Ultra-high sensitivity photonic crystal fiber sensor based on dispersion turning point sensitization of surface plasmonic polariton modes for low RI liquid detection

A surface plasmon resonance (SPR) sensor composed of photonic crystal fibers (PCFs) is designed for the detection of low-concentration liquids. The excellent sensing properties are attributed to the sensitivity enhancement at the dispersion turning point (DTP) of the surface plasmon polariton (SPP) modes. The sensor consists of two identical and structurally simple D-shaped PCFs together with a plasmonic thin film coated on the polished surface in direct contact with the analyte. Changes in the refractive index (RI) result in the splitting of the degenerate plasma peaks, thus allowing the monitoring of the analyte concentration variations by measuring the peak separation. In the RI range between 1.328 RIU and 1.33 RIU, the sensor exhibits an ultra-high sensitivity of 129,800 nm/RIU, which is 37.22 times higher than that of the unsensitized single D-shaped structure. Compared to fiber grating sensors operating in the vicinity of the cladding mode DTP, the spliced dual D-shaped PCF still has a predictably high mechanical strength. In addition, the RI detection range of the sensor can be changed by adjusting the slit width. The average sensitivities are 4.38 nm/g·L^-1 in the sodium chloride concentration range of 0 g/L to 100 g/L and 20.85 nm/g·L^-1 in the hemoglobin concentration range of 0 g/L to 20 g/L. Our results reveal that this SPR sensor based on PCFs has large potential in a variety of applications, especially biochemistry, due to its exceptional sensitivity, structural simplicity, and adjustable detection range.