title: Valley-selective optical Stark effect in monolayer WS2.
abstract: 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 WS2. 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.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Edbert J
authors.initials: EJ
authors.lastname: Sie
authors.affiliation: 1] Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA [2] Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: McIver
authors.affiliation: Material Sciences and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan.
authors.forename: Yi-Hsien
authors.initials: YH
authors.lastname: Lee
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Liang
authors.initials: L
authors.lastname: Fu
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Jing
authors.initials: J
authors.lastname: Kong
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Nuh
authors.initials: N
authors.lastname: Gedik