Modeling of High-Performance Fiber Optic SPR Sensor for Colorectal Cancer Detection Designed Using Amorphous Silicon and TiO<sub>2</sub> Layers Under Optimum Radiation Damping
Adarsh Chandra Mishra, D. K. Dwivedi, Anuj K. Sharma, Pooja Lohia, Baljinder Kaur
Abstract
The performance of fused silica core and perfluorinated (PF) polymer clad-based fiber optic sensor is simulated and investigated in near-infrared (NIR) region under optimum radiation damping (ORD) condition of plasmonic excitation. The sensor structure consists of an amorphous silicon (a-Si) layer over the polymer clad as a high-refractive index (RI) substrate for Ag-titanium dioxide (TiO2) heterojunction. Normal and pathological (cancerous) colorectal tissues have been considered as analytes. By keeping the thickness of the a-Si layer (100 nm) constant, the figure of merit (FOM) of the sensor is optimized by judiciously coordinating the thicknesses of Ag layer (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{\text {Ag}}$ </tex-math></inline-formula>) in the range of 30–60 nm and TiO2 layer (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{\text {TiO2}}$ </tex-math></inline-formula>) in the range of 0–10 nm at 1000-nm NIR wavelength using 2-D simulation under principal ORD condition. An optimized FOM as high as 12 810 RIU<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> is achieved for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{\text {Ag}} =40.4$ </tex-math></inline-formula> nm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{\text {TiO2}} =8.6$ </tex-math></inline-formula> nm. Moreover, there appear more such combinations (called secondary ORD) leading to slightly smaller FOM. Furthermore, the effect of coordinated variation of wavelength and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{\text {TiO2}}$ </tex-math></inline-formula> on the sensor’s FOM is also analyzed for further optimization. The power loss (PL) ratio and field enhancement factor are also calculated for optimized thicknesses of Ag and TiO2 layers. Finally, the combined FOM (CFOM) for the proposed sensor is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$128073.09~\mu $ </tex-math></inline-formula>m4/RIU, which is substantially higher than the existing FOSPR sensors to the best of the authors’ knowledge. The introduction of the bovine serum albumin (BSA) layer improves the selectivity and prevents cross-sensitivity, however, with a slight decrease in FOM and sensitivity. The findings are crucial for the development of high-performance plasmonic sensors in NIR.