Optimized film thicknesses for maximum refractive index sensitivity and figure of merit of a bimetallic film surface plasmon resonance sensor

Abstract

In this paper, a simple surface plasmon resonance (SPR) optical sensor with a bimetallic gold-silver film is proposed and its parameters are optimized based on reflectance spectra analysis. In comparison with most SPR sensors, which use single gold films, the evanescent field of this bimetallic configuration at the analyte-metal interface enables the sensing with substantially enhanced refractive index (RI) sensitivity and figure of merit (FOM). The proposed structure comprising a BK7 glass substrate, an adhesion layer of chromium, a gold-silver plasmonic layer, and a protective layer of silicon dioxide is employed in the Kretschmann configuration for liquid analyte sensing. In order to achieve the best sensing properties, the gold-silver layer thickness ratio is varied, and a thorough analysis of reflectance spectra is performed based on RI sensitivity and FOM. For this purpose, a liquid analyte of aqueous solutions of NaCl is employed for angles of incidence 63 degrees and 64 degrees, respectively. Moreover, based on the analysis, an optimized gold-silver layer thickness ratio is selected, and a sensitivity of 41,592 nm per RI unit (RIU) and FOM of 424 RIU-1 are reached. The designed bimetallic SPR structure proves to be advantageous compared to single-gold-film structures employed for sensing applications.