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The Role of Particle Size in the Dispersion Engineering of Plasmonic Arrays

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journal contribution
posted on 13.01.2020, 19:43 by Veronika Tretnak, Ulrich Hohenester, Joachim R. Krenn, Andreas Hohenau
Grazing diffraction orders on metal gratings give rise to peculiar optical effects that were contemplated by Wood, Rayleigh, and Fano. With plasmonic nanoparticles as resonant grating elements, the phenomenology of such surface lattice resonances becomes quite rich, including spectrally narrow extinction peaks and optical band gap formation. It has been observed that at perpendicular incidence, either the higher- or lower-energy branch corresponding to the first grazing diffraction orders is bright, that is, couples strongly to light. Reviewing the literature, it appears that particle size is the factor determining which dispersion branch lights up. However, a consistent explanation for this effect is lacking. After revisiting the effect experimentally and by numerical simulation, we clarify the underlying physics by analyzing nanoparticle gratings in terms of, first, an oscillator model and, second, a photonic crystal description. Both approaches reveal the central role of a particle size-dependent phase shift in the back-scattering of grazing light fields by the particle grating. This phase shift determines the symmetry of the resulting field profiles corresponding to the dispersion branches and thus their ability to couple to the exciting light. This physical understanding could considerably simplify the dispersion engineering of plasmonic nanoparticle gratings for specific applications as sensing or lasing.