Abstract:The regular arrangement of metal nanoparticles influences their plasmonic behavior. It has been previously demonstrated that the coupling between diffracted waves and plasmon modes can give rise to extremely narrow plasmon resonances. This is the case when the singleparticle localized surface plasmon resonance (λ LSP ) is very close in value to the Rayleigh anomaly wavelength (λ RA ) of the nanoparticles array. In this paper, we performed angle-resolved extinction measurements on a 2D array of gold nano-cylinders designed to fulfil the condition λ RA < λ LSP . Varying the angle of excitation offers a unique possibility to finely modify the value of λ RA , thus gradually approaching the condition of coupling between diffracted waves and plasmon modes. The experimental observation of a collective dipolar resonance has been interpreted by exploiting a simplified model based on the coupling of evanescent diffracted waves with plasmon modes. Among other plasmon modes, the measurement technique has also evidenced and allowed the study of a vertical plasmon mode, only visible in TM polarization at off-normal excitation incidence. The results of numerical simulations, based on the periodic Green's tensor formalism, match well with the experimental transmission spectra and show fine details that could go unnoticed by considering only experimental data.

We study theoretically and numerically bidimensional square gratings of monomers and dimers of gold nanocylinders supported on a dielectric substrate, under plane wave illumination as a function of the angle of incidence and of the polarization. The number of parameters investigated makes that system a rich platform for the investigation of how grating coupling, and in particular edge diffraction which corresponds to the grazing propagation of a particular diffracted order, influence the surface plasmons response of nanoparticles. In particular, the considered periods are comparable to the range of incident wavelength, which makes the interpretation of the observed phenomena complex due to the large number of diffraction orders coming into play. In order to analyze those systems, we perform exact numerical simulations using Green's tensor method, and compare them to a simplified approach based on the coupled-dipole approximation. The systematic identification of the grazing diffracted orders, combined with the computation of the S-matrix components, leads to better understanding of the different types of profiles (sharp maxima or angular minima) observed in the extinction spectra around the Rayleigh wavelengths associated with grazing diffraction in air or glass. The analysis is supported by computation of several electric field distributions computed for selected parameters.