Dual-Polarization SPR Sensor of U-Shaped Photonic Crystal Fiber Coated with Au-TiO₂

We propose a surface plasmon resonance (SPR) sensor based on the U-shaped photonic crystal fiber (PCF) coated with Au-TiO₂ layers, which can detect the refractive index (RI) of the analyte. We introduce elliptical air holes near the fiber core, which can break the symmetry of PCF structural and lead to a strong birefringence for achieving dual-polarization sensors. Besides, the TiO₂ layer not only enhances the adhesion of Au and quartz but also improves the SPR effect. By using finite element method (FEM) numerical analysis, geometrical parameters are optimized to enhance sensors’ performances, and dual polarization demonstrates superior performance in different detection ranges, expanding the range of analyte detection. Finally, simulation results show that the detection range under the Y-polarization (X-polarization) is 1.20–1.30 (1.36–1.43), with a maximum wavelength sensitivity of 10200 nm/RIU (5900 nm/RIU). This sensor offers broad RI detection range and high sensitivity, promising extensive applications in environmental and medical diagnostics.