Metal nanohole arrays are a famous example of plasmonic nanostructured materials, which are crucial plasmonic devices that display resonances and high electromagnetic confinement in the visible and near-infrared range. Therefore, they have been suggested for use in many applications, including communications and biosensing. In this work, we present the asymmetry in nanoholes and examine its impact on the electromagnetic response using numerical models and broadband experimental measurements. We fabricated a 2D hexagonal array of asymmetric nanoholes in Ag using a low-cost production method called nanosphere lithography combined with tilted silver evaporation. Our experimental setup is based on a laser with fine input and output polarization control that is broadly controllable in the near-infrared spectrum. When the nanohole array is activated with linear polarization, we next determine the circular polarization degree of the transmitted light. We explain the asymmetry of the nanohole, which is supported by numerical simulations, as the cause of the imbalance between left and right transmitted light. We propose that such straightforward plasmonic shape could be optimized to create multipurpose flat-optic devices.
The nanoscale community has proposed various nanostructures for the enhancement of near- and far-field chiro-optical effects. Here we study such effects in asymmetric metasurfaces which can be produced by means of nanosphere lithography (NSL). NSL, combined with tilted plasmonic deposition, is a versatile, self-assembling method for fabrication of different asymmetric nanogeometries. Polystyrene nanospheres (PSN) are first self-assembled on glass, then reduced in diameter, and subsequently covered with a plasmonic layer. By controlling fabrication parameters, we can obtain three types of samples. First sample is based on PSN asymmetrically covered by metal under 45deg. This sample has a high contribution of the plasmonic elliptical nanohole array on the glass. Second sample is a plasmonic elliptical nanohole array obtained by simply removing the PSN from the first sample. Third sample is obtained by increasing the metallic deposition angle to 60deg; this way, nanohole array contribution vanishes, and the metasurface is based on asymmetric plasmonic nanoshells. We report on numerical studies on these three samples, when excited by oblique left or right circular polarization in the near-infrared range. The simulations are in good agreement with previously obtained experimental results, which gives a route to possible optimization of fabrication parameters for different applications. Finally, we comment on the follow-up application for each geometry. We believe that this technique can be used to produce high quality and low-cost substrates for chiral sensing; moreover, with the inclusion of near-field light emitting layer, these metasurfaces could lead to tunable circularly polarized visible or near-infrared light emission.
Optical enantioselectivity of chiral molecules could be enhanced by depositing them on suitable nanostructured substrates. Different kind of chiral substrates can be developed, but chiral features are in general difficult to fabricate or costly. Self-assembled approach allows realizing plasmonic metasurfaces with a low cost reliable procedure. In this case asymmetric fabrication parameters can induce chiral optical response of the realised substrate. Self-organized polystyrene spheres deposited on glass substrate, are utilised to produce asymmetric hole array on a metal thin film. In our case the spheres (518 nm in diameter) where reduced by selective reactive ion etching and then covered by gold (and other metals), that is evaporated at a glancing angle. After the removing of the spheres an elliptical-hole array is produced forming a circular-dichroic substrate. The circular dichroic response of light interacting with the substrate can be tuned by choosing proper incidence angle and excitation wavelength, while the flat nature of the metasurface is very useful for easy molecular deposition processes. Two new enantiomers (right-handed and left-handed molecules) have been synthesized in order to present a good circular dichroism in the visible range and to be tested on the realized metasurfaces. Different tests were carried out on the samples, investigating the spectral optical properties of the structures with and without chiral molecules on top of them. The results are very promising due to the possibility of easily tuning and optimizing the optical response.
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