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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 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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Figure 1: The optical Stark effect and its observation in monolayer WS2.
Figure 2: The valley selectivity of the optical Stark effect.
Figure 3: Fluence and detuning dependences of the optical Stark shift.
Figure 4: Valley-specific Floquet topological phase.

References

  1. Xiao, D., Liu, G-B., Feng, W., Xu, X. & Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 108, 196802 (2012).

    Article  Google Scholar 

  2. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech. 7, 494–498 (2012).

    CAS  Article  Google Scholar 

  3. Zeng, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech. 7, 490–493 (2012).

    CAS  Article  Google Scholar 

  4. Cao, T. et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nature Commun. 3, 887 (2012).

    Article  Google Scholar 

  5. Inoue, J. & Tanaka, A. Photoinduced transition between conventional and topological insulators in two-dimensional electronic systems. Phys. Rev. Lett. 105, 017401 (2010).

    Article  Google Scholar 

  6. Kitagawa, T., Oka, T., Brataas, A., Fu, L. & Demler, E. Transport properties of nonequilibrium systems under the application of light: Photoinduced quantum Hall insulators without Landau levels. Phys. Rev. B 84, 235108 (2011).

    Article  Google Scholar 

  7. Lindner, N. H., Refael, G. & Galitski, V. Floquet topological insulator in semiconductor quantum wells. Nature Phys. 7, 490–495 (2011).

    CAS  Article  Google Scholar 

  8. Shirley, J. H. Solution of Schrodinger equation with a Hamiltonian periodic in time. Phys. Rev. 138, B979–B987 (1965).

    Article  Google Scholar 

  9. Autler, S. H. & Townes, C. H. Stark effect in rapidly varying fields. Phys. Rev. 100, 703–722 (1955).

    Article  Google Scholar 

  10. Bakos, J. S. AC Stark effect and multiphoton processes in atoms. Phys. Rep. 31, 209–235 (1977).

    Article  Google Scholar 

  11. Cohen-Tannoudji, C. N. & Phillips, W. D. New mechanisms for laser cooling. Phys. Today 43, 33–40 (1990).

    CAS  Article  Google Scholar 

  12. Wang, Y. H., Steinberg, H., Jarillo-Herrero, P. & Gedik, N. Observation of Floquet–Bloch states on the surface of a topological insulator. Science 342, 453–457 (2013).

    CAS  Article  Google Scholar 

  13. Joffre, M., Hulin, D., Migus, A. & Antonetti, A. Dynamics of the optical Stark effect in semiconductors. J. Mod. Opt. 35, 1951–1964 (1988).

    CAS  Article  Google Scholar 

  14. Fröhlich, D., Nöthe, A. & Reimann, K. Observation of the resonant optical Stark effect in a semiconductor. Phys. Rev. Lett. 55, 1335–1337 (1985).

    Article  Google Scholar 

  15. Mysyrowicz, A. et al. “Dressed excitons” in a multiple-quantum-well structure: Evidence for an optical Stark effect with femtosecond response time. Phys. Rev. Lett. 56, 2748–2751 (1986).

    CAS  Article  Google Scholar 

  16. Chemla, D. S. et al. The excitonic optical Stark effect in semiconductor quantum wells probed with femtosecond optical pulses. J. Lumin. 44, 233–246 (1989).

    CAS  Article  Google Scholar 

  17. Sieh, C. et al. Coulomb memory signatures in the excitonic optical Stark effect. Phys. Rev. Lett. 82, 3112–3115 (1999).

    CAS  Article  Google Scholar 

  18. Hayat, A. et al. Dynamic Stark effect in strongly coupled microcavity exciton polaritons. Phys. Rev. Lett. 109, 033605 (2012).

    Article  Google Scholar 

  19. Koster, N. S. et al. Giant dynamical Stark shift in germanium quantum wells. Appl. Phys. Lett. 98, 161103 (2011).

    Article  Google Scholar 

  20. Sie, E. J., Lee, Y-H., Frenzel, A. J., Kong, J. & Gedik, N. Biexciton formation in monolayer MoS2 observed by transient absorption spectroscopy. Preprint at http://arxiv.org/abs/1312.2918 (2013)

  21. Combescot, M. Optical Stark effect of the exciton. II. Polarization effects and exciton splitting. Phys. Rev. B 41, 3517–3533 (1990).

    CAS  Article  Google Scholar 

  22. Bernevig, B. A., Hughes, T. L. & Zhang, S. C. Quantum spin Hall effect and topological phase transition in HgTe quantum wells. Science 314, 1757–1761 (2006).

    CAS  Article  Google Scholar 

  23. Perez-Piskunow, P. M., Usaj, G., Balseiro, C. A. & Torres, L. E. F. F. Floquet chiral edge states in graphene. Phys. Rev. B 89, 121401(R) (2014).

    Article  Google Scholar 

  24. Liu, X. et al. Strong light–matter coupling in two-dimensional atomic crystals. Preprint at http://arxiv.org/abs/1406.4826 (2014)

  25. Mai, C. et al. Many-body effects in valleytronics: Direct measurement of valley lifetimes in single-layer MoS2 . Nano Lett. 14, 202–206 (2014).

    CAS  Article  Google Scholar 

  26. Mai, C. et al. Exciton valley relaxation in a single layer of WS2 measured by ultrafast spectroscopy. Phys. Rev. B 90, 041414(R) (2014).

    Article  Google Scholar 

  27. Kim, J. et al. Ultrafast generation of pseudo-magnetic field for valley exciton in WSe2 monolayers. Preprint at http://arxiv.org/abs/1407.2347 (2014)

  28. MacNeill, D. et al. Valley degeneracy breaking by magnetic field in monolayer MoSe2. Preprint at http://arxiv.org/abs/1407.0686 (2014)

  29. Aivazian, G. et al. Magnetic control of valley pseudospin in monolayer WSe2. Preprint at http://arxiv.org/abs/1407.2645 (2014)

  30. Srivastava, A. et al. Valley Zeeman effect in elementary optical excitations of a monolayer WSe2. Preprint at http://arxiv.org/abs/1407.2624 (2014)

  31. Li, Y. et al. Valley splitting and polarization by the Zeeman effect in monolayer MoSe2. Preprint at http://arxiv.org/abs/1409.8538 (2014)

  32. Lee, Y-H. et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Adv. Mater. 24, 2320–2325 (2012).

    CAS  Article  Google Scholar 

  33. Lee, Y-H. et al. Synthesis and transfer of single-layer transition metal disulfides on diverse surfaces. Nano Lett. 13, 1852–1857 (2013).

    CAS  Article  Google Scholar 

  34. Gutiérrez, H. R. et al. Extraordinary room-temperature photoluminescence in triangular WS2 monolayers. Nano Lett. 13, 3447–3454 (2012).

    Article  Google Scholar 

Download references

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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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.

Corresponding author

Correspondence to Nuh Gedik.

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The authors declare no competing financial interests.

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Sie, E., McIver, J., Lee, YH. et al. Valley-selective optical Stark effect in monolayer WS2. Nature Mater 14, 290–294 (2015). https://doi.org/10.1038/nmat4156

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