Surface-Physics-Based Numerical Analysis of a Defective 1D Photonic Crystal Sensor Using Refractive Index Variations

This study presents a one-dimensional defective photonic crystal sensor for the refractometric detection of albumin, urea, and glucose concentrations in human urine. The proposed structure has the configuration [air/(Si/Al 2 O 3 ) N C(Si/Al 2 O 3 ) N /glass], where a central analyte-filled cavity is symmetrically confined between two Si/Al 2 O 3 Bragg reflectors. The optical response of the sensor is theoretically investigated using the transfer matrix method and MATLAB simulations. Variations in the concentrations of albumin, urea, and glucose modify the refractive index of the urine sample inside the cavity, producing measurable redshifts in the localized defect mode within the photonic band gap. The cavity thickness is further optimized to enhance the resonance shift and improve the sensing performance. The reflectance spectra are analyzed in terms of resonant wavelength, sensitivity, figure of merit, and quality factor. The results demonstrate that the proposed design provides a simple, label-free, and highly sensitive refractometric platform for urine-based biomarker detection. In addition, its multilayer configuration is compatible with conventional thin-film fabrication techniques, supporting its potential for future photonic biosensing applications.