Internal specular reflection from nanoparticle layers on the end face of optical fibers

Abstract

Metal nanoparticles deposited on a dielectric substrate exhibit an absorption peak due to localized surface plasmon resonance (LSPR). Changes in the refractive index of the surrounding medium cause the LSPR band to shift in wavelength. In this paper, gold nanoparticles were placed on the end face of an optical fiber spliced to a reflection setup. As expected in reflection geometries, the back-reflected signal exhibits a dip at wavelengths in the LSPR band when the fiber tip is surrounded by ambient air (n = 1). However, as the refractive index of the surrounding medium is increased, there is a gradual inversion from a dip (n = 1.0) to a peak (n = 2.0). To explain these results, a model based on absorption and interference between the specular reflections from the fiber/nanoparticle and nanoparticle/ambient interfaces is proposed, showing that our reflection configuration can be used in sensing applications.