# Optical chirality from dark-field illumination of planar plasmonic nanostructures

## TL;DR

This paper demonstrates that dark-field illumination can induce optical chirality in planar plasmonic nanostructures, enabling circular dichroism effects similar to true chiral scatterers, with potential for planar device engineering.

## Contribution

The study reveals that dark-field illumination induces optical chirality in planar nanostructures through geometric projection effects, providing a new method to engineer optical chirality in planar devices.

## Key findings

- Circular dichroism observed in single oligomers under dark-field illumination.
- The peak chiral response wavelength shifts with nanoparticle separation.
- Amplification of circular dichroism in closely packed planar arrays.

## Abstract

Dark-field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction analogous to true chiral scatterers. Circular dichrosim is experimentally observed at the maximum scattering of single oligomers consisting rotationally symmetric arrangements of gold nanorods, with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the geometric projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark-field that is normally incident on a glass substrate. Owing to this geometric origin, the wavelength of the peak chiral response is also experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. The realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark-field illumination.