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

- Near-field and far-field spectral properties of gold nanostructures.
- Multipole field expansion and T-matrix formalism.
- Spectral shift in terms of near and far-field radial and transverse components.
- For dimers the red-shift universality is broken and a blue-shift can also appear.

We investigate the shift between near- and far-field spectral properties in a set of different gold nanostructures 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 enhanced 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 enhanced spectroscopies.