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

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, f_{CD}, a magnitude describing the electromagnetic near-field enhancement of scatterers associated with a given helicity. However, an exact analytical expression of f_{CD} remains elusive. Here, we derive an exact multipolar expansion of f_{CD}, 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. Our exact analytical findings extend previous approximate expressions of f_{CD} that are restricted to the dipolar regime. In addition to this, and based on our exact analytical findings, we show that the near-field f_{CD} 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. Strikingly, we show that in the case of lossless cylindrically symmetric samples, the near-field f_{CD} 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 from far-field measurements.