Abstract
Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom1,2,3,4. Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark5,6,7,8. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment9, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film10, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication11.
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Acknowledgements
The authors acknowledge support from the DoD Vannevar Bush Faculty Fellowship (N00014-16-1-2825), AFOSR MURI (FA9550-12-1-0024 and FA9550-17-1-0002), the NSF (PHY-1506284), NSF CUA (PHY-1125846), the Gordon and Betty Moore Foundation and Samsung Electronics. All film deposition and device fabrication was carried out at the Harvard Center for Nanoscale Systems.
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H.P., P.K., M.D.L., A.A.H., Y.Z., A.D. and L.A.J. conceived the study and Y.Z., G.S., A.A.H., A.D., L.A.J., K.P. and A.Y.J. developed the fabrication procedure. Y.Z., G.S., A.A.H., A.D., C.S. and K.D.G. performed experiments and T.T. and K.W. performed hBN growth. D.S.W. and A.A.H. performed computational analyses and simulations. D.S.W., M.D.L. and H.P. contributed to theoretical descriptions. Y.Z., G.S., D.S.W., A.A.H., P.K., M.D.L. and H.P. wrote the manuscript, with extensive input from all authors.
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Zhou, Y., Scuri, G., Wild, D. et al. Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons. Nature Nanotech 12, 856–860 (2017). https://doi.org/10.1038/nnano.2017.106
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DOI: https://doi.org/10.1038/nnano.2017.106
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