Multiple Brillouin zone folding based broadband topological slow light in valley photonic crystals.

Topological photonic crystals (TPCs) have attracted considerable attention in integrated photonics community for the intriguing prospect of robust light transport. As a photonic analogue of quantum valley Hall insulators, valley photonic crystals (VPCs) support topologically protected directional edge modes without broken time-reversal symmetry. However, the observation of VPCs-based broadband slow light remains challenging due to the frequency discontinuity of valley kink modes resulting from the broken symmetry at the domain wall. In this work, we achieve continuous broadband topological slow light in two-dimensional VPCs. Based on the coupling of size-perturbed resonator periodic array, the resulting kink mode has been dramatically slow down through multiple Brillouin zone folding and separated with mini-gaps. To circumvent the mini-gaps issue, we propose a continuous broadband topological slow light waveguide consisting of two combined and frequency complementary VPCs waveguides. We also obtain a novel topological frequency-selective beam splitter based on the valley-contrasting directional slow-light propagation. Our results provide exciting opportunities in topological slow-light applications.