This study conducts an in-depth numerical investigation of a D-shaped butterfly-core photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor, specifically designed for the precise and highly sensitive detection of refractive indices (RI) over a wide range. A key and notable feature of this study is the comparative analysis of three adhesive overlayers—titanium dioxide \((TiO_2)\) , magnesium fluoride \((MgF_2)\) , and tantalum pentoxide \((Ta_2O_5)\) to optimize the sensor performance. Among these materials, the \((Ta_2O_5)\) layer proved most effective, significantly enhancing sensor performance when paired with gold (Au) as the primary plasmonic material. The sensor’s response was rigorously analyzed for both x and y-polarizations of the guided mode to ensure optimal functionality. In the RI range of 1.34–1.43, the proposed sensor demonstrates optimal performance, achieving a maximum wavelength sensitivity of 34,000 nm/RIU and an amplitude sensitivity of 493 \(RIU^{-1}\) within the RI range of 1.42–1.43. Furthermore, the highest birefringence obtained was \(2.8\times 10^{-3}\) , while the figure of merit (FOM) and sensor resolution (SR) were optimized to 157 \( RIU^{-1}\) and \(2.94\times 10^{-6}\) RIU, respectively. Operating in the near-infrared region, this sensor features a compact and straightforward design, making it well-suited for practical applications. The findings of this study contribute to the advancement of efficient, high-performance, and cost-effective SPR sensors for diverse applications, including chemical analysis, environmental monitoring, and medical diagnostics.
