"Spin" and "Orbital" Flows in a Circularly Polarized Paraxial Beam: Orbital Rotation without Orbital Angular Momentum

TL;DR

This paper analytically investigates the transverse energy flows in circularly polarized beams, revealing complex spin and orbital flow interactions that influence microparticle motion and can vary across the beam cross section.

Contribution

It provides a detailed analytical study of spin and orbital energy flows in Laguerre-Gaussian beams, highlighting their complex interactions and effects on microparticle dynamics.

Findings

Spin circulatory flow can oppose orbital flow and polarization handedness.

Total transverse energy circulation varies across the beam cross section.

Microparticles can exhibit orbital, spinning, or combined motions depending on position.

Abstract

In light beams with circular or elliptic polarization, the transverse energy flow consists of the "spin" and "orbital" parts. Both of them can induce the orbital motion of microparticles suspended within the field of a light beam, and this should be taken into account in experiments on the spin-to-orbital angular momentum conversion. The character of the spin, orbital and total transverse energy flows in circular Laguerre-Gaussian beams is studied analytically; graphical representations of the flows in the beam cross section (flow maps) are calculated and analyzed. The spin circulatory flow can be directed oppositely to the orbital one and/or to the polarization handedness. As a result, the total transverse energy circulation of a beam with homogeneous circular polarization can be of different handedness in different regions of the beam cross section, which are separated by the contours of zero transverse energy flow. Regarding the particle position within the beam cross section, it can perform orbital, spinning or combined spinning-orbital motion with variable parameters. Possible applications to optical driving of microparticles are discussed.