Spiral surface plasmon modes on uniform and tapered metallic nanorods

Spiral surface plasmon (SSP) modes on uniform and tapered silver nanorods are explored by performing both simulations and theoretical analyses. On a uniform nanorod with a radius equal to 240 nm, the SSP modes can be generated by linearly superposed higher-order HE₁ and H₂ eigenmodes. Both the single- and triple-stranded SSP modes are produced by controlling the relative rotation direction of the two component modes. On a tapered nanorod, the spiral pitch of the SSP mode decreases with the reduction of nanorod radius. However, the field energy density along the nanorod axis increases to a maximum value and then falls.