Extreme Optical Chirality from Plasmonic Nanocrystals on a Mirror

Area of Science:

• Nanophotonics
• Plasmonics
• Chiral Metamaterials

Background:

• Typically, achiral nanostructures lack chiroptical properties.
• Chiroptical effects are crucial for sensing and optical technologies.
• Controlling chirality in nanomaterials is a significant challenge.

Purpose of the Study:

• To investigate the emergence of chiroptical effects in nominally achiral metal nanocrystals.
• To explore the mechanism behind giant chiroptical responses.
• To demonstrate the generalizability of this phenomenon across different nanocrystal geometries.

Main Methods:

• Synthesizing metal nanocrystals with controlled dimensions.
• Fabricating nanostructure-on-mirror (NoM) configurations with precise gaps.
• Characterizing chiroptical responses using spectroscopy.
• Employing theoretical modeling and simulations to elucidate the underlying physics.

Main Results:

• Observed giant chiroptical effects (anisotropy factors up to g ≈ 0.9) in achiral nanocrystals on gold films.
• Demonstrated the phenomenon for various nanocrystal shapes, including nanodecahedra.
• Identified the enhancement of tiny chiral imperfections by gap-edge modes as the key mechanism.
• Revealed the generation of strong chiroptical signatures through coherent superposition of edge modes and dipoles.

Conclusions:

• Placing achiral nanocrystals near plasmonic surfaces can induce significant chiroptical activity.
• The gap geometry and edge modes play a critical role in amplifying chiral signals.
• This approach offers a novel route to engineer chiroptical responses in nanomaterials.
• The findings have implications for photonic spin Hall effects and chiral light-matter interactions.