Generating optical angular momentum through wavefront curvature

TL;DR

This paper reveals how wavefront curvature in strongly focused light beams can generate and control optical angular momentum, including transverse spin and orbital angular momentum, even from simple linearly polarized Gaussian beams.

Contribution

It introduces a novel mechanism linking wavefront curvature gradients to angular momentum generation in focused beams, expanding control over light-matter interactions.

Findings

Circularly polarized beams develop helicity-dependent transverse spin.

Linearly polarized Gaussian beams produce longitudinal spin and orbital angular momenta after focusing.

Higher-order paraxial terms are responsible for non-trivial angular momenta.

Abstract

Recent developments in the understanding of optical angular momentum have resulted in many demonstrations of unusual optical phenomena, such as optical beams with orbital angular momentum and transverse spinning light. Here we detail novel contributions to spin and orbital angular momentum generated by the gradient of wavefront curvature that becomes relevant in strongly focused beams of light. While circularly polarized beams are shown to develop helicity-dependent transverse spin, a linearly polarized Gaussian beam produces longitudinal spin and orbital angular momenta in the focal region, even if lacking both of these before focusing. Analytical treatment of a nonparaxial electromagnetic field, validated with vectorial diffraction modelling, shows that the terms related to higher orders of a paraxial parameter are responsible for the appearance of non-trivial angular momenta. The obtained dependences relate these quantities to the gradient of the wavefront curvature, showing how it can be used as a novel degree of freedom for applications in optical manipulation and light-matter interactions at subwavelength scales, enabling angular momentum transfer even from a simple Gaussian beam with linear polarization.