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 HE 1 and HE 2 eigenmodes. Both the single-and triplestranded 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.

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Featured Application: The main contribution of this work is to propose an electromagnetic finite-difference time-domain method that applies transformation optics for the generation of arbitrary non-uniform physical grids in a simulation system. This method can be applied to the simulation study of light interaction with sub-wavelength structures with efficiency.Abstract: With sub-wavelength scaled structures in a large system, the conventional finite-difference time-domain method can consume much computational resources since it includes both the spatial and temporal dimension in the scheme. In order to reduce the computational cost, we combine the novel methodology "transformation optics" in the simulation to map a physical coordinate with designated non-uniform grids to a uniform numerical coordinate. For a demonstration, the transmission spectrum through a sub-wavelength metallic aperture with one-dimensional and two-dimensional coordinate transformation is simulated, and compared with uniform-grid cases. We show that the proposed method is accurate, and the computational cost can be reduced remarkably to at most 5.31%, in comparison with the simulation of the finest uniform grids demonstrated. We are confident that it should be helpful to the simulation study in sub-wavelength optics due to its verified accuracy and efficiency.

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