Strong Nanoscale Optical Activity Localized in Two-Dimensional Chiral Metal Nanostructures

Nanostructures with chiral shapes behave like chiral molecules and show optical activity. The optical activity of the chiral nanostructure should be given approximately by the summation of the local optical activities over the entire nanostructure. In this work, we report 100-nm-scale spatially resolved circular dichroism (CD) and compare the results with the macroscopically obtained CD spectrum of S-shaped gold nanostructures. Local CD signals with both directions of handedness coexisted in the individual nanostructures, and the spatial distribution of the CD reflected the chiral symmetry of the nanostructure. When integrated over the entire nanostructure, the local CD signal was approximately 1% of the maximum of the local CD signal, which approximately coincided with the macroscopic CD signal. This indicates that prominent nanoscale local CD signals might exist even if only a tiny CD signal is observed as the macroscopic optical activity of the nanostructured sample.