Design and Optimization of Novel Heterostructure SPR Sensor for Wide-Range Refractive Index Sensing

Surface plasmon resonance (SPR) has emerged as a cutting-edge, highly reliable, and widely employed method in modern biosensing applications. However, most designs are found to work within a very restricted refractive index (RI) range (1.33-1.37). In fact, such designs are not useful for biomolecules at higher concentrations if their RIs are above 1.40. These include proteins, lipids, and cellular bodies. This paper proposes the design and optimization of a heterostructure SPR sensor that is capable of detecting minute changes in various bio-analytes having a refractive index$(\mathbf{R I})$falling within a wider range ($\mathbf{1. 3 3}$to$\mathbf{1. 4 3}$). The proposed sensor is based on the Kretschmann configuration with an optical glass (H-ZF62) prism, silver (Ag), tungsten disulfide (WS2), nickel (Ni), and indium tin oxide (ITO) layers. The sensor layers have been systematically optimized through finite difference time domain (FDTD) simulations. The final optimized design incorporates a 40 nm Ag film, five layers of WS${ }_{2}, 35 \text{nm} \text{Ni}$film, and a 5 nm ITO film. This arrangement exhibited a high sensitivity of$173.58 \text{deg} /$RIU, a detection accuracy (DA) of 4.44, and a quality factor (QF) of$44.42 \text{RIU}^{-1}$. The achieved performance was found to be better than the conventional configurations, with the sensitivity increasing by up to 212.47%. The findings enable the usage of this structure in various label-free biosensing applications.