Coupling of topological interface states in one-dimensional photonic crystals is controlled by spatial distance and cap-layer symmetry, dictating the coupling quality

This work investigates coupled topological interface states in a one-dimensional triple-photonic-crystal structure featuring two independently controllable cap layers. Using the transfer matrix method, we demonstrate that coupling between two topological interface states is governed by two critical factors: the spatial separation between interfaces, controlled by the period number of the middle photonic crystal, and the spectral alignment of individual states, controlled by cap layer parameters. When the interfaces are sufficiently separated, the two states remain degenerate, whereas reducing the spatial separation induces evanescent coupling that lifts the degeneracy. Critically, strong coupling and high transmittance require perfect spectral alignment: when identical cap layers are employed at both interfaces, robust mode hybridization occurs with near-unity transmission, and the transmittance exhibits a Lorentzian dependence on the refractive index mismatch between the cap layers. Conversely, when cap layer parameters differ, the resulting detuning suppresses coupling efficiency, substantially reducing transmission. For closely spaced interfaces, significant field penetration into the middle photonic crystal enhances light-matter interaction—a feature particularly advantageous for sensing applications. A comprehensive tolerance analysis reveals that the proposed structure is remarkably robust against fabrication-induced thickness variations. This work establishes a versatile platform for engineering coupled topological resonators with controllable mode splitting and field distribution for narrowband filtering and high-sensitivity optical sensing.