Design of a Dual Polarized Gold-Coated Four-Channel PCF-SPR Sensor with Ultra-High Sensitivity and Broad RI Coverage
Surface plasmon resonance (SPR) sensors built on photonic crystal fibers (PCFs) have recently become a promising category of optical sensing platforms. This is primarily due to their high sensitivity, compact structural design and ability to work with a wide variety of analytes. The overall sensing performance can significantly be enhanced by carefully tailoring the fiber geometry to strengthen the coupling between the guided core mode and the plasmon mode. This study designs and thoroughly investigates a polarization-dependent SPR-based PCF sensor that delivers strong mode coupling, wide detection range and high refractive-index (RI) sensitivity using finite element method (FEM). In the proposed configuration, air holes with different diameters and a rectangular core slot are used to create birefringence along with four outer microchannels coated with a 30 nm gold film create distinct plasmonic regions that support multiple resonances. The simulated results yield maximum wavelength sensitivities of 77500 $nmRIU^-{1}$ and 92500 $nmRIU^-{1}$, along with amplitude sensitivities of 3930 $RIU^-{1}$ and 3139 $RIU^-{1}$ and figures of merit of 517 $RIU^-{1}$ and 356 $RIU^-{1}$ for the x- and y-polarizations respectively, over an analyte RI range of 1.32-1.42. The parametric study verifies that the design is highly tolerant to fabrication deviations and the coupling behavior is primarily sensitive to the smallest air holes and the height of the core slot. Since the sensing region is external, analytes can be reused conveniently. Consequently, the proposed sensor provides a compact, highly sensitive and practically realizable platform for biochemical and chemical sensing applications.