Confining photons in a finite volume is highly desirable in modern photonic devices, such as waveguides, lasers and cavities. Decades ago, this motivated the study and application of photonic crystals, which have a photonic bandgap that forbids light propagation in all directions1,2,3. Recently, inspired by the discoveries of topological insulators4,5, the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic topological insulators6,7,8, with promising applications in topological lasers9,10 and robust optical delay lines11. However, a fully three-dimensional (3D) topological photonic bandgap has not been achieved. Here we experimentally demonstrate a 3D photonic topological insulator with an extremely wide (more than 25 per cent bandwidth) 3D topological bandgap. The composite material (metallic patterns on printed circuit boards) consists of split-ring resonators (classical electromagnetic artificial atoms) with strong magneto-electric coupling and behaves like a ‘weak’ topological insulator (that is, with an even number of surface Dirac cones), or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk band structure and the Dirac-like dispersion of the photonic surface states, and demonstrate robust photonic propagation along a non-planar surface. Our work extends the family of 3D topological insulators from fermions to bosons and paves the way for applications in topological photonic cavities, circuits and lasers in 3D geometries.
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
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We thank Q. Yan at Zhejiang University, L. Lu at the Chinese Academy of Sciences and J. C. W. Song at Nanyang Technological University for discussions. The work at Zhejiang University was sponsored by the National Natural Science Foundation of China under grant numbers 61625502, 61574127, 61601408, 61775193 and 11704332, the ZJNSF under grant number LY17F010008, the Top-Notch Young Talents Program of China, the Fundamental Research Funds for the Central Universities and the Innovation Joint Research Center for Cyber-Physical-Society System. Y.C. and B.Z. acknowledge the support of Singapore Ministry of Education under grant numbers MOE2015-T2-1-070, MOE2015-T2-2-008, MOE2016-T3-1-006 and Tier 1 RG174/16 (S). Y.Y. and R.S. acknowledge the support of the Singapore Ministry of Education under grant number MOE2015-T2-2-103.
Nature thanks J. Bravo-Abad and the other anonymous reviewer(s) for their contribution to the peer review of this work.