Valley-selective optical Stark effect in monolayer WS[subscript 2]

Breaking space–time symmetries in two-dimensional crystals can markedly influence their macroscopic electronic properties. Monolayer transition metal dichalcogenides (TMDs) are prime examples where the intrinsically broken crystal inversion symmetry permits the generation of valley-selective electron populations, 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 offer 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 ​WS[subscript 2]. By applying intense circularly polarized light, which breaks time-reversal symmetry, we demonstrate that the exciton level in each valley can be selectively tuned by as much as 18 meV through the optical Stark effect. These results offer a new way to control the valley degree of freedom, and may provide a means to realize new Floquet topological phases in two-dimensional TMDs.United States. Dept. of Energy (Award DE-FG02-08ER46521)United States. Dept. of Energy (Award DE-SC0006423)National Science Foundation (U.S.) (DMR 0845358)National Science Foundation (U.S.) (STC Center for Integrated Quantum Materials Grant DMR-1231319