Valley-selective optical Stark shift of exciton-polaritons in a monolayer semiconductor

Light provides a high-speed coherent medium for measurement and manipulation of electronic quantum states. Exploiting the optical selection rules of transition metal dicalchogenide monolayers (TMDs), the optical Stark effect allows for valley-selective control of energy levels using sub-resonant optical pulses. Recent discoveries have revealed that microcavity exciton-polaritons in TMDs preserve valley features while also incorporating many of the favorable properties of light. Here, we use polarization-dependent transient reflectance to demonstrate that the optical Stark effect can also be used for valley-selective manipulation of energy levels in WS2 exciton-polaritons. In the reflectance spectra we observe a simultaneous shift of both polariton branches when pump and probe are co-polarized, and no appreciable shift when they are cross-polarized. We find excellent agreement between measured data and a Lorentz oscillator model over a wide range of experimental parameters. The extracted polariton shift confirms the expected linear power dependence of the optical Stark effect. The polarization-dependent Stark shift of TMD exciton-polaritons provides a new tool for state control in coherent valleytronics.