Giant chiroptical effect caused by the electric quadrupole

TL;DR

This paper reveals that electric quadrupolar contributions significantly enhance plasmonic circular dichroism in chiral media, enabling ultrasensitive detection of molecular chirality beyond traditional dipole-based theories.

Contribution

It demonstrates the crucial role of electric quadrupoles in plasmonic CD, challenging the dipole approximation and proposing new nanostructure designs for enhanced chiroptical effects.

Findings

Electric quadrupoles can increase CD signals by two orders of magnitude.

Plasmonic CD activity linked to electric quadrupoles appears mainly at resonance.

Design strategies for giant chiroptical effects using quadrupole contributions.

Abstract

Recently, there is a great interest in studying ultrasensitive detection and characterization of biomolecules using plasmonic particles, because they are of considerable importance in biomedical science and pharmaceutics. So far, all the theories on plasmoninduced circular dichroism (CD) are based on the dipole approximation, the electric quadrupolar contribution is generally considered to be relatively small and neglected. Here we demonstrate that the electric quadrupolar contribution not only cannot be ignored, but also plays a key role in many cases. Particularly, for the chiral medium that possesses preferential molecular orientations and is located at the hotspot of plasmonic nanostructures, the plasmonic CD strength contributed by molecular electric quadrupoles (EQ) can be two orders of magnitude higher than that contributed by molecular electric/magnetic dipoles. Unlike the case of dipole approximation, molecular EQ associated plasmonic CD activity appears mainly at the plasmonic resonance absorptions that facilitate the optically enhanced near-field with steep electric filed gradients, and is correlated with the boosted emission rate of a molecular EQ. Based on such physical understandings, we can design nanostructures to realize giant chiroptical effect using the EQ contribution according to the requirements,2 which provide a new strategy for ultrasensitive detection and quantification of molecular chirality.