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|Title: ||Vertical split-ring resonator based nanoplasmonic sensor|
|Authors: ||Pin Chieh Wu;Greg Sun;Wei Ting Chen;Kuang-Yu Yang;Yao-Wei Huang;Yi-Hao Chen;Hsiang Lin Huang;Wei-Lun Hsu;Hai Pang Chiang;Din Ping Tsai|
|Issue Date: ||2017-02-09T08:10:48Z
|Publisher: ||Applied Physics Letters|
|Abstract: ||Abstract: Split-ring resonators (SRRs) have been the subject of investigation as plasmonic sensors that operate by sensing plasmon resonance shift δλ when exposed to a medium with a refractive index change δn. However, conventional planar SRRs have their plasmon fields spread into the substrates, reducing accessible sensing volume and its sensing performance. Such a limitation can be eradicated with vertical SRRs in which the plasmon fields localized in SRR gaps are lifted off from the substrate, allowing for greatly enhanced sensitivity. Here, we demonstrate the highest sensitivity among reported SRR-based sensors in optical frequencies.
Plasmonic metamaterials are artificially constructed media with extraordinary properties that are derived from periodical sub-wavelength constituent elements typically involving metal with hosting dielectric.1 These properties of the 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.2–4 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.5–7 Such a property can be explored for a variety of applications,8–13 one of which is sensing based on the following general design principle. Depending on the wavelength range of interest, a metamaterial consisting of subwavelength metal elements of specific shape and sizes is fabricated on a given substrate. These metal elements exhibit different resonance shifts when exposed to media of different indexes of refraction, and by optically probing the shift, the media to which the metamaterial is exposed is then identified.14 The motivation of exploring metamaterials for the sensing application is the potential for achieving high sensitivity.15–18 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.21–23 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 (δλ/δn), 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. This reality is the direct consequence of significant technical challenges in the fabrication because metamaterials are far more easily constructed with planar sub-wavelength elements on substrates. 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). It has been theoretically predicted that the sensitivity approaching 1400 nm/RIU can be achieved with VSRRs.27 Using Fourier transform infrared spectroscopy (FTIR) measurement and numerical simulation software (COMSOL Multiphysics), we demonstrate that plasmonic refractive index sensors constructed of VSRRs deliver significantly improved sensitivity over their planar counterparts reported in the literature.
|Appears in Collections:||[光電科學研究所] 期刊論文|
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