Enantiomorphing Chiral Plasmonic Nanostructures:A Counterintuitive Sign Reversal of the Nonlinear Circular Dichroism

Plasmonic nanostructures have demonstrated a remarkable ability to control light in ways never observed in nature, as the optical response is closely linked to their flexible geometric design. Due to lack of mirror symmetry, chiral nanostructures allow twisted electric field hotspots to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Understanding the properties of these chiral hotspots is crucial for their applications; for instance, in enhancing the optical interactions with chiral molecules. Here, the results of an elegant experiment are presented: by designing 35 intermediate geometries, the structure is enantiomorphed from one handedness to the other, passing through an achiral geometry. Nonlinear multiphoton microscopy is used to demonstrate a new kind of double-bisignate circular dichroism due to enantiomorphing, rather than wavelength change. From group theory, a fundamental origin of this plasmonic chiroptical response is proposed. The analysis allows the optimization of plasmonic chiroptical materials. V.K.V. acknowledges support from the Royal Society through the University Research Fellowships. Nuno Braz acknowledges support from the EPSRC grant reference EP/G037256/1. The authors would like to acknowledge the C2 project (C24/15/015) and the PDMK/14/126 project of KU Leuven, the FWO long term stay abroad project grant V405115N, and the Methusalem project funded by the Flemish government. The authors are grateful to Hongjian Tao, Deputy Director of Department of International Cooperation and Exchanges in the Ministry of Education of the People's Republic of China, for facilitating the collaboration between the UK, Chinese, and Belgian teams. All data supporting this study are openly available from the University of Bath data archive at https://doi.org/10.15125/BATH-00483.