Electric and magnetic toroidal dipole excitations in core-shell spheres

Abstract: Toroidal dipole moments, which consist of enclosed circulating currents aligned along paths within a torus shape, can be experimentally achieved in metamaterials using various geometrical configurations. Here, we investigate the excitation of toroidal dipole moments in a core-shell nanosphere composed of dispersive materials within the framework of the Lorenz-Mie theory. By isolating the contributions related to volumetric oscillations of electric charges from those of transverse and radial current oscillations, we derive closed-form analytic expressions for the scattering coefficients associated with electric and magnetic toroidal dipole moments. These analytic expressions, obtained beyond the Rayleigh scattering regime and previously concealed within the Lorenz-Mie theory, agree with the Cartesian toroidal dipoles determined from the radial and transverse current oscillations. By exploring the resonances in these calculated coefficients, we demonstrate that the electric and magnetic toroidal dipole excitations can be manipulated and tuned for near- and far-field applications involving plasmonic core-shell nanoparticles and engineered metamaterials. We believe that our analytic results could shed light on some of the extraordinary effects obtained in Lorenz-Mie theory that depend on the interference of multipole coefficients, such as toroidal dipole-induced transparency, Fano resonances and superscattering of light.