Mechanistic understanding of excitation of surface plasmons in a fiber-optic SPR sensor utilizing Al/Cu bimetallic configuration: mode field approach

Abstract

This work reports on a novel approach of incorporating effective refractive indices of the guided modes obtained through wave optics to optimize the thickness of plasmonic metal in a fiber–optic surface plasmon resonance (SPR) sensor based on Kretschmann–Raether configuration. Using optical fibers with significantly smaller dimensions than used in conventional fiber–optic SPR sensors, a rigorous mathematical analysis has been carried out to conclude the existence of a single guided mode propagating in multi–layered fiber–optic sensing structure responsible for the excitation of SP mode. The effective refractive indices of the guided modes and modal attenuation for different thickness combinations of Al and Cu used in bimetallic layer configuration have been determined using wave optics. The optimized thickness of Al/Cu bimetallic layer, thus obtained, is subsequently used to simulate a fiber–optic SPR refractive index sensor. Operating in a refractive index range of 1.330–1.380 with sensitivity 3.16 µm/RIU, key attributes of the designed sensor include larger shift in resonance wavelength supplemented with simultaneous narrowing of SPR spectrum culminating in enhancing figure of merit. First and unique of its kind, this work is an attempt at the visualization of SP excitation in fiber–optic SPR sensor using wave optics, and thus, brings innovative insights towards exploration of fiber–optic SPR sensors using modal field approach.