Abstract
We study the plasmonic resonances of double nanoholes (DNHs) in metal films. These apertures exhibit the usual gap-mode Fabry-Pérot resonances, where the zeroth order resonance is determined by the waveguide cut-off and the first order resonance shows sensitivity to the film thickness. An additional wedge resonance is observed, which is sensitive to the curvature of the cusps in the DNHs, analogous to the wedge modes of single wedges. While the gap mode intensity increases dramatically with decreasing gap-width, the wedge mode intensity saturates since its field enhancement arises from the curvature of the metal film, like cylindrical Sommerfeld waves. Experimental transmission spectra agree well with finite-difference time-domain simulations showing these separate resonances. The controlled design of these resonances is critical for applications including optical tweezers, nonlinear conversion, sensing and spectroscopy.
Highlights
Nanoapertures in metallic thin films have shown the ability to significantly enhance optical fields in the nanoscale region that they define at the resonance frequency [1,2]
We study the plasmonic resonances of double nanoholes (DNHs) in metal films
An additional wedge resonance is observed, which is sensitive to the curvature of the cusps in the DNHs, analogous to the wedge modes of single wedges
Summary
Nanoapertures in metallic thin films have shown the ability to significantly enhance optical fields in the nanoscale region that they define at the resonance frequency [1,2]. Our lab has already shown the trapping of particles of 12 nm in size [38] and even the single proteins [39] using a double nanohole (DNH) optical aperture For these applications, we aim to understand the role of aperture geometry on the resonant transmission and near-field properties. We perform finite-difference time-domain (FDTD) simulations to investigate the usual gap-mode FP resonances and the wedge resonance not studied in those past works on apertures. Past works on optical tweezers have investigated the role of the resonant transmission modes on the trapping performance [16,17,18]; it is important to identify the modes present in the aperture and to quantify their influence on trapping. Since the wedge modes are tightly confined to the wedge, they are good candidates for trapping experiments; this remains to be investigated in future works
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