Plasmonic properties of a metallic torus.

Using the plasmon hybridization method, we investigate the optical properties of metallic tori of different shapes and for different polarizations. The plasmon energies are found to be strongly dependent on polarization and on the aspect ratio of the torus, which we define as the ratio of the radii of the two circles that define the structure. For incident light polarized in the plane of the torus, the optical spectrum is characterized by two features, a long wavelength highly tunable dipolar plasmon resonance, and a short wavelength mode corresponding to excitation of several higher order torus modes. For aspect ratios smaller than 0.8, we find that the energy of the tunable dipolar torus mode can be described analytically as an infinite cylinder plasmon of a wavelength equal to the length of the tube. For perpendicular polarization, the spectrum exhibits a single feature made up of several closely spaced higher order torus modes which are only weakly dependent on the aspect ratio. The calculated optical properties are found to be in excellent agreement with results from numerical finite difference time domain calculations and with results from other groups.