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Experimental observation of optical Weyl points and Fermi arc-like surface states

Abstract

Weyl fermions are hypothetical two-component massless relativistic particles in three-dimensional (3D) space, proposed by Hermann Weyl in 1929. Their band-crossing points, called ‘Weyl points’, carry a topological charge and are therefore highly robust. There has been much excitement over recent observations of Weyl points in microwave photonic crystals and the semimetal TaAs. Here, we report on the experimental observation of ‘type-II’ Weyl points of light at optical frequencies, with the photons having a strictly positive group velocity along one spatial direction. We use a 3D structure consisting of laser-written waveguides, and show the presence of type-II Weyl points by observing conical diffraction along one axis when the frequency is tuned to the Weyl point; and observing the associated Fermi arc-like surface states. The realization of Weyl points at optical frequencies allows these novel electromagnetic modes to be further explored in the context of linear, nonlinear, and quantum optics.

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Figure 1: Schematic and corresponding band structure of a waveguide array that supports type-II Weyl points.
Figure 2: Theoretical and numerical demonstration of topological phase transition associated with type-II Weyl points.
Figure 3: Conical diffraction as a signature of the existence of type-II Weyl points.
Figure 4: Direct observation of Fermi arc-like surface states.

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Acknowledgements

M.C.R. acknowledges the National Science Foundation under award number ECCS-1509546, the Penn State MRSEC, Center for Nanoscale Science, under award number NSF DMR-1420620, and the Alfred P. Sloan Foundation under fellowship number FG-2016-6418. K.P.C. acknowledges the National Science Foundation under award numbers ECCS-1509199 and DMS-1620218. D.L. and C.Y.D. acknowledge support by the Singapore National Research Foundation under grant No. NRFF2012-02, by the Singapore MOE Academic Research Fund Tier 2 Grant No. MOE2015-T2-2-008, and by the Singapore MOE Academic Research Fund Tier 3 grant MOE2011-T3-1-005.

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J.N. carried out experimental measurements and performed the data analysis; S.H. developed the laser fabrication process and characterized the samples under the supervision of K.P.C. and with guidance from M.C.R.; D.L., C.Y.D. and M.C.R. conceived the idea and performed theoretical analysis and calculations; M.C.R. supervised the project.

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Correspondence to Mikael C. Rechtsman.

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Noh, J., Huang, S., Leykam, D. et al. Experimental observation of optical Weyl points and Fermi arc-like surface states . Nature Phys 13, 611–617 (2017). https://doi.org/10.1038/nphys4072

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