The nanoscaling of metamaterial structures represents a technological challenge toward their application in the optical frequency range. In this work we demonstrate tailored chiro-optical effects in plasmonic nanohelices, by a fabrication process providing a nanometer scale control on geometrical features, that leads to a fine tuning of operation band even in the visible range. Helicoidal 3D nanostructures have been prototyped by a bottom-up approach based on focused ion and electron beam induced deposition, investigating resolution limits, growth control and 3D proximity effects as a function of the interactions between writing beam and deposition environment. The fabricated arrays show chiro-optical properties at the optical frequencies and extremely high operation bandwidth tailoring dependent on the dimensional features of these 3D nanostructures: with the focused ion beam we obtained a broadband polarization selection of about 600 nm and maximum dissymmetry factor up to 40% in the near-infrared region, while with the reduced dimensions obtained by the focused electron beam a highly selective dichroic band shifted toward shorter wavelengths is obtained, with a maximum dissymmetry factor up to 26% in the visible range. A detailed finite difference time domain model highlighted the role of geometrical and compositional parameters on the optical response of fabricated nanohelices, in good agreement with experimental results.
Silver is the ideal material for plasmonics because of its low loss at optical frequencies, though it is often replaced by a lossier metal, gold. This is because of silver’s tendency to tarnish, an effect which is enhanced at the nanoscale due to the large surface-to-volume ratio. Despite chemical tarnishing of Ag nanoparticles (NPs) has been extensively studied for decades, it has not been well understood whether resulted by sulfidation or oxidation processes. This intriguing quest is herein rationalized by studying the atmospheric corrosion of electron beam lithography-fabricated Ag NPs, through nanoscale investigation performed by high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss (EEL) and energy dispersive X-ray (EDX) spectroscopies. We demonstrate that tarnishing of Ag NPs upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur-based contaminants rather than via an oxidation process. Furthermore, we show that the sulfidation occurs through the formation of crystalline Ag2S bumps onto Ag surface in place of a homogeneous growth of a silver sulfide film. From a single 2D Z-contrast scanning transmission electron microscopy image, a method for 3D reconstruction of silver nanoparticles with extremely high spatial resolution has been derived thus establishing the preferential nucleation of Ag2S bumps in proximity of lattice defects located on the NP surface. Finally, we also provide a straightforward and low-cost solution to achieve stable Ag NPs by passivating them with a self-assembled monolayer of hexanethiols. The sulfidation mechanism inhibition allows to prevent the increased material damping and scattering losses.
In this work, we experimentally investigate the chiro-optical properties of 3D metallic helical systems at optical frequencies. Both single and triple-nanowire geometries have been studied. In particular, we found that in single-helical nanostructures, the enhancement of chiro-optical effects achievable by geometrical design is limited, especially with respect to the operation wavelength and the circular polarization conversion purity. Conversely, in the triple-helical nanowire configuration, the dominant interaction is the coupling among the intertwined coaxial helices which is driven by a symmetric spatial arrangement. Consequently, a general improvement in the g-factor, extinction ratio and signal-to-noise-ratio is achieved in a broad spectral range. Moreover, while in single-helical nanowires a mixed linear and circular birefringence results in an optical activity strongly dependent on the sample orientation and wavelength, in the triple-helical nanowire configuration, the obtained purely circular birefringence leads to a large optical activity up to 8°, independent of the sample angle, and extending in a broad band of 500 nm in the visible range. These results demonstrate a strong correlation between the configurational internal interactions and the chiral feature designation, which can be effectively exploited for nanoscale chiral device engineering.
In this paper, we report on the effect of metal oxidation on strong coupling interactions between silver nanostructures and a J-aggregated cyanine dye. We show that metal oxidation can sensibly affect the plexcitonic system, inducing a change in the coupling strength.In particular, we demonstrate that the presence of oxide prevents the appearance of Rabi splitting in the extinction spectra for thick spacers. In contrast, below a threshold percentage, the oxide layer results in an higher coupling strength between the plasmon and the Frenkel exciton.Contrary to common belief, a thin oxide layer seems thus to act, under certain conditions, as a coupling mediator between an emitter and a localized surface plasmon excited in a metallic nanostructure. This suggests that metal oxidation can be exploited as a means to enhance lightÀmatter interactions in strong coupling applications.
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