Enhanced Nanoplasmonic Optical Sensors with Reduced Substrate Effect

Abstract

We present a straightforward method to double the refractive index sensitivity of surface-supported nanoplasmonic optical sensors by lifting the metal nanoparticles above the substrate by a dielectric nanopillar. The role of the pillar is to substantially decrease the spatial overlap between the substrate and the enhanced fields generated at plasmon resonance. Data presented for nanodisks and nanoellipsoids supported by pillars of varying heights are found to be in excellent agreement with electrodynamics simulations. The described concepts apply to multitude of plasmonic nanostructures, fabricated by top-down or bottom-up techniques, and are likely to further facilitate the development of novel nanooptical sensors for biomedicine and diagnostics.

Plasmonic gold mushroom arrays with refractive index sensing figures of merit approaching the theoretical limit

“…As a result, more spatial regions around the gold caps with plasmon-enhanced electric fields will be accessible by the surrounding environment. Therefore, the apparent refractive index sensitivity of the lifted metal caps will be increased 10 . Moreover, a Fano resonance caused by the interference between WA of the periodic array and the LSPR of the individual gold units is simultaneously introduced.…”

“…By lifting metal nanostructures above substrates with dielectric pillars, the index sensitivity of the resultant LSPR sensors can be increased because a large fraction of the spatial region with enhanced electric fields is exposed to the environment and accessible by molecular species 10,11 . More efforts have been made to reduce the FWHM values of LSPRs and therefore increase the FOM values [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] .…”

Vertical split-ring resonator based nanoplasmonic sensor

“…[15][16][17][18] To this end, metamaterials require to possess strong plasmon resonance features that are sensitive to environment change. 19,20 The split-ring resonator (SRR) is such a metal structure that is typically used as a building block for metamaterials because of its strong resonance accompanied with strong field enhancement within the SRR gap. [21][22][23] One important measure of a metamaterial sensor is its sensitivity characterized as the ratio of LSPR shift to the change in refractive index of its nearby sensing medium (dk/dn), or spectral shift per refractive index unit (RIU).…”

“…24 Obviously, in order to obtain high sensitivity, it is important to have significant portion of the localized fields associated with the plasmon excitation exposed to the sensing medium for a greater resonance shift. 19 Unfortunately, a majority of the metamaterials reported so far have planar SRRs that lie flat on substrates, resulting in a rather appreciable fraction of the plasmon energy distributed in the dielectric substrate below as shown in Fig. 1 which limits the effective sensing volume as well as the sensing performance.…”

“…Although a couple of published works show that the sensing volume can be increased by raising up the plasmonic elements on dielectric pillars, a fraction of the localized fields still leaks into the dielectric material underneath. 19,25,26 In this work, we report the fabrication of vertical SRRs (VSRRs) capable of lifting essentially all of the localized fields above the supporting substrate they stand on as illustrated in Fig. 2(a).…”