Optical Activity Governed by Local Chiral Structures in Two-Dimensional Curved Metallic Nanostructures

Chiral nanostructures show macroscopic optical activity. Local optical activity and its handedness are not uniform in the nanostructure, and are spatially distributed depending on the shape of the nanostructure. In this study we fabricated curved chain nanostructures made of gold by connecting linearly two or more arc structures in a two-dimensional plane. Spatial features of local optical activity in the chain structures were evaluated with near-field circular dichroism (CD) imaging, and analyzed with the aid of classical electromagnetic simulation. The electromagnetic simulation predicted that local optical activity appears at inflection points where arc structures are connected. The handedness of the local optical activity was dependent on the handedness of the local chirality at the inflection point. Chiral chain structures have odd inflection points and the local optical activity distributed symmetrically with respect to structural centers. In contrast, achiral chain structures have even inflection points and showed antisymmetric distribution. In the near-field CD images of fabricated chain nanostructures, the symmetric and antisymmetric distributions of local CD were observed for chiral and achiral chain structures, respectively, consistent with the simulated results. The handedness of the local optical activity was found to be determined by the handedness of the inflection point, for the fabricated chain structures having two or more inflection points. The local optical activity was thus governed primarily by the local chirality of the inflection points for the gold chain structures. The total effect of all the inflection points in the chain structure is considered to be a predominant factor that determines the macroscopic optical activity. Chirality 28:540-544, 2016. © 2016 Wiley Periodicals, Inc.