An axisymmetric metal and metal-oxide grating-structured self-reference fiber optic SPR sensor

Abstract

An optical-fiber-based surface plasmon resonance (SPR) sensing probe consisting of axisymmetrical grating is proposed and studied theoretically for the detection of the refractive index (RI). The sensing probe is formed by depositing a plasmonic metal layer on a bare core of optical fiber, followed by an axisymmetric grating layer and an analyte. The grating consists of metal and metal oxide in an alternate manner. The proposed structure of a single channel provides two resonance modes in the transmission SPR spectra. It is found that the mode present at the lower wavelength remains fixed, and the mode present at the higher wavelength is varied when the RI of the analyte is varied, so that the previous one works as a reference mode while the latter one works as a sensing mode. These properties provide the self-referencing capability of the proposed RI sensor in ambient conditions. We have calculated the sensitivity, detection accuracy (DA) and signal to width ratio (SWR) using the SPR spectra of various probes of different combinations of plasmonic metal and metal-oxide gratings to enhance the study. The best performing probe is found to be Ag/Cu-ITO. This optimum probe attained a maximum SWR of 42, sensitivity of 5400 nm RIU−1 and DA of 0.08 nm−1 with the self-referencing capability. The optimum probe also works well for a dispersive analyte within the spectral range. The use of metal oxide provides better adsorption of the analyte as well as high sensitivity. The cumulative properties of self-referencing and the metal oxide of the designed probe make it accurate and useful for the detection of biological elements in the ambient atmosphere.