Abstract
We report a sensing platform for surface-enhanced infrared absorption (SEIRA) spectroscopy, based on Fano metamaterials (FMMs) on dielectric nanopedestals. FMMs consist of two parallel gold (Au) nanorod antennas, with a small horizontal coupler attached to one of the nanorod antenna. When placed on SiO2 dielectric nanopedestals, which exhibit strong field enhancements caused by the interference between subradiant and superradiant plasmonic resonances, they provide the highly enhanced E-field intensities formed near the Au nanoantenna, which can provide more enhanced molecular detection signals. Here, the sensing characteristics of FMMs on nanopedestals structure was confirmed by comparison with FMMs on an unetched SiO2 substrate as a control sample. The control FMMs and the FMMs on nanopedestals were carefully designed to excite Fano resonance near the target 1-octadecanethiol (ODT) fingerprint vibrations. The FMMs were fabricated by using nanoimprint lithography and the nanopedestal structures were formed by isotropic dry-etching. The experimental reflection spectra containing the enhanced absorption signals of the ODT monolayer molecules was analyzed using temporal coupled-mode theory. The FMMs on nanopedestals achieved over 7% of reflection difference signal, which was 1.7 times higher signal than the one from the control FMMs. Based on the FMMs on nanopedestal structures proposed in this study, it may be widely applied to future spectroscopy and sensor applications requiring ultrasensitive detection capability.
Highlights
(d) with ODT monolayer coated around the Au nanoantennas are shown
As a result of the interference of the two plasmon modes, Fano resonance should be formed with significant near-field enhancement of the two nanoantennas
We present a surface-enhanced infrared absorption (SEIRA) sensing platform based on the Fano metamaterials (FMMs) on the nanopedestal structure and experimentally demonstrate the ODT monolayer detection using the proposed structure
Summary
(d) with ODT monolayer coated around the Au nanoantennas are shown. The undercut etching depth (U) of the FMM on the nanopedestal structure is 30 nm and the vertical etching depth (ΔZ) is 50 nm. New SEIRA detection strategies based on Fano resonances have been developed for bio-sensing applications; they can obtain a sharp spectral response with high near-field intensities[22,23,24,25]. By employing an asymmetric arrangement of nanoantennas, a weak coupling between the two resonant modes can be introduced, allowing indirect energy transfer of incident waves to the dark resonant mode. This indirect excitation of the dark mode yields sharp transmission and reflection spectra, with high quality (Q) factors and large near-field intensities[26,27]. The nanopedestals supporting the metallic nanoantennas are fabricated through an isotropic dry-etching process
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