Optical vortex dichroism in chiral particles

TL;DR

This paper demonstrates that chiral particles can differentially absorb optical vortices depending on vortex twist, revealing a new mechanism for optical activity involving longitudinal fields and orbital angular momentum transfer.

Contribution

It introduces a novel dipole coupling mechanism that accounts for longitudinal fields, enabling vortex dichroism in randomly oriented chiral particles, unlike previous paraxial studies.

Findings

Chiral particles absorb vortex beams differently based on vortex twist.

Longitudinal fields enable OAM transfer during dipole interactions.

Vortex dichroism persists in randomly oriented particles, useful for liquid-phase applications.

Abstract

Circular dichroism is the differential rate of absorption of right- and left-handed circularly polarized light by chiral particles. Optical vortices which convey orbital angular momentum (OAM) possess a chirality associated with the clockwise or anti-clockwise twisting of their wavefront. Here it is highlighted that both oriented and randomly oriented chiral particles absorb photons from twisted beams at different rates depending on whether the vortex twists to the right or the left through a dipole coupling scheme. This is in contrast to previous studies that investigated dipole couplings with vortex modes in the paraxial approximation and showed no such chiral sensitivity to the vortex handedness: only in oriented media where electric quadrupole coupling contributes to optical activity effects due to absorption does such a mechanism exist for paraxial vortices. The distinct difference in the scheme highlighted in this work is that longitudinal fields are taken account of and these allow for OAM transfer even during dipole interactions. Due to the vortex dichroism persisting in randomly oriented collections of chiral particles, the mechanism has a distinct advantage in its potential applicability in chemical and biochemical applications where the systems under study are invariably in the liquid phase. Additionally, the result is put into context in terms of the quantifiable optical chirality, highlighting that optical OAM can in fact increase the optical chirality density of an electromagnetic field.