Coupled Edge-state Modes for Bandgap Engineering and Terahertz Topological Duplexer Integration

Topological photonic crystals (TPCs) provide a robust platform for terahertz (THz) applications demanding precise manipulation of topological band structures. This study demonstrates a planar valley TPC operating in the THz regime, facilitating tunable coupling between adjacent edge states. When two domain walls supporting different topological edge modes are positioned in close proximity, the spatial overlap of their evanescent wavefunction tails induces mode coupling, resulting in the emergence of a bandgap within the edge-state continuum. A semi-analytical model, inspired by frameworks in quantum mechanics and condensed matter physics, quantitatively correlates the induced bandgap width with the modal decay constants and spatial separation of the edge states. Full-wave simulations of diverse supercell architectures, including those incorporating Dirac photonic crystals (DPCs), corroborate the theoretical predictions. Exploiting this coupling mechanism, an on-chip THz topological duplexer is designed and experimentally realized, demonstrating low insertion loss, high isolation, and strong immunity to fabrication imperfections, withstanding geometric deviations of up to approximately 20% without performance degradation. These findings establish a unified framework for bandgap engineering via edge-state interactions and open a new avenue toward high-performance, frequency-selective, and integration-compatible topological photonic devices in the THz regime.