The excitation of plasmonic dark modes via a radiative channel is a phenomenon strongly hindered in the subwavelength regime. Recently, for achieving this purpose it has been proposed to exploit near-field interactions between radiating (bright) modes and lossless dark modes. However, this approach unveils challenging difficulties related to the excitation of dark modes through the near-field coupling with a bright mode. Here, it is experimentally and numerically shown how symmetry breaking applied to a nanoantenna dimer can conversely induce the excitation of plasmonic resonances, which play a key role for the dark modes' activation in more complex nanoassemblies. On the basis of this study, a Tshaped nanoantenna trimer has been introduced as an elemental unit for the energy transfer between bright and dark modes in plasmonic nanostructures. Finally, we implemented an analytical perturbative model to further investigate the plasmonic hybridization of subwavelength systems.
The possibility to develop nanosystems with appreciable magnetic response at optical frequencies has been a matter of intense study in the past few years. This aim was strongly hindered by the saturation of the magnetic response of "natural" materials beyond the THz regime. Recently, in order to overcome such limitation, it has been considered to enhance the magnetic fields through the induction of displacement currents triggered by plasmonic resonances. Here we investigate a nanoassembly supporting the hybridization of an electric and magnetic plasmonic mode in Fano resonance conditions. Taking advantage of the enhancement properties owned by such interferential resonance, we have been able to generate an intense and localized magnetic hot-spot in the near-infrared spectral region.
In this letter, we present a comparative experimental–simulation study of Au-nanodisc-enhanced upconversion of 1500 nm light in an Er3+ doped TiO2 thin film. The geometry of the Au nanodiscs was guided by finite-element simulations based on a single nanodisc in a finite computational domain and controlled experimentally using electron-beam lithography. The surface-plasmon resonances (SPRs) exhibited a well-known spectral red shift with increasing diameter, well explained by the model. However, an experimentally observed double-peak SPR, which resulted from inter-particle interactions, was expectedly not captured by the single-particle model. At resonance, the model predicted a local-field enhancement of the upconversion yield, and experimentally, the luminescence measurements showed such enhancement up to nearly 7 fold from a nanodisc with 315 nm diameter and 50 nm height. The upconversion enhancement agreed qualitatively with the theoretical predictions, however with 3–5 times higher enhancement, which was attributed to scattered light from neighboring particles.
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