Abstract

The optical properties of the plasmonic metamaterials are often times intrinsically connected to the localized surface plasmon (SP) resonances (LSPR) arising from the collective oscillations of free electrons which induce strong electromagnetic fields adjacent to the artificial sub-wavelength metallic elements in the metamaterials. The resonance wavelengths are determined by feature geometries of metamaterial elements and their surrounding environment, and thus can be tuned by either changing the element dimensions or the surrounding dielectric. Such a property can be explored for a variety of applications, one of which is sensing based on the following general design principle. The motivation of exploring metamaterials for the sensing application is the potential for achieving high sensitivity. To this end, metamaterials require to possess strong plasmon resonance features that are sensitive to environment change. The split-ring resonator (SRR) is such a metal structure that is typically used as a building block for metamaterials because of its strong magnetic resonance accompanied with strong field enhancement within the SRR gap [1]. 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 (δλ/δn). Unfortunately, a majority of the met-amaterials reported so far have planar SRRs that lay flat on substrates, resulting in a rather appreciable fraction of the plasmon energy distributed in the dielectric substrate below which limits the effective sensing volume as well as the sensing performance [2]. 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. 1(a). Using Fourier transform infrared spectroscopy measurement and numerical simulation software, we demonstrate that plasmonic refractive index sensors constructed of VSRRs deliver significantly improved sensitivity over their planar counterparts reported in the literature.

© 2014 Japan Society of Applied Physics, Optical Society of America