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.