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.
Highlights
The possibility to exert new types of forces on chiral objects by chiral optical fields has recently initiated a lot of interest [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
It is important to realize that the limitations put by this unfavorable ratio can be evaded by developing specific optical schemes where the influence of chiral forces on the mechanical action is clearly identified, either by minimizing, if not merely canceling, non-chiral forces [1, 8] or by forcing the chiral-dependent component of the optical force acting on a different direction than the non-chiral action [2, 3, 9, 11]
We show that the transverse spin angular momentum density, characteristic of surface plasmon (SP) modes, is exactly what is needed to induce a lateral component in the optical forces via an angular-to-linear crossed momentum transfer mediated by the chirality of the scatterer
Summary
The possibility to exert new types of forces on chiral objects by chiral optical fields has recently initiated a lot of interest [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. We consider an original configuration where a finite size chiral sphere is immersed in a plasmonic near field and we predict a purely chiral optical force component in the lateral direction, with opposite directions for opposite enantiomers. For the case of a sphere with circular dichroism, we reveal that the progressive inclusion of multipoles in the lateral force evaluation can lead to a sign inversion of the force when the sphere radius increases This sign inversion is clearly reminiscent of the pulling effect we described recently involving angular-to-linear crossed momentum transfers mediated by chirality [14] and could have important consequences when discussing discriminatory protocols based on the lateral chiral force effect. In a third part we present a multipolar calculation of the plasmonic force exerted on a chiral sphere, which enables to confirm the existence of this chiral lateral force and to discuss the validity of the recoil force in the small sphere limit
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