Capturing near-field circular dichroism enhancements from far-field measurements

TL;DR

This paper develops a theoretical framework linking near-field circular dichroism enhancements to far-field measurements, enabling easier experimental characterization of chiroptical devices.

Contribution

It derives an exact multipolar expansion of the near-field CD enhancement factor and relates it to measurable far-field quantities, facilitating practical device analysis.

Findings

Near-field CD enhancement factor can be expressed via far-field scattering and helicity.

For lossless cylindrically symmetric samples, near-field enhancements can be inferred from two far-field measurements.

The framework enables experimental characterization of chiroptical enhancement devices.

Abstract

Molecular Circular dichroism (CD) spectroscopy faces significant limitations due to the inherent weakness of chiroptical light-matter interactions. In this view, resonant optical antennas constitute a promising solution to this problem since they can be tuned to increase the CD enhancement factor, fCD, a magnitude describing the electromagnetic near-field enhancement of scatterers associated with a given helicity. Here, we derive an exact multipolar expansion of fCD, which is valid to deduce the integrated near-field CD enhancements of chiral molecules in the presence of scatterers of any size and shape under general illumination conditions. Based on our analytical findings, we show that the near-field fCD factor can be related to magnitudes that can be computed in the far-field, i.e., the scattering cross-section and the helicity expectation value. Moreover, we show that in the case of lossless cylindrically symmetric samples, the near-field fCD factor can be inferred experimentally only from two far-field measurements at specific scattering angles. Our contribution paves the way for the experimental characterization of devices capable of enhancing molecular CD spectroscopy.