Extraordinary spin density and energy back-flow under interference

TL;DR

This paper investigates the counter-intuitive behaviors of local optical spin density and energy flux in interference fields, revealing phenomena like energy back-flow and transverse spin densities even with uniform polarization.

Contribution

It uncovers new effects of interference on optical spin density and energy flow, including transverse spin and vortex-like energy back-flow, expanding understanding of light-matter interactions.

Findings

Local optical spin density can oppose the integrated spin orientation.

Interference can induce transverse spin density perpendicular to the propagation plane.

Energy flux exhibits back-flow and vortex behavior in interference fields.

Abstract

A novel phenomenon was reported recently that the "local optical spin density" based on Poynting vector might be counter-intuitively opposite to the integrated spin orientation while the one related to the canonical expression might not [Leader, Phys. Lett. B 779, 385-387 (2018)]. However, the "local optical spin density" of the canonical expression can also be counter-intuitively opposite to the integrated spin orientation under the interference of plane waves, even if all of the plane waves possess the same polarization handedness. Moreover, the interference fields might acquire a transverse spin density (perpendicular to the propagation plane), which can have more well-controlled relations with the polarization. Meantime, in such a case its energy flux exhibits counter-intuitive back-flow and a circular motion (vortex) in the propagation plane locally, which implies a transverse local orbital angular momentum density.