Imaging Chirality of Optical Fields near Achiral Metal Nanostructures Excited with Linearly Polarized Light

Chiral systems (consisting of materials and incident radiation) respond differently to left- and right-handed circularly polarized light macroscopically. As a consequence, only chiral materials show intrinsic macroscopic optical activity, and only chiral systems generate circularly polarized light from linearly polarized incident light. In the nanoscopic regime, in contrast to this general rule for macroscopic cases, it is theoretically expected that achiral (nonchiral) systems can locally generate circularly polarized fields. Here, we report experimental evidence for that situation in achiral systems consisting of gold nanostructures and linearly polarized incident light. The local circularly polarized fields were visualized by near-field polarimetry imaging, and the spatial features of the observed circularly polarized fields were qualitatively reproduced by a simple dipole model. The present results may provide a novel technique to produce controllable circularly polarized optical fields in nanospaces.