Spin and orbital angular momentum dynamics in counterpropagating vectorially structured light
Hang Li, Valeria Rodríguez-Fajardo, Peifeng Chen, Andrew Forbes
Abstract
Here we study spin angular momentum (SAM) and orbital angular momentum (OAM) in counter propagating vectorial light fields, where the polarization forms a longitudinal standing wave structure, oscillating from vector to scalar. We outline the dynamics of the exchange between the electric and magnetic components in such vectorial longitudinal standing waves, highlighting regimes in which the individual components are constant along the $z$ axis. We confirm the theoretical findings with a simple experimental setup that reveals all the salient features, allowing us to measure the SAM and OAM densities, including through the ``peaks'' and ``troughs'' of the standing wave at subwavelength step intervals where polarization structure changes from homogeneous to nonhomogenous. The apparent spin-orbit coupling in such counterpropagating paraxial light is explained as an artefact of considering only one component of the field, electric or magnetic. This work generalizes previous findings based on traveling vectorial structured light waves, explains the apparent spin-orbit coupling in counterpropagating paraxial light, and broadens our understanding of angular momentum conservation in arbitrary structured light waves.