Spiral surface plasmon modes inside metallic nanoholes

Spiral surface plasmon (SSP) modes that propagate inside a silver (Ag) nanohole are investigated by performing both simulations and theoretical analyses. The SSP modes are formed by a linear combination of two rotating SP eigenmodes of the Ag nanohole in the fast-wave branch. Inside a uniform Ag nanohole, the handedness and the number of strands of the SSP modes are determined by both the component SP eigenmodes and their rotation directions. The spiral pitch of the SSP mode increases with the nanohole radius for a fixed wavelength and is inversely related to the incident wavelength for a fixed nanohole radius. Inside a tapered Ag nanohole, the spiral pitch decreases with the reduction of nanohole radius. However, the azimuth-integrated field energy density increases to a maximum value and then falls. For a tapered Ag-clad fiber capped by a tapered Ag nanorod, the SSP mode reverses its handedness when it passes through the fiber-nanorod interface. Furthermore, using this composite structure, the field energy density of SSP mode that arrives at the tip of the tapered nanorod is largely increased.