Curvature-Modulated Fiber-Optic SPR Sensor With Au/GST Composite Coating for Tunable Resonance and Enhanced Refractive Index Sensitivity

This paper presents a novel approach for modulating the surface plasmon resonance (SPR) response through optical fiber curvature. A plastic-clad silica fiber (PCSF) is used as the sensing platform, with the cladding removed to expose the core. A 50 nm Au layer is deposited followed by a 30 nm Ge2Sb2Te5 (GST) layer to create a composite plasmonic interface. The inclusion of the GST layer enhances the local dielectric contrast near the metal surface, improving the refractive index (RI) sensitivity of the SPR sensor. Unlike conventional fiber-optic SPR sensors, this method allows continuous and reversible modulation of optical coupling by mechanically adjusting the fiber curvature within a range of −1/70 m−1 to 1/70 m−1. This modulation facilitates dynamic tuning of essential resonance characteristics, including resonance wavelength and full width at half maximum (FWHM), without requiring structural modifications or complex functionalization. Experimental results show a resonance wavelength tuning range of 186 nm at a fixed RI of 1.403 RIU. For RI values between 1.333 and 1.403 RIU, the Au/GST-coated sensor exhibits enhanced performance under curvature modulation, with the sensitivity increasing from 5877.1 nm/RIU to 7971.4 nm/RIU and the figure of merit (FOM) improving from 7.9 (−70 m-1) to 45.7 (70 m-1), accompanied by a reduction in FWHM from 746.3 nm to 174.6 nm. This curvature modulation technique offers a flexible, compact, and cost effective means of tuning SPR performance, making it a promising solution for miniaturized, high-performance SPR sensing applications.