize with equilibrium states, leading to transient band renormalization, a phenomenon known as the optical Stark effect (OSE). Extensive research has been conducted on the optical control of the OSE by modulating the optical pump fluence, detuning energy, and polarization. Herein, we present the electrical tuning of valley-selective OSE in a back-gated monolayer WS₂ device using helicity-resolved transient absorption spectroscopy (TAS). The mechanism involves an enhanced transition dipole moment and exciton oscillator strength under electrical control. The OSE blue shift deviates from the exciton intensity trend at high negative voltages, arguing against a simple interpretation from the steady-state exciton density of states and suggesting a more complex underlying origin. Our study demonstrates electrical tuning as an additional degree of freedom for Floquet engineering, offering profound insights into coherent light-matter interaction in 2D materials.