Theoretical model of the influence of oxide overlayer thickness on the performance of a surface plasmon fibre-optic sensor

Abstract

Fibre-optic surface plasmon resonance (SPR) sensors have been proven to be a useful tool in various measurement tasks where the miniaturization, compactness and low cost are the essential requirements. Recently, it has been shown that the sensitivity of a SPR sensor can be improved if the metallic layer used for surface plasmon excitation is covered by a high-refractive index overlayer (for example, oxide or semiconductor). The semiconductor layer can protect the metal against oxidation, but when it is exposed to the atmosphere, a thin oxide layer can be formed on its top surface. This fact leads to a general question: how an oxide layer (naturally or thermally grown) affects the sensing? This paper deals with the influence of the oxide layer on the sensor performance. The analysed structure consists of a thin gold film covered by the partially oxidized silicon overlayer. The presented theoretical model of the fibre-optic SPR sensor is based on the optics of multilayered media and optical dispersion of all involved media is taken into account. The influence of oxide layer thickness change on the sensor performance in the wavelength domain is studied in detail. The computation of optical power transmitted through the sensing part of a multimode fibre is carried out in order to evaluate the response of the sensor in terms of sensitivity and detection accuracy.