Angular plasmon response of gold nanoparticles arrays: approaching the Rayleigh limit

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 single-particle localized surface plasmon resonance (lambda(LSP)) is very close in value to the Rayleigh anomaly wavelength (lambda(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 lambda(RA) < lambda(LSP). Varying the angle of excitation offers a unique possibility to finely modify the value of lambda(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.