Defect-engineered one-dimensional topological photonic crystal for enhanced gas sensing applications

In this work, a highly sensitive refractive-index (RI) and gas sensing platform based on a one-dimensional (1D) topological photonic crystal (TPC) is proposed and analyzed. The structure consists of a defect layer embedded between two topologically distinct photonic crystals composed of TiO 2 and SiO 2 . The designed configuration supports topological edge state (TES) modes, enabling precise detection of minute variations in the RI of the defect medium. The sensing characteristics are systematically investigated using the transfer matrix method (TMM). The results reveal that the existence of two distinct resonant modes (Mode 1 and Mode 2) is governed primarily by the thickness of the defect layer. Mode 1, observed for defect thicknesses in the range of 50–600 nm, exhibits moderate sensitivity with relatively broadened resonance peaks. In contrast, Mode 2, corresponding to defect thicknesses exceeding 700 nm, demonstrates significantly enhanced performance, characterized by stronger optical confinement, higher quality factor (Q-factor), and improved sensitivity. An optimized defect-layer thickness of 1138 nm provides a maximum sensitivity of approximately 806 nm/RIU, which is further enhanced to 2265.37 nm/RIU after structural optimization. Furthermore, the resonance wavelength exhibits a linear redshift with increasing RI of the gaseous medium, accompanied by slight broadening in the full width at half-maximum (FWHM) and corresponding variations in the Q-factor, figure of merit (FOM), and detection limit (LoD). The proposed sensor exhibits excellent characteristics for both RI and gas sensing applications while maintaining high Q -factor and FOM values. These results demonstrate that precise engineering of the defect-layer thickness in topological photonic crystal heterostructures offers an effective and compact strategy for the realization of high-resolution, low-cost optical gas sensing platforms.