Electromagnetic trapping of chiral molecules: orientational effects of the irradiating beam

Abstract

The photonic interaction generally responsible for the electromagnetic trapping of molecules is forward-Rayleigh scattering, a process that is mediated by transition electric dipoles connecting the ground electronic state and virtual excited states. Higher order electric and magnetic multipole contributions to the scattering amplitude are usually negligible. However, on consideration of chiral discrimination effects (in which an input light of left-handed circular polarization can present different observables compared to right-handed polarization, or molecules of opposite enantiomeric form respond differently to a set circular polarization), the mechanism must be extended to specifically accommodate transition magnetic dipoles. Moreover, it is important to account for the fact that chiral molecules are necessarily nonspherical, so that their interactions with a laser beam will have an orientational dependence. Using quantum electrodynamics, this article quantifies the extent of the energetic discrimination that arises when chiral molecules are optically trapped, placing particular emphasis on the orientational effects of the trapping beam. An in-depth description of the intricate ensemble-weighted method used to incorporate the latter is presented. It is thus shown that, when a mixture of molecular enantiomers is irradiated by a continuous beam of circularly polarized light, a difference arises in the relative rates of migration of each enantiomer in and out of the most intense regions of the beam. As a consequence, optical trapping can be used as a means of achieving enantiomer separation.

Quantum electrodynamics in modern optics and photonics: tutorial

“…A clear example of the powerful predictive capacity of QED is that optical binding was originally predicted by Thirunamachandran using QED methods [118], almost a decade before it was first observed experimentally [119]. Further QED studies concerned with optical forces have highlighted discriminatory trapping and binding forces that offer novel possibilities for separating enantiomers by all-optical means [48][49][50]120,121]. A review on the latest research on such binding forces at the nanoscale is to be found in Ref.…”

Chiral separation and twin-beam photonics

“…Therefore, equation (3) corresponds to a non-zero trapping force for spatial separation of enantiomers in a fixed configuration; the corresponding results for freely rotating chiral molecules are found elsewhere. 8,15,16 3.…”

“…If such works prove to be practicable at an industrial scale 14 a significant impact on the health, food and medical sectors is certainly possible. Our previous research 8,[15][16][17][18] worked on the principle of optical trapping which, at the nanoscale, is described by forward-Rayleigh scattering of off-resonant input light; the resultant optical force is dependent on the position of the particle within the beam due to the non-uniform intensity profile of the beam. We established that a slightly different optical force arises when left-handed circularly polarized light is scattered by chiral molecules compared to right-handed light and, equivalently, when circular light of one type is scattered by a left-handed enantiomer compared to a right-handed one.…”

Chirality in Optical Trapping and Optical Binding

“…One possible implementation for enantiomer separation, based on optical trapping, may involve enantiomers in a solution. In such a scenario, while all the enantiomers are attracted to the high-intensity part of a circularly polarized trapping beam (for example, the centre of a Gaussian beam), the left-handed molecule may be more inclined, in relation to its mirror image, to reside in the high intensity region of the beam [105]. This prospect does not account for the intermolecular interactions known as optical binding, which is the subject of the following section.…”