Topological interface state-based photonic crystal sensor with porous cap layer for high-performance biosensing

In this study, we introduce a refractive index (RI) sensor based on topological interface states (TISs) in one-dimensional photonic crystals. The design incorporates a porous cap layer positioned at the interface between two photonic crystals, enabling the support of TIS modes. Owing to its porosity, the cap layer makes the device particularly suitable for aqueous and biological analytes, and the reported analysis focuses on the RI range relevant for such biosensing applications. The behavior of TISs is investigated through the transfer matrix method by tuning the physical parameters of the cap layer, and the findings are further validated using FEM simulations. The sensing characteristics are examined as a function of structural parameters, showing robust performance across the RI range of 1.33-1.43 with an almost constant sensitivity of ~ 745 nm/RIU. Although performance optimization was not the main goal, results reveal that in the fully porous case-where the analyte itself serves as the defect layer-the sensitivity can reach up to 941 nm/RIU. As a practical demonstration, the sensor was applied to glucose detection. For concentrations up to 36 g/L and at 85% porosity, the sensitivity, figure of merit, and detection limit were found to be 803 nm/RIU, 6.5 × 10⁶, and 1.4 × 10⁻⁷ RIU, respectively, surpassing previously reported works in all major metrics. These findings highlight the potential of the proposed structure for diverse biosensing applications, while noting that similar principles could be applied to other RI ranges.