Traditional prism-based surface plasmon resonance (SPR) sensors face challenges in large-scale applications due to their bulky size, high costs, and complex structures. Photonic crystal fibers (PCFs) offer advantages such as compactness, ease of operation, and strong anti-interference capabilities, making them promising replacements for prism substrates. To broaden sensor applications, various sensing media have been used to measure parameters like temperature, magnetic fields, and gases. This necessitates SPR sensors to adapt to various environments. Gold (Au), with its stable chemical properties, is widely utilized as a sensing layer, but reports often overlook the issue of poor adhesion of Au films. Using metals as adhesive layers significantly affects sensor performance due to the imaginary part of their refractive index (RI) of metals. Titanium dioxide (TiO₂), with a purely real RI, minimizes light absorption, but theoretical analyses reveal it can still reduce sensitivity in traditional sensors. While adding enhancement layers can mitigate this issue, it also increases complexity and cost. In this paper, we address sensitivity loss in SPR sensors caused by adhesive layers by proposing a D-shaped SPR-PCF dual-open-circular structure. The design reduces the distance between the sensing layer and the core, thus enhancing sensor performance.

The system, comprising the fiber core, TiO₂, Au, and analyte, is modeled as an equivalent thin-film structure to evaluate the influence of film thicknesses on sensor performance. The PCF features three layers of air holes arranged in a triangular lattice. The outer two layers contain large air holes, while the innermost layer consists of two smaller air holes with varying diameters. A dual-open-circular channel is located at the center of the D-shaped PCF surface, coated with a TiO₂-Au bilayer. This external sensing structure simplifies coating processes, reduces film area, and lowers costs. In addition, the open-circular channel shortens the distance between the sensing layer and the core, improving performance of the sensor with the adhesive layer. SPR occurs when the frequency of the evanescent wave matches that of the surface plasmon wave (SPW), causing strong absorption of incident light by the surface plasmon polariton (SPP) mode. By detecting resonance wavelength shifts, the analyte’s RI can be determined.

For an equivalent film system with TiO₂ as the adhesive layer and Au as the sensing layer, numerical analysis is conducted using the Fresnel optical formula under the simplified condition of a single reflection (Fig. 2). The results show that thinner Au films are more affected by variations in TiO₂ thickness. Without an adhesive layer, the optimal thickness range for a single Au film is 43?59 nm. When a 10-nm-thick TiO₂ adhesive layer is added, this range shifts to 40?56 nm, resulting in a sensitivity reduction of 2.33% and 0.54% for 43 nm and 59 nm Au films, respectively (Table 1). Considering both sensitivity and minimum reflectivity, the optimal configuration is achieved when the Au film thickness is between 45?55 nm and the TiO₂ film thickness is below 15 nm. To address the sensitivity reduction caused by adhesive layers, a D-shaped PCF-SPR sensor with an enhanced open-circular structure is designed. At an analyte RI of 1.25, analysis of the core-SPP mode interaction reveals an anti-crossing effect, with the y-polarization mode proving more effective for sensing than the x-polarization (Figs. 4 and 5). Finite element method (FEM) simulations are used to evaluate the effects of internal and external structural parameters on sensor performance, leading to the determination of optimal parameters (Figs. 6 and 7). Compared to standard D-shaped structure, this enhanced design increases sensitivity by 217%. In addition, when compared with semi-circular and U-shaped designs, the proposed structure demonstrates the most significant enhancement among the compared designs (Figs. 8?10). In the RI range of 1.22?1.32, the sensor achieves a maximum wavelength sensitivity of 26700 nm/RIU, an optimal resolution of 3.75×10^-6 RIU, and a quality factor of 125 RIU^-1 (Fig. 11). For precise measurement, the sensing range is divided into two subranges: 1.22?1.27 and 1.28?1.32. Linear fitting yields a high correlation coefficient of 0.97835 for the lower range, while second-order polynomial fitting achieves a correlation coefficient of 0.99256 for the higher range (Fig. 12). These results enable accurate RI measurements across different subranges, tailored to specific application requirements.

In this paper, we explore the influence of TiO₂ adhesive layers on the performance of traditional SPR sensors and identify the optimal thickness range. To address sensitivity reductions caused by adhesive layers, a D-shaped PCF-SPR sensor with a dual-open-circular structure is introduced. The TiO₂-Au bilayer improves adhesion strength without requiring additional enhancement layers. FEM analysis validates the open-circular design’s superior performance, achieving a sensitivity increase of 217% compared to traditional D-shaped designs. The sensor demonstrates excellent performance within the low RI range of 1.22?1.32, with a maximum wavelength sensitivity of 26700 nm/RIU, an optimal resolution of 3.75×10^-6 RIU, and a quality factor of 125 RIU^-1. Compared to previously reported low RI sensors, this design offers superior stability, sensitivity, and resolution, offering promising potential for applications in chemical detection, biometric recognition, and low RI measurement.