The ubiquitous photonic wheel

TL;DR

This paper develops a comprehensive theory for electromagnetic waves with transverse spin angular momentum, revealing how their shape influences spin orientation and introducing tools to quantify and manipulate such optical fields.

Contribution

The work establishes a general theoretical framework for transverse spin in electromagnetic waves, including new concepts like meridional Stokes parameters and analysis of non-diffracting beams.

Findings

Transverse spin can be either perpendicular to mean linear momentum or to linear momentum density.

Non-diffracting beams like Bessel beams exhibit locally transverse spin.

The theory applies to various optical fields, enabling new manipulation techniques.

Abstract

A circularly polarized electromagnetic plane wave carries an electric field that rotates clockwise or counterclockwise around the propagation direction of the wave. According to the handedness of this rotation, its \emph{longitudinal} spin angular momentum density is either parallel or antiparallel to the propagation of light. However, there are also light waves that are not simply plane and carry an electric field that rotates around an axis perpendicular to the propagation direction, thus yielding \emph{transverse} spin angular momentum density. Electric field configurations of this kind have been suggestively dubbed "photonic wheels". It has been recently shown that photonic wheels are commonplace in optics as they occur in electromagnetic fields confined by waveguides, in strongly focused beams, in plasmonic and evanescent waves. In this work we establish a general theory of electromagnetic waves {propagating along a well defined direction, which carry} transverse spin angular momentum density. We show that depending on the shape of {these waves, the} spin density may be either perpendicular to the \emph{mean} linear momentum (globally transverse spin) or to the linear momentum \emph{density} (locally transverse spin). We find that the latter case generically occurs only for non-diffracting beams, such as the Bessel beams. Moreover, we introduce the concept of \emph{meridional} Stokes parameters to operationally quantify the transverse spin density. To illustrate our theory, we apply it to the exemplary cases of Bessel beams and evanescent waves. These results open a new and accessible route to the understanding, generation and manipulation of optical beams with transverse spin angular momentum density.