Insulator-free topological photonic multi-lane highways

Abstract Topological photonics has revolutionized light transport by enabling robust unidirectional propagation immune to structural disorder. However, conventional topological designs confine light to narrow interfacial channels, necessitating optically unoccupied topological-insulator regions and imposing a fundamental trade-off between topological protection and spatial efficiency. 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, we achieve four inequivalent photonic valley half-semimetals at distinct critical transition boundaries between trivial and Chern insulator phases. We arrange these four structures parallelly and cyclically, such that each domain simultaneously functions as a valley-selective waveguide and a topological barrier for the other valley in neighboring channels, circumventing the need for additional 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 drastic shape variations. This work establishes a paradigm for ultracompact topological photonic circuits, with direct implications for high-density integrated optics.