Extraordinary transverse spin: Hidden vorticity of the energy flow and momentum distributions in propagating light fields

TL;DR

This paper explores the complex dynamical properties of inhomogeneous electromagnetic fields, revealing a fundamental relationship between spin and momentum that enhances understanding of light behavior and potential applications in optical manipulation.

Contribution

It investigates the spin-momentum relationship in guided light fields, clarifying its applicability and limitations, and connects transverse spin with energy flow in inhomogeneous wave fields.

Findings

Spin-momentum equation applies to guided waves with defined propagation direction.

Transverse spin is linked to the curl of the longitudinal momentum density.

Results have implications for optical trapping and data processing.

Abstract

Spatially inhomogeneous fields of electromagnetic guided modes exhibit a complex of extraordinary dynamical properties such as the polarization-dependent transverse momentum, helicity-independent transverse spin, spin-associated non-reciprocity and unidirectional propagation, etc. Recently, the remarkable relationship has been established between the spin and propagation features of such fields, expressed through the spin-momentum equations [Proc. Natl. Acad. Sci. 118 (2021) e2018816118] connecting the wave spin with the curl of momentum. Here, the meaning, limitations and specific forms of this correspondence are further investigated, involving the physically transparent and consistent examples of paraxial light fields, plane-wave superpositions and evanescent waves. The conclusion is inferred that the spin-momentum equation is an attribute of guided waves with well defined direction of propagation, and it unites the helicity-independent "extraordinary" transverse spin with the spatially-inhomogeneous longitudinal field momentum (energy flow) density. Physical analogies with the layered hydrodynamic flows and possible generalizations for other wave fields are discussed. The results can be useful in optical trapping, manipulation and the data processing techniques.