Letter | Published:

Valley-selective optical Stark effect in monolayer WS2

Nature Materials volume 14, pages 290294 (2015) | Download Citation

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 populations1,2,3,4, 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 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 phases5,6,7 in two-dimensional TMDs.

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Acknowledgements

The authors acknowledge technical assistance from Q. Ma and Y. Bie during the measurement of the equilibrium absorption of monolayer WS2, and helpful discussions with Z. Alpichshev, I. M. Vishik and Y. H. Wang. This work is supported by US Department of Energy (DOE) award numbers DE-FG02-08ER46521 and DE-SC0006423 (data acquisition and analysis). Y-H.L. and J.K. acknowledge support from NSF DMR 0845358 (material growth and characterization). Y-H.L. also acknowledges partial support from the Ministry of Science and Technology of the Republic of China (103-2112-M-007-001-MY3). L.F. acknowledges support from the STC Center for Integrated Quantum Materials (CIQM), NSF Grant No. DMR-1231319 (theory).

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Affiliations

  1. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • Edbert J. Sie
    • , James W. McIver
    • , Liang Fu
    •  & Nuh Gedik
  2. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

    • James W. McIver
  3. Material Sciences and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan

    • Yi-Hsien Lee
  4. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • Jing Kong

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Contributions

E.J.S. performed the experiments and the data analysis, and wrote the manuscript with crucial inputs from J.W.M., L.F. and N.G. The Floquet topological phase in TMDs was initially proposed by L.F. The monolayers of WS2 were synthesized by Y-H.L., supervised by J.K. This project is supervised by N.G.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Nuh Gedik.

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DOI

https://doi.org/10.1038/nmat4156

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