Photonic Crystal Fiber Surface Plasmon Resonance (PCF-SPR) Design for Low Concentration Biomolecule Detection

Abstract

This study aims to design, model, and optimize a Photonic Crystal Fiber Surface Plasmon Resonance (PCF-SPR) sensor capable of detecting biomolecules at very low concentrations, particularly in the picomolar to femtomolar range, which are generally difficult to detect using conventional biosensing methods. Optimization efforts were carried out through a numerical simulation approach using a standard hexagonal PCF structure with an air hole diameter of 0.8–1.2 μm and a pitch of 1.8–2.2 μm. A gold layer with a thickness of 35–50 nm was deposited on the central capillary as the sensing core to produce strong coupling between the fundamental mode of the fiber and the surface plasmon polariton (SPP). TM mode was selected because it provides the most optimal evanescent field interaction with the plasmonic layer. The sensor performance evaluation process included measurements of resonance wavelength shift, confinement loss, full width at half maximum (FWHM), sensitivity, and figure of merit (FOM) against variations in the analyte refractive index. The simulation results show that a gold layer thickness of 40 nm produces the best performance, characterized by a sensitivity of 4500 nm/RIU, FWHM of 38 nm, confinement loss of 210.5 cm⁻¹, and the highest FOM of 118.4 RIU⁻¹. These values indicate an ideal balance between evanescent field penetration, optical attenuation, and plasmon coupling efficiency. This design has also been proven capable of detecting changes in refractive index down to 10⁻⁶ RIU, making it potentially useful for detecting protein, DNA, or antigen biomarkers at ultra-low concentrations. In conclusion, this PCF-SPR design offers highly sensitive, stable biosensing performance and is worth developing for early medical diagnostics, environmental monitoring, and food safety applications. Further development is recommended by integrating 2D materials, microfluidic systems, and D-shaped structure-based metal deposition techniques to improve long-term stability and ease of fabrication.