Mechanical action of inhomogeneously polarized optical fields and detection of the internal energy flows

TL;DR

This paper numerically investigates how inhomogeneous polarized optical fields exert mechanical forces on particles, revealing the roles of spin and orbital energy flows and highlighting the complexity of force interactions for optical studies.

Contribution

It demonstrates that both spin and orbital energy flows contribute to mechanical forces on particles, emphasizing the influence of field inhomogeneity and polarization on optical force measurements.

Findings

Spin and orbital energy flows can produce measurable mechanical forces.

Force direction and magnitude depend on detailed field-particle interactions.

Gradient and dipole forces can significantly alter the observed mechanical action.

Abstract

We analyze numerically the correspondence between the mechanical action, experienced by a spherical microparticle, and the internal energy flows as well as spatial and polarization inhomogeneity of the light field incident on the particle. The inhomogeneous incident field is modelled by superposition of two plane waves, the mechanical action is calculated via the Mie theory for dielectric and conducting particles of different sizes and optical properties. It is shown that both spin and orbital components of the field momentum can produce the mechanical action, which can thus be used for experimental study of the internal energy flows in light beams. However, exact value and even direction of the force applied to a particle depends on many details of the field-particle interaction. Besides, forces that are not associated with any sort of the energy flow (we attribute them to the gradient force owing to the inhomogeneous intensity and to the dipole force emerging due to inhomogeneous polarization) can strongly modify the observed mechanical action. The results should be taken into account in experiments employing the motion of probing particles suspended within optical fields. We suppose that situations where an optical field exerts the polarization-dependent mechanical action on particles can be treated as a new form of the spin-orbit interaction of light.