Helicity maximization below the diffraction limit

Optimally chiral electromagnetic fields with maximized helicity density, recently introduced by Hanifeh et al. [M. Hanifeh, M. Albooyeh, and F. Capolino, ACS Photonics 7, 2682 (2020)10.1021/acsphotonics.0c00304], enable chirality characterization of optically small nanoparticles. Here we demonstrate a technique to obtain optimally chiral near fields 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 near fields. In particular, we explore helicity and energy densities in the near field of a spherical dielectric nanoantenna illuminated by an optimally chiral combination of azimuthally and radially polarized beams. This beam combination generates parallel induced electric and magnetic dipole moments in the nanoantenna that in turn generate an optimally chiral scattered field with the same helicity sign of the incident field. The application of helicity maximization to near fields results in helicity enhancement at the nanoscale, which is of great advantage in the detection of nanoscale chiral samples, microscopy, and optical manipulation of chiral nanoparticles.