Spectral shift between the near-field and far-field optoplasmonic response in gold nanospheres, nanoshells, homo- and hetero-dimers

We investigate the shift between near- and far-field spectral properties in a set of different gold na- nostructures using the multipole expansion of the electromagnetic fields. The optical behavior of isolated spheres, gold-silica nanoshells, homo- and hetero-di7mers is studied computationally as a function of the particle size. We show that in isolated nanospheres, both homogeneous and shell-like, the red-shift between near- and far-field peak intensities increases with nanoparticle dimension and has a universal character that can be attributed to the different spectral behavior of the radial component of the en- hanced field, dominating in the near-field, with respect to its transverse part, which determines the far- field properties. Calculations on dimers of closely separated nanospheres, instead, highlight that the spectral shift between the maximum field intensity, calculated at the dimer nanocavity center, and the maximum extinction or scattering, computed in the far-field, strongly depends on both the radius of the monomers and the gap distance. The shift can turn from positive (i.e. red-shift) to negative (i.e.blue-shift) depending on the values of the nanostructure parameters. Our results call into question the universality of the energy red-shift, with relevant implications for the optimization of the efficiency in surface en- hanced spectroscopies.