Optimally chiral optical trapping

Abstract

Our investigation focuses on studying the forces induced on a chiral dipolar particle when exposed to optimally chiral light (OCL). The concept of OCL encompasses all structured light fields that achieve maximum helicity density at a given energy density. Examples of OCL include circularly polarized light (CPL) and the optimally chiral configurations of the azimuthally-radially polarized beam (ARPB), which is a phase-shifted superposition of an azimuthally and a radially polarized beams. The optimal chirality condition requires that light's magnetic and electric fields along the same direction be phase-shifted by a quarter of a period and that the ratio of their magnitudes equals the characteristic impedance in free space [Hanifeh, Albooyeh, Capolino, ACS Phot 2020, 7, 10, 2682-2691. By meeting these conditions, the resulting field also exhibits electric-magnetic symmetry in its energy and spin densities. Consequently, OCL simplifies the computation of forces induced on a chiral dipolar particle, while simultaneously boosting its ability to discern chirality. Notably, the gradient force depends exclusively on the gradient of the energy density (as opposed to a combination of gradients), whereas the remaining forces can be expressed using the Poynting vector and the field's orbital momentum. Given these properties, optimally chiral fields represent a promising avenue for the precise manipulation of chiral nanoparticles. Additionally, the choice of the beam is dependent on the geometry of the optical trap, as different optimally chiral fields have different spatial features. While a beam with CPL induces more pronounced transverse forces that discriminate chirality perpendicular to the propagation direction, the optimally chiral ARPB produces strong longitudinal forces that discriminate chirality along the propagation direction, eliminating the problem of false chirality detection due to electric anisotropy of the nanoparticle.

The Combination of the Azimuthally and Radially Polarized Beams: Helicity and Momentum Densities, Generation, and Optimal Chiral Light

“…Additionally, the different chirality densities displayed by the ARPB and CPL affect possible designs of chirality‐discriminating optical traps, as discussed in ref. [47]. Preliminary theoretical results also show that illuminating dielectric nanospheres with an OC‐ARPB results in almost twice the helicity density enhancement than polarized Gaussian beam.…”