Valley-selective Optical Stark Effect Probed by Kerr Rotation*

The valley pseudospin in transition metal dichalcogenide (TMDC) monolayers is a promising degree of freedom for coherent control. The optical Stark effect allows for valley-selective manipulation of energy levels in WS₂ and WSe₂ using ultrafast optical pulses. Despite these advances, understanding of valley-selective optical Stark shifts in TMDCs has been limited by reflectance-based detection methods where the signal is small and prone to background effects. We show that polarization-sensitive, time-resolved Kerr rotation is less sensitive to background effects, providing a five-fold improvement in the signal-to-noise ratio of the Stark effect optical signal and a more precise estimate of the energy shift. This increased sensitivity allows for observation of an optical Stark shift in monolayer MoS₂ that exhibits both valley- and energy-selectivity, demonstrating the promise of this method for investigating this effect in other layered materials and heterostructures.