High-sensitivity SPR refractive index sensor based on a PbF2-coated U-shaped optical fiber

A PbF 2 -assisted dielectric modulation strategy was proposed for enhancing SPR response in U-shaped fiber sensors. • The PbF 2 layer achieved a 1.97-fold enhancement in local electric-field intensity through interfacial dielectric modulation. • The experimentally fabricated coating showed excellent agreement with the optimized 70 nm design predicted by FEM simulations. • The proposed sensor achieved an average RI sensitivity of 8483.58 nm/RIU over 1.3381–1.3778. • A maximum sensitivity of 14195.32 nm/RIU and an experimental FOM of 40.42 RIU −1 were obtained. To improve the sensitivity of fiber-optic SPR sensors, a lead fluoride (PbF 2 ) sensitizing layer was introduced into a U-shaped fiber SPR configuration as a fluoride-based dielectric modulation layer. The proposed sensor was designed using a U-bent multimode fiber integrated with a PbF 2 -modified layer and was evaluated by FEM simulation and experimental testing. In the FEM simulation, the local electric field near the sensing interface became stronger after adding the PbF 2 -containing layer, with a peak intensity about 1.97 times higher than that of the pure-Au structure. The stronger electric field is mainly associated with the change in the effective dielectric environment and the redistribution of the evanescent field at the Au/analyte interface after introducing the PbF 2 -containing layer. The wide band gap and optical transparency of PbF 2 also make it suitable as a low-loss dielectric sensitizing layer. In the simulation, a PbF 2 thickness of 70 nm gave the best overall response. Experimentally, the sensor showed an average RI sensitivity of 8483.58 nm/RIU over the RI range of 1.3381–1.3778. For comparison, the pure-Au U-shaped sensor had a sensitivity of 4207.54 nm/RIU, corresponding to a 2.02-fold improvement after PbF 2 modification. In the high-RI range of 1.3669–1.3778, the sensitivity reached 14,195.32 nm/RIU, which was 3.37 times higher than that of the pure-Au sensor. The sensor also showed good stability under the tested conditions, with a thermal drift coefficient of 0.4867 nm/°C from 30 °C to 60 °C and an average sensitivity RSD of 0.6181% in repeated RI measurements. The PbF 2 -containing layer therefore improves the RI response of the U-shaped fiber SPR sensor through interfacial dielectric modulation.