Insulator-free topological photonic multi-lane highways.

Abstract

Topological protection in photonic structures enables robust unidirectional propagation immune to structural disorder^1-20. However, conventional implementations obtain this protection from topological-insulator domains whose interfaces host narrow guiding channels, leaving much of the insulating bulk inactive for transport. This imposes a fundamental trade-off between topological robustness and spatial footprint^21-25. Here we introduce an insulator-free topological waveguide architecture that eliminates this trade-off, enabling multi-lane unidirectional light guiding with both 100% spatial utilization efficiency and topological protection. By strategically combining time-reversal and inversion-symmetry breaking in gyromagnetic honeycomb photonic crystals (PCs), we achieve four inequivalent photonic valley half-semimetals (PVHSMs)^26,27 at distinct critical transition boundaries between trivial and Chern insulator phases. We arrange these four structures in a parallel, cyclic configuration, such that each domain simultaneously functions as a valley-selective waveguide and a topological barrier for the other valley in adjacent domains, circumventing the need for further topological insulating layers. Our experimental and theoretical results demonstrate that this multi-lane configuration transforms conventional edge states into densely packed, large-area one-way modes. These modes exhibit alternating unidirectionality across the four domains while maintaining robustness even under arbitrary sharp bends and pronounced shape variations. This work exemplifies a design strategy for ultracompact topological photonic circuits, with potential for high-density integrated optics.