Nanoshells to nanoeggs to nanocups: optical properties of reduced symmetry core-shell nanoparticles beyond the quasistatic limit

The plasmonic properties of metallodielectric nanoparticles exhibit a highly sensitive dependence on geometry, due to the interaction between primitive plasmon modes associated with the surfaces of the nanoparticle. Changes in nanoparticle geometry that reduce symmetry alter the interactions between plasmon modes and give rise to modified, and altogether new, plasmonic features. Here, we have examined the near- and far-field optical properties of three variants of a core-shell nanoparticle: nanoshells, nanoeggs and nanocups. Nanoshells, consisting of a spherical silica core coated with a thin gold shell, convert to 'nanoeggs' by offsetting the core within the shell. Offsets of the core greater than the thickness of the shell layer, where the core pierces the shell, result in 'nanocups'. The absorption and scattering spectra of a nanoegg reveal the emergence of multipolar peaks strongly redshifted relative to those of nanoshells and larger near- field enhancements. The wavelength of maximum field enhancement increases with increasing core offset, distinct from the dipole resonance of the nanoparticle. For larger nanoeggs beyond the quasistatic regime, variations in the relative contribution of scattering and absorption to the nanoparticle extinction depend upon both the core-shell offset and on overall particle size. These observations may lead to new opportunities to tailor near- and far-field properties of plasmonic nanoparticles for specific applications, such as high performance surface-enhanced spectroscopy, bioimaging and nanoparticle-based therapeutics.