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Direct measurement of exciton valley coherence in monolayer WSe2

Nature Physics volume 12, pages 677–682 (2016)Cite this article

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Abstract

In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin1,2,3,4. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides5,6,7. However, the valley coherence time—a crucial quantity for valley pseudospin manipulation—is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron–hole pairs) in monolayer WSe2 (refs 8,9). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron–hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

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Figure 1: Coupled spin and valley degrees of freedom at the band extrema.
Figure 2: Ultrafast resonant generation and detection of exciton valley coherence in monolayer TMDs.
Figure 3: Exciton population relaxation and valley coherence dynamics measured with 2DCS.
Figure 4: Theory of valley decoherence.

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Acknowledgements

The theoretical and experimental collaboration is made possible by SHINES, an Energy Frontier Research Center funded by the US Department of Energy (DoE), Office of Science, Basic Energy Science (BES) under award # DE-SC0012670. K.H., F.W., L.X., X.L. and A.H.M. have all received support from SHINES. Optical spectroscopy studies performed by K.H., C.K.D., L.S. and X.L. have been partially supported by NSF DMR-1306878 and Welch Foundation F-1662. A.H.M. also acknowledges support from Welch Foundation F-1473. L.J.L. is grateful for support from KAUST Saudi Arabia, Academia Sinica Taiwan, and AOARD FA23861510001 USA. C.-H.C. is grateful for support from the Ministry of Science and Technology Taiwan (MOST 104-2218-E-035-010 and 104-2628-E-035-002-MY3).

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Author notes

  1. Galan Moody

    Present address: Present address: National Institute of Standards & Technology, Boulder, Colorado 80305, USA.,

  2. Kai Hao and Galan Moody: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics and Center for Complex Quantum Systems, University of Texas at Austin, Austin, Texas 78712, USA

    Kai Hao, Galan Moody, Fengcheng Wu, Chandriker Kavir Dass, Lixiang Xu, Liuyang Sun, Allan H. MacDonald & Xiaoqin Li

  2. Department of Automatic Control Engineering, Feng Chia University, Taichung 40724, Taiwan

    Chang-Hsiao Chen

  3. Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia

    Ming-Yang Li & Lain-Jong Li

Authors
  1. Kai Hao
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Contributions

K.H. and G.M. contributed equally to this work. G.M. and X.L. conceived the concept. K.H. led the experimental effort. All co-authors at the University of Texas ran the experiments, acquired the data, and analysed the results. C.-H.C., M.-Y.L. and L.-J.L. provided the samples. F.W. and A.H.M. performed the theoretical studies. G.M., F.W. and X.L. wrote the manuscript. All authors discussed the results and commented on the manuscript at all stages.

Corresponding author

Correspondence to Xiaoqin Li.

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The authors declare no competing financial interests.

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Hao, K., Moody, G., Wu, F. et al. Direct measurement of exciton valley coherence in monolayer WSe2. Nature Phys 12, 677–682 (2016). https://doi.org/10.1038/nphys3674

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