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, fCD, a magnitude describing the electromagnetic near-field enhancement of scatterers associated with a given helicity. However, an exact analytical expression of fCD remains elusive. 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. Our exact analytical findings extend previous approximate expressions of fCD that are restricted to the dipolar regime. In addition to this, and based on our exact 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. Strikingly, 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 from far-field measurements.