Empowering structured light to enhance chirality detection and characterization at nanoscale

Abstract

We demonstrate that the detection of material chirality is possible by employing a large category of electromagnetic fields rather than using the standard method based on two plane waves with circular polarization. We illustrate that any set of two fields with equal electric and magnetic energy densities and non-zero difference between their helicity densities lie within the proposed category. This suggests a one-to-one relation between maximizing the helicity density of fields and maximizing the probability of chirality detection of inclusions. Therefore, with the goal of maximizing detection sensitivity, we find an upper bound for helicity density of generic electromagnetic fields which is attributed to a specific polarization of the fields. Then, we elaborate that chirality characterization, i.e., determining the strength of chirality, of nanoparticles samples is achieved only within a subcategory of convenient fields introduced for detection. Briefly, this subcategory consists of electromagnetic fields with helicity densities that reach the proposed upper bound. We further offer two examples of structured lights, i.e., an optical beam composed of properly phase shifted azimuthally and radially polarized beams, and also specific nearfields, which belong to the proposed category and demonstrate how a specially engineered beams and/or nanoantennas substantially improve the chirality detection of nanoparticles by locally maximizing the helicity density of the excitation fields. We finally show that both the introduced optical beam and the proposed nanoantenna enable chirality characterization by offering fields reaching the upper bound of the helicity density.