Analyses of the resonances of both symmetric and antisymmetric polarization states in pairs of tightly coupled
nanoshells, made of either a gold-core/dielectric-shell or a dielectric-core/gold-shell, are carried out at optical
frequencies. The nanoparticles are modeled as single electric dipoles, at first considering only the static (non retarded)
field terms and resorting to closed-form expressions to investigate the transverse and longitudinal (with respect to the
pair axis) plasmonic resonance frequencies of the nanoshell pair. These approximate resonance values are then compared
to the ones obtained including all dynamical retarded field terms, and with full wave simulations. We also show how the
additional degree of freedom provided by using nanoshells, in contrast to using solid metallic nanoparticles, can be
exploited for tuning the symmetric and antisymmetric resonance frequencies in pairs of tightly coupled nanoshells.
Indeed, optical resonances of nanoshells can be varied over hundreds of nanometers in wavelength, across the visible
and into the infrared region of the spectrum, by varying the relative dimensions of the core and shell. This makes the pair
suitable as a constituent for metamaterials since it supports an antisymmetric mode that can be interpreted as an effective
magnetic dipole; therefore, it is useful for providing artificial magnetism in metamaterials that may support backward
propagation or have equivalent high/low characteristic wave impedance. Furthermore, we show the field enhancement
between the two nanoparticles which may find applications in surface enhanced Raman scattering. We also show how an
incident field excites the transversal and longitudinal modes supported by the pair.
Resonances of symmetric and antisymmetric polarization states in tightly coupled nanoshell particles made of either a metallic core and a dielectric shell or, vice versa, a dielectric core and a metallic shell were analyzed at optical frequencies. The investigation was performed by using the single dipole approximation (SDA) with all the dynamical retarded field terms included. Furthermore, analytic formulas for the four possible resonances were derived for the first time by retaining only the static (non-retarded) term in the dipolar field expression. The image principle was used to distinguish a priori between symmetric and antisymmetric modes and for full-wave simulations performed to confirm the identification of resonances achieved by the SDA. It was observed that the resonance frequencies of a pair of nanoshells can be tuned over a wide range of wavelength/frequencies by varying the relative dimensions of the core and shell. This makes this kind of particle pairs suited very well to be adopted either as constituents of metamaterials or to enhance local fields when operating frequencies range from the visible to the infrared spectral regions.