Extrinsic Chirality by Interference between Two Plasmonic Modes on an Achiral Rectangular Nanostructure

The optical near field (NF) induced by circularly polarized light (CPL) is a hot scientific topic. We observed a chiral NF intensity distribution on a series of achiral gold nanorectangular structures (Au-NRs) under CPL irradiation by using multiphoton photoemission electron microscopy (MP-PEEM). Additionally, the differential NF spectra under left and right CPL irradiation, which represent the asymmetry of the NF intensity distribution, were investigated. We propose an interpretation that the chiral NF intensity distribution on an achiral metallic nanostructure is extrinsically generated by the interference between two plasmonic modes by combining state-of-the-art MP-PEEM techniques and the classical oscillator model. Our interpretation well explains both the experimental and simulation results. Furthermore, the intensity of the NF and its phase angle of each mode under linearly polarized light irradiation were revealed to be critical factors for the generation of extrinsic chirality in the NF intensity distribution.