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

Joel T Collins,Ventsislav K Valev,Marcel Ameloot,Victor V Moshchalkov,Shuai Zu,Nuno V S Braz,Paul A Warburton,Guy A E Vandenbosch,Zheyu Fang,Eli Slenders,Xuezhi Zheng

doi:10.1002/adom.201800153

Abstract

AbstractPlasmonic 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.

Highlights

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
We propose a fundamental origin of this plasmonic chiroptical response
We demonstrate a new kind of bisignate circular dichroism due to enantiomorphing, rather than wavelength change, in the nonlinear emission from near-field hotspots

Summary

MP RCP

There are more red-dominant pairs of correlated modes than, to the point that the overall calculated CD is reversed, as in the experimental observation In these plasmonic nanostructures, the chiroptical response originates from the superposition of all the individual modal responses. We further verify this agreement with a second set of intermediate structures (Supplementary Figure 5) in which the nanostripe length is varied, for a constant width of 200 nm. We observe in both experiments and simulations that the CD emerges away from the achiral structure, and subsequently plateaus.

Discussion

Methods