Visualization of a Unidirectional Electromagnetic Waveguide Using Topological Photonic Crystals Made of Dielectric Materials

TLDR

A deformation of the photonic crystal from the ideal honeycomb lattice creates a pseudospin degree of freedom representing different orbital angular momentum states, serving as the photonic analogue of the electronic Kramers pair. Using only conventional dielectric materials and local real‑space manipulations, the authors propose a scheme extendable to visible light that could inspire future photonic applications. Experimentally, an Al₂O₃ photonic crystal shows time‑reversal symmetric topological propagation akin to the quantum spin Hall effect, and microwaves of a specific pseudospin were observed to propagate unidirectionally along the interface between the topological and trivial crystals.

Abstract

We demonstrate experimentally that a photonic crystal made of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ cylinders exhibits topological time-reversal symmetric electromagnetic propagation, similar to the quantum spin Hall effect in electronic systems. A pseudospin degree of freedom in the electromagnetic system representing different states of orbital angular momentum arises due to a deformation of the photonic crystal from the ideal honeycomb lattice. It serves as the photonic analogue to the electronic Kramers pair. We visualized qualitatively and measured quantitatively that microwaves of a specific pseudospin propagate only in one direction along the interface between a topological photonic crystal and a trivial one. As only a conventional dielectric material is used and only local real-space manipulations are required, our scheme can be extended to visible light to inspire many future applications in the field of photonics and beyond.