Helicity and Angular Momentum. A Symmetry based framework for the study of Light-Matter Interactions

TL;DR

This paper introduces a symmetry-based framework for analyzing light-matter interactions that unifies the treatment of angular momentum using helicity and symmetries, offering clearer physical insights.

Contribution

It presents a novel approach that avoids spin and orbital angular momentum separation by leveraging symmetries, improving theoretical understanding and practical analysis.

Findings

Reveals that spin-to-orbit transfer phenomena are linked to different symmetry breakings.

Provides a unified framework based on helicity and total angular momentum.

Simplifies the analysis of light-matter interactions through symmetry considerations.

Abstract

We propose a new theoretical and practical framework for the study of light-matter interactions and the angular momentum of light. Our proposal is based on helicity, total angular momentum, and the use of symmetries. We compare the new framework to the current treatment, which is based on separately considering spin angular momentum and orbital angular momentum and using the transfer between the two in physical explanations. In our proposal, the fundamental problem of spin and orbital angular momentum separability is avoided, predictions are made based on the symmetries of the systems, and the practical application of the concepts is straightforward. Finally, the framework is used to show that the concept of spin to orbit transfer applied to focusing and scattering is masking two completely different physical phenomena related to the breaking of different fundamental symmetries: transverse translational symmetry in focusing and electromagnetic duality symmetry in scattering.