Plasmonic lateral forces on chiral spheres

TL;DR

This paper demonstrates that chiral spheres experience a lateral optical force from surface plasmons, which depends on their chirality and size, revealing new aspects of light-matter interaction involving angular momentum transfer.

Contribution

It introduces a multipolar analysis of lateral forces on chiral spheres induced by surface plasmons, highlighting size-dependent sign inversions and the role of transverse spin angular momentum.

Findings

Lateral force depends on sphere chirality and plasmon momentum.

Sign of the lateral force can invert with increasing sphere size.

Recoil effects are significant in the dipolar limit.

Abstract

We show that the optical force exerted on a finite size chiral sphere by a surface plasmon mode has a component along a direction perpendicular to the plasmon linear momentum. We reveal how this chiral lateral force, pointing in opposite directions for opposite enantiomers, stems from an angular-to-linear crossed momentum transfer involving the plasmon transverse spin angular momentum density and mediated by the chirality of the sphere. Our multipolar approach allows us discussing the inclusion of the recoil term in the force on a small sphere taken in the dipolar limit and observing sign inversions of the lateral chiral force when the size of the sphere increases.