Helicity Maximization Below the Diffraction Limit

TL;DR

This paper presents a method to generate maximally-helical electromagnetic nearfields beyond the diffraction limit, enhancing nanoscale chiral detection and manipulation by optimizing the helicity density of light.

Contribution

It introduces a technique to produce optimally-chiral nearfields with constant energy density that maximizes helicity density at the nanoscale, surpassing diffraction constraints.

Findings

Helicity density is maximized using azimuthally and radially polarized beams.

Achieves balanced electric and magnetic dipole moments in dielectric nanoantennas.

Enhanced helicity at the nanoscale improves chiral sample detection and optical manipulation.

Abstract

Optimally-chiral electromagnetic fields with maximized helicity density, recently introduced in [1], enable chirality characterization of optically small nanoparticles. Here, we demonstrate a technique to obtain optimally-chiral nearfields that leads to the maximization of helicity density, under the constraint of constant energy density, beyond the diffraction limit. We show how optimally-chiral illumination induces balanced electric and magnetic dipole moments in an achiral dielectric nanoantenna which leads to generating optimally-chiral scattered and total nearfield. In particular, we explore helicity and energy densities in nearfield of a spherical dielectric nanoantenna illuminated by an optimally-chiral combination of azimuthally and radially polarized beams that generates parallel induced electric and magnetic dipole moments that in turn also generate optimally-chiral scattered field with the same handedness of the incident field. The application of helicity maximization to nearfields results in helicity enhancement at nanoscale which is of great advantage in the detection of nanoscale chiral samples, microscopy, and optical manipulation of chiral nanoparticles.