Chirality Dependent Spatially Discriminated Excitations Using Vogel Spirals

The spatially discriminated enhancements are achieved by introducing anisotropic Au nanorods in Fibonacci spirals. Building from Vogel's formula [1], where the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{n}^{\mathrm{t}\mathrm{h}}$</tex> nanorod is located at polar coordinate as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$r_n=\mathrm{k} \sqrt{n}, \phi_n= n\varphi$</tex> with the scaling factor k=32, and the divergence angle set to the golden angle <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varphi=137.508^{\circ}$</tex>, intertwined spirals with packing efficiency as high as 80% are formed. In our FDTD simulations, the elliptical nanorods has a dimension with short axis a=20 nm, and long axis b=22 nm. Total-field scattered field (TFSF) source and 8 layer PML boundary conditions are used. As compared to circular nanorods, exploiting elliptical nanorods can selectively enhance certain spiral groups, depending on their orientations. As a result, chirality dependent spatial excitations can be achieved.