Highly sensitive dual-side polished SPR PCF sensor for ultra-wide analyte range in the visible to near-IR operating band

In this article, a surface plasmon resonance (SPR) sensor based on photonic crystal fiber (PCF) is simulated and analyzed by finite element method (FEM). Gold and titanium oxide (TiO₂) are used as plasmonic layers. The TiO₂ layer is placed under the gold layer to increase the sensitivity of this sensor. The sensor's top and bottom surfaces are polished, and the plasmonic layer is placed on both these surfaces. Unlike sensors with typical external circular coating, less plasmonic material is used. Additionally, unlike internal coating sensors, there is no issue of partial air holes coating and achieving a uniform thickness of the plasmonic layer. Also using two plasmonic levels can expand the range of refractive index (RI) detection. The sensor's structure creates birefringence, allowing complete coupling between the core mode and both plasmonic layers. This results in more reliable detection ability. By optimizing the dimensions and arrangement of air holes and the sensor’s structure, the ability to detect unknown analytes in the ultra-wide range of refractive index of 1.21–1.41 with the maximum wavelength sensitivity (WS) of 61,000 \({\text{nm}}/{\text{RIU}}\) $\text{nm}/\text{RIU}$ , the maximum amplitude sensitivity (AS) of 1294.29 \({\text{RIU}}^{ - 1}\) ${\text{RIU}}^{-1}$ , the maximum figure of merit (FOM) of 657.14 \({\text{RIU}}^{ - 1} { }\) ${\text{RIU}}^{-1}$ and the sensor resolution (SR) of 4.35 × 10^–6 \({\text{RIU}}\) $\text{RIU}$ have been obtained. This sensor operates in the visible to near-infrared wavelength range. During its design, feasibility in construction was taken into consideration, along with reducing the complexity and multiplicity of structural parameters. As a result, the sensor delivers excellent performance in detection parameters and is a suitable option for RI detection applications like organic chemicals and biosensing.