SPR-based sensor for ultra-high sensitivity high refractive index measurements in the near-infrared

To achieve high refractive index (RI) sensing in the near-infrared (NIR) region, a surface plasmon resonance (SPR) RI sensor was designed using a D-shaped photonic crystal fiber (PCF) as a substrate, and the upper limit of RI detection was improved by cutting rectangular grooves on the structure surface. To improve the performance of the PCF-SPR sensor, the structural parameters of the sensor were initially optimized, and then the effects of the common metal materials gold (Au), silver (Ag), and copper (Cu) and their thicknesses on the sensor performance were investigated. The results show that changing the materials will shift the resonance wavelength position, thus affecting the linearity of resonance wavelength fitting and sensitivity. To solve the problem of poor adhesion of metal directly coated on the surface of the structure, a layer of TiO 2 film was added on the surface of the metal coating and PCF structure, and the effect of its thickness on the sensing performance was investigated.The calculation results showed that, TiO 2 films can enhance the sensor's confinement loss, significantly improve the resonance wavelength fitting linearity and tuned resonance wavelength position, while also reducing the sensitivity. Our detailed results provide guidance for the selection of a suitable plasma coating according to the light source requirements. The sensor can measure the RI in the range of 1.44 ~ 1.48 in the NIR region with a wavelength of 1100 ~ 2100 nm. When gold is used as the plasma coating, the highest RI sensitivity can reach 32,000 nm/RIU; the average RI sensitivity is 17,000 nm/RIU;the resolution is 3.1 × 10 −6 RIU; R 2 = 0.91277; the maximum figure of merit (FOM) is 704.3 RIU −1 , the sensor has broad application prospects in the fields of biomedicine, environmental monitoring and chemical industry.