Abstract
Due to unique properties and great design flexibilities, Fano resonances represent one of the most promising optical features mediated by metallic nanostructures, while the excitation of some Fano modes is impossible due to symmetry reasons. The aim of this work is to show that dense lattice arrangements can have a profound impact on the optical properties of nanostructures and, in particular, can enable the excitation of otherwise dark modes. Here, we demonstrate this concept using the example of rectangular arrays of symmetric trimers packed so densely that the coupling between neighbouring unit cells imposes a symmetry break, enabling the excitation of magnetic Fano resonances. We found that in experiments as well as in simulations, electric and magnetic Fano resonances can be simultaneously formed in cases where the inter-trimer distances are sufficiently small. By analysing the transition from an isolated trimer mode into a regime of strong near-field coupling, we show that by modifying the rectangular unit cell lengths due to the symmetry mismatch between lattice and trimer, two types of Fano resonances can be found, especially magnetic Fano resonances with loop-type magnetic field distributions within the centre of each trimer, which can be either enhanced or suppressed. In addition, the influence of the refractive index environment was measured, showing sensitivity values of approximately 300 nm/RIU. Our work provides fundamental insights into the interaction of the lattice and nanostructure response and paves the way towards the observation of novel optical excitations.
Highlights
In recent years, nano-optics has attracted substantial attention, with a large number of studies[1,2,3] carried out to understand the interaction between light and matter on the nanoscale
We conducted a set of experiments as well as numerical calculations with a focus on the trimer modes and found that magnetic Fano resonances can be excited for the symmetric trimer geometry when the trimers are arranged in densely packed arrays, with the symmetry breaking induced by the lattice arrangement and not by the individual trimer unit
We have studied the impact of the lattice geometry on the optical response of nanotrimers arranged in densely packed square arrays at infrared wavelengths
Summary
Nano-optics has attracted substantial attention, with a large number of studies[1,2,3] carried out to understand the interaction between light and matter on the nanoscale. Due to the low damping of the dark mode, the electromagnetic near-field at the wavelength of the Fano resonance is greatly enhanced, which has the consequence that the scattered light is enhanced [12] To achieve such a unique effect, numerous types of nanostructures such as single split nanodots[12,13], nanocavities[14], nanosphere dimers[15], dolmen-like nanostructures[14,16], plasmonic clusters[17] and oligomers[18,19,20] are currently being employed. We conducted a set of experiments as well as numerical calculations with a focus on the trimer modes and found that magnetic Fano resonances can be excited for the symmetric trimer geometry when the trimers are arranged in densely packed arrays, with the symmetry breaking induced by the lattice arrangement and not by the individual trimer unit. The results indicate that an asymmetric refractive index arrangement between substrate and superstrate weakens the formation of magnetic Fano resonances
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