Near-field optical patterns of dielectric nanoparticles deposited on a metallic surface

Spatial structures of electromagnetic near-fields generated by plasmonic resonances are studied through numerical simulations. Resonances can appear in silver nanoplates onto which nanoparticles of various shapes are deposited. For forthcoming biophysical applications, nanoparticles are considered here as irregular aggregates of grains made of DNA material. The Discrete Dipole Approximation technique is used to calculate the electromagnetic field profiles. In certain controlled physical situations, the plasmonic pattern appears to be the glowing anti-shadow of the deposited nanoparticle, and such a pattern locally produces strong increase in the electromagnetic fields. Even when the nanoparticle size is much smaller than the wavelength, fine (sub-wavelength) details of the anti-shadow are directly related to the shape of the nanoparticle. These observations should result in a better understanding of the Surface-Enhanced Raman Scattering process and an improvement in nanocharacterization techniques.