Defect-engineered 1D photonic crystals with topological edge states for optical humidity sensing

Photonic crystals (PCs) provide an attractive platform for self-display colorimetric sensing. This work presents a theoretically optimized one-dimensional (1D) optical humidity sensor based on photonic crystal containing alternating TiO₂/SiO₂ layers incorporating SiO₂ defect layer. By exploiting topological photonic edge states, the structure supports a sharply confined defect resonance within the photonic bandgap, enhancing sensitivity to humidity-induced refractive-index variations. The effective refractive index of the SiO₂ defect layer was modeled using the Bruggeman effective medium approximation, while transmission and reflectance spectra were analyzed using the transfer matrix method. The proposed 1D defective photonic crystal exhibits a maximum sensitivity of 1.46 nm per percent relative humidity (%RH) and a figure of merit of 0.27. These results demonstrate that combining topological edge states with defect-layer engineering significantly improves resonance sharpness and spectral stability, highlighting the potential of topological photonic crystals for compact, high-precision optical humidity sensing.