Tunable Plasmonic Resonances in Hexagonally Ordered Gold Nanostructure Arrays with Increasing Interstice Size

Abstract

Gold nanostructure arrays were fabricated by combining colloidal lithography with inclined reactive ion etching and inclinded sputtering. Field emission scanning electron microscopy studies of this substrate show uniaxial hexagonally ordered graded nanostructure arrays with increasing interstice at different positions. Optical spectroscopy studies in visible range reveal that these structures support localized surface plasmon resonance, surface plasmon polaritons, and hybridized modes. Optical spectra were found at each position along the direction of increasing interstice size, and the resonances tend to redshift with position. Different morphologies lead to 50 nm spectral tunability across 10 mm length of the substrate. Based on spectral resolution, optimum peak arising due to localized surface plasmon resonance in reflectance was selected for refractive index sensing application. The sensitivity and figure of merit were evaluated at each position. The sensitivity was found to increase accompanied by decrease in figure of merit with position. The highest sensitivity of 621.6 nm/RIU is obtained for the structure in which interstice size is maximum.