Realization of a three-dimensional photonic topological insulator

TL;DR

This paper reports the first experimental realization of a three-dimensional photonic topological insulator with a wide topological bandgap, enabling robust 3D photonic surface states for advanced optical applications.

Contribution

The authors demonstrate a 3D photonic topological insulator with a large bandgap, extending topological photonics from 2D to 3D structures with practical robustness.

Findings

Achieved a >25% bandwidth 3D topological bandgap.

Mapped bulk bandstructure and surface states via direct measurements.

Demonstrated robust photonic surface propagation on non-planar surfaces.

Abstract

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.