Optical properties of helical and multiring Ag nanostructures: The effect of pitch height.

The pitch height–dependent optical extinction spectra and the electric field enhancement of helical and multiring nanostructures are studied by the discrete dipole approximation method. For light incident along the axis of the helix, with the increase of the pitch height, the plasmon peak wavelengths redshift monotonically and the electric coupling between two adjacent pitches weakens. The plasmon peak can also be tuned by changing the polarization of the incident light, particularly by circularly polarized light. In most cases, the maximum electric field is distributed near the surface of the helix. As a comparison, since multiring structures do not have helicity, the extinction spectra are polarization independent and the maximum electrical fields always distribute on the top ring. These results show that the optical properties of helical structures are different when irradiated by left or right circular polarized light. Understanding and manipulating pitch height allow one to tune the plasmon peak and rearrange the electric field enhancement distribution of helical nanostructures. [ABSTRACT FROM AUTHOR]