Valley-selective optical Stark effect in monolayer WS2

TL;DR

This paper demonstrates that intense circularly polarized light can induce a valley-selective optical Stark effect in monolayer WS2, lifting valley degeneracy and enabling control over valley-specific electronic states.

Contribution

First experimental observation of valley-selective optical Stark effect in monolayer WS2 using circularly polarized light.

Findings

Valley degeneracy in WS2 can be lifted by up to 18 meV.

Circularly polarized light breaks time-reversal symmetry in monolayer WS2.

The optical Stark effect enables valley-specific energy tuning.

Abstract

Breaking space-time symmetries in two-dimensional crystals (2D) can dramatically 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. While applying a magnetic field should in principle accomplish this task, experiments to date have observed no 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 via the optical Stark effect. These results offer a novel way to control valley degree of freedom, and may provide a means to realize new valley-selective Floquet topological phases in 2D TMDs.