Local Optical Chirality Induced by Near-Field Mode Interference in Achiral Plasmonic Metamolecules

TL;DR

This paper demonstrates that achiral plasmonic metamolecules can exhibit local optical chirality in their near-field response due to mode interference, enabling nanoscale imaging of optical chirality.

Contribution

It reveals that achiral nanostructures can have chiral near-field responses caused by mode interference, which can be imaged and utilized in polarization-sensitive applications.

Findings

Achiral plasmonic structures show near-field optical chirality.

Near-field chirality can be imprinted into photosensitive polymers.

Experimental evidence of local optical chirality at nanoscale.

Abstract

When circularly polarized light interacts with a nanostructure, the optical response depends on the geometry of the structure. If the nanostructure is chiral (i.e., it cannot be superimposed on its mirror image), then its optical response, both in near-field and far-field, depends on the handedness of the incident light. In contrast, achiral structures exhibit identical far-field responses for left- and right-circular polarization. Here, we show that a perfectly achiral nanostructure, a plasmonic metamolecule with trigonal D3h symmetry, exhibits a near-field response that is sensitive to the handedness of light. This effect stems from the near-field interference between the different plasmonic modes sustained by the plasmonic metamolecule under circularly polarized light excitation. The local chirality in a plasmonic trimer is then experimentally evidenced with nanoscale resolution using a molecular probe. Our experiments demonstrate that the optical near-field chirality can be imprinted into the photosensitive polymer, turning an optical chirality into a geometrical chirality that can be imaged using atomic force microscopy. These results are of interest for the field of polarization-sensitive photochemistry.