Experimental realization of a Weyl exceptional ring

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Abstract

Weyl points are isolated degeneracies in reciprocal space that are monopoles of the Berry curvature. This topological charge makes them inherently robust to Hermitian perturbations of the system. However, non-Hermitian effects, usually inaccessible in condensed-matter systems, are an important feature of photonic systems, and when added to an otherwise Hermitian Weyl material have been predicted to spread the Berry charge of the Weyl point out onto a ring of exceptional points, creating a Weyl exceptional ring and fundamentally altering its properties. Here, we observe the implications of the Weyl exceptional ring using real-space measurements of an evanescently coupled bipartite optical waveguide array by probing its effects on the Fermi arc surface states and bulk diffraction properties of the two constituent sublattices in an experimental realization of a distributed Berry charge in a topological material.

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Fig. 1: Helical waveguide array and corresponding band structure supporting a WER.
Fig. 2: Surface states connecting the projection of the Berry charges.
Fig. 3: Direct observation of a topological transition through the emergence of Fermi arc surface states.
Fig. 4: Distinguishing a WER from a Weyl point by observing the transverse radial propagation.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

Code availability

The code used in this study is available from the corresponding author on reasonable request.

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Acknowledgements

The authors thank J. Noh for discussions about experimental techniques. M.C.R. and A.C. acknowledge support from the National Science Foundation under grants ECCS-1509546 and DMS-1620422 as well as the Packard Foundation via fellowship no. 2017-66821, and the Charles E. Kaufman foundation under grant no. KA2017-91788. K.P.C., S.H. and M.W. acknowledge National Science Foundation grants ECCS-1509199 and DMS-1620218. Y.D.C. is supported by Singapore MOE Academic Research Fund Tier 2 Grants MOE2015-T2-2-008 and MOE2016-T2-1-128, and Singapore MOE Academic Research Fund Tier 3 Grant MOE2016-T3-1-006.

Author information

A.C. conceived of the idea, carried out the experimental measurements, and performed the data analysis and numerical simulations. A.C. and M.C.R. designed the experiments. A.C., Y.D.C. and M.C.R. performed the theoretical analysis and wrote the manuscript. S.H. and M.W. developed the laser fabrication process and characterized the samples under the supervision of K.P.C. The project was supervised by M.C.R. All authors contributed to discussions and to finalizing the manuscript.

Correspondence to Alexander Cerjan.

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Supplementary Information

Supplementary derivations and discussion, Supplementary Figs. 1–4 and Supplementary references 1–14.

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