Enhanced urine refractive index sensing using a defect-engineered one-dimensional photonic crystal.

In this work, a defect-engineered one-dimensional photonic crystal (1D PhC) sensor is proposed for high-resolution urine refractive-index detection. The structure consists of alternating TiO₂/MgF₂ layers forming Bragg mirrors with a central urine-filled defect cavity, where a sharp localized resonance is generated inside the photonic band gap. Numerical analysis based on the transfer matrix method shows a stable red shift of the defect mode as the urine refractive index increases from 1.333 to 1.360. The proposed sensor achieves a sensitivity of 388.57 nm/RIU with excellent linearity [Formula: see text], together with an average FWHM of 0.0305 nm, an average Q-factor of [Formula: see text] and an average FOM of [Formula: see text]. A tolerance analysis under ± 2.5% thickness variation confirms that the resonance remains narrow and well defined, demonstrating good fabrication robustness. The results also highlight an important principle in photonic sensing: sensor performance should not be judged by sensitivity alone, but by a balanced combination of sensitivity, linewidth, resonance quality, figure of merit, and tolerance against structural deviations. These findings indicate that the proposed design is a promising candidate for practical urine-based biomedical sensing applications. A comparison with representative previously reported photonic-crystal sensing platforms further confirms the balanced overall performance of the proposed design.