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Optical generation of excitonic valley coherence in monolayer WSe2

Abstract

As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges (‘valleys’) to have additional spin-like quantum numbers1,2,3,4,5,6. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies7,8. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence9,10, in addition to valley polarization11,12,13,14,15, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

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Figure 1: Device and photoluminescence characterization.
Figure 2: Electrical control of valley excitons.
Figure 3: Optical generation of valley exciton and trion polarization.
Figure 4: Signature of excitonic valley quantum coherence.

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Acknowledgements

The authors thank B. Spivak, D. Cobden, A. Andreev and K-M. Fu for helpful discussions. This work was mainly supported by the National Science Foundation (NSF, DMR-1150719). The experimental set-up and device fabrication was partially supported by a Defense Advanced Research Projects Agency (DARPA) Young Faculty Award (YFA) (N66001-11-1-4124). H.Y. and W.Y. were supported by the Research Grant Council (HKU705513P) and the University Grant Council (AoE/P-04/08) of the government of Hong Kong, and the Croucher Foundation under the Croucher Innovation Award. N.G., J.Y., D.M. and D.X. were supported by the US Department of Energy (DoE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Device fabrication was performed at the University of Washington Microfabrication Facility and the NSF-funded Nanotech User Facility.

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Authors

Contributions

X.X. conceived the experiments. A.M.J. performed the experiments, assisted by S.W., G.A. and B.Z. The devices were fabricated by J.S.R., assisted by A.M.J. The theoretical explanation was provided by H.Y. and W.Y., with input from D.X. The WSe2 crystals were synthesized by N.J.G., J.Y. and D.G.M., who also performed bulk characterization measurements. A.M.J., X.X., H.Y. and W.Y. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Wang Yao or Xiaodong Xu.

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Jones, A., Yu, H., Ghimire, N. et al. Optical generation of excitonic valley coherence in monolayer WSe2. Nature Nanotech 8, 634–638 (2013). https://doi.org/10.1038/nnano.2013.151

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