Valley-selective optical Stark eect in

Breaking space–time symmetries in two-dimensional crystals can markedly influence their macroscopic electronic proper-ties. Monolayer transition metal dichalcogenides (TMDs) are primeexampleswhere the intrinsicallybrokencrystal inversion symmetry permits the generation of valley-selective electron populations1–4, even though the two valleys are energetically degenerate, locked by time-reversal symmetry. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley-specific band engineering and oer additional control in valleytronic applications. Although applying a magnetic field should, in principle, accomplish this task, experiments so far have not shown valley-selective energy level shifts in fields accessible in the laboratory. Here, we show the first direct evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying intense circularl