Vertically stacked plasmonic nanoparticles in a circular arrangement: a key to colorimetric refractive index sensing

True colorimetric sensing produces a linear spectral response of a single peak within the visible light range with various surrounding media refractive indices. We demonstrate how the circular arrangement of hemispheric silver nanoparticles achieves colorimetric properties without modifying the associated full-width-half-maximum values in a broad range of surrounding media refractive indices. We also show that the vertical out-of-plane arrangement of each circular array in nanoholes enhances the signal-to-noise ratio. High electric field confinement at the interface of the nanoparticles and the supporting substrate reveals the effect of the dielectric constant of the substrate and the morphology of the 3D nanoparticle arrays on achieving a single resonance peak in the visible range with a change in the surrounding refractive index. This study opens up the pathway to top-down fabricated nanostructure platform based plasmonic colorimetric sensing with a single resonance peak in the visible range. The studied rich set of tunable geometrical nanostructures enables broadening of the working optical range of the device.