Separation of valley excitons in a MoS2 monolayer using a subwavelength asymmetric groove array

Abstract

Excitons in monolayer transition metal dichalcogenides are formed at K and K′ points at the boundary of the Brillouin zone. They acquire a valley degree of freedom, which has been explored as an alternative information carrier, analogous to charge or spin. Two opposite valleys in transition metal dichalcogenides can be optically addressed using light with different helicity. Here, we demonstrate that valley-polarized excitons can be sorted and spatially separated at room temperature by coupling a MoS2 monolayer to a subwavelength asymmetric groove array. In addition to separation of valley excitons in real space, emission from valley excitons is also separated in photon momentum-space; that is, the helicity of photons determines a preferential emission direction. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the field of valleytronics.

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Fig. 1: Schematics of optically addressable valleys and spatial separation of valley excitons by a metasurface.
Fig. 2: Metasurface design principle.
Fig. 3: Experimental observation of separation of valley excitons in real space.
Fig. 4: Experimental observation of valley exciton emission separation in momentum space.

Data availability

The data that support the figures in this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

L.S., A.Z., C.-K.S. and X.L. were supported by NSF EFMA-1542747 and NSF DMR-1306878. J.C., C.-K.S., X.L. and A.A. are supported by NSF MRSEC programme DMR-1720595. X.L. also acknowledges support from the Welch Foundation via grant F-1662. A.A., A.K. and J.S.G.-D. were partially supported by the Air Force Office of Scientific Research (MURI grant no. FA9550-17-1-0002), the DARPA Nascent program and the Welch Foundation (grant no. F-1802). S.G. and C.-Y.W. acknowledge support from the Ministry of Science and Technology (MOST) in Taiwan (MOST 105-2112-M-007-011-MY3). Collaboration between National Tsing-Hua University and The University of Texas at Austin is facilitated by the Global Networking Talent (NT 3.0) Program, Ministry of Education in Taiwan. J.S.G.-D. is supported by NSF CAREER grant no. ECCS-1749177. J.S. acknowledges support from NSFC (11774035 and 11674032).

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L.S. led the optical experiments. C.-Y.W. synthesized the silver plates and fabricated the metasurface. A.K. performed the simulations. J.C., J.S. and A.Z. assisted with experiments. L.S., C.-Y.W., A.K., X.L. and A.A. wrote the manuscript. X.L., A.A., C.-K.S. and S.G. designed and supervised the project. All authors discussed the results and commented on the manuscript at all stages.

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Correspondence to Andrea Alù or Xiaoqin Li.

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Sun, L., Wang, CY., Krasnok, A. et al. Separation of valley excitons in a MoS2 monolayer using a subwavelength asymmetric groove array. Nature Photon 13, 180–184 (2019). https://doi.org/10.1038/s41566-019-0348-z

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