Orbital rotation without orbital angular momentum: mechanical action of the spin part of the internal energy flow

TL;DR

This paper introduces a novel interference method to observe the mechanical action of the spin part of internal energy flow in light beams, demonstrating its effects without orbital angular momentum influence.

Contribution

It presents the first experimental demonstration of the mechanical action of the spin energy flow in light fields using a two-beam interference technique.

Findings

Successful detection of spin flow-induced orbital motion of probe particles

Demonstration that spin flow can be isolated from orbital flow in focused beams

First experimental evidence of mechanical action of the spin component in light energy flow

Abstract

It is known that internal energy flow in a light beam can be divided into the orbital flow, associated with the macroscopic energy redistribution within the beam, and the spin flow originating from instantaneous rotation of the field vectors inherent in circular or elliptic polarization. In contrast to the orbital one, experimental observation of the spin flow constituent seemed problematic because (i) it does not manifest itself in the visible transformation of the beam profile and (ii) it converts into the orbital flow upon tight focusing of the beam, usually employed for the energy flow detection by the mechanical action on probe particles. We propose a two-beam interference technique that permits to obtain appreciable level of the spin flow in moderately focused beams and to detect the orbital motion of probe particles within a field where the transverse energy circulation is associated exclusively with the spin flow. This result can be treated as the first demonstration of mechanical action of the spin flow of a light field.