Realization of a three-dimensional photonic topological insulator

• DOI: 10.1038/s41586-018-0829-0
• Publication Date: 2019-01-08T19:21:54Z
• Authors: Yihao Yang; Zhen Gao; Haoran Xue; Li Zhang; Mengjia He; Zhaoju Yang; Ranjan Singh; Yidong Chong; Baile Zhang; Hongsheng Chen
• Keywords: Condensed Matter - Mesoscale and Nanoscale Physics; Classical Physics (physics.class-ph); FOS: Physical sciences; Physics - Classical Physics; 01 natural sciences; Topological Insulators; Metamaterials; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); 0103 physical sciences; :Physics::Optics and light [Science]; Physics - Optics; Optics (physics.optics)

Confining photons in a finite volume is in high demand in modern photonic devices. This motivated decades ago the invention of photonic crystals, featured with a photonic bandgap forbidding light propagation in all directions. Recently, inspired by the discoveries of topological insulators (TIs), the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic TIs, with promising applications in topological lasers and robust optical delay lines. However, a fully three-dimensional (3D) topological photonic bandgap has never before been achieved. Here, we experimentally demonstrate a 3D photonic TI with an extremely wide (> 25% bandwidth) 3D topological bandgap. The sample consists of split-ring resonators (SRRs) with strong magneto-electric coupling and behaves as a 'weak TI', or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk bandstructure 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 TIs from fermions to bosons and paves the way for applications in topological photonic cavities, circuits, and lasers in 3D geometries.