Optical Properties of Metallic Nanotubes with Broken Symmetry: Detection of Eccentricity

The optical properties of dielectric core-metallic concentric shell nanoparticles show a highly sensitive dependence on geometrical parameters of the system. It is due to the strong interaction between plasmon modes excited on the surfaces of the metallic shell. Perturbations in the symmetries of the system modify these interactions and enable new ones, giving rise to dramatic variations in the far field spectrum. In this paper, we examine the electromagnetic response (far and near fields) of hollow metallic nanotubes (nts) with empty or dielectric core as a function of the offset parameter between the core and the shell. The evolution of extinction spectra shows a strong redshift for the dipolar resonance when the offset parameter is increasing, whereas new multipole interactions can be observed at high energy. As dimers, the extinction cross section for nts with nonhomogeneous shell thickness depends on incidence angle with respect to offset axis. We show that for a given offset, the lowest energy peak in extinction curves experiences both, a blueshift and increase of intensity when the angle of incidence is varied with respect to the axis of eccentricity. Maxima of both intensity and blueshift are achieved for an incidence perpendicular to the axis of eccentricity. We show that the optical properties provide sufficient information to unambiguously characterize the system. To compute the electromagnetic field at any point in space, we implemented a rigorous integral formalism (2D geometry) for the two fundamental modes of polarization ( s and p modes).