Both orbital and spin torques originate from r x g

TL;DR

This paper demonstrates that both orbital and spin torques in light-matter interactions originate from the cross product of position and electromagnetic momentum density, g, confirming the role of optical spin momentum.

Contribution

It provides explicit evaluation showing that the cross product r x g accounts for optical spin momentum and associated torques, resolving a long-standing question.

Findings

r x g includes optical spin momentum

Optical spin momentum contributes to torque exerted on particles

Explicit calculations confirm the origin of spin and orbital torques

Abstract

Does the cross-product of the position and the electromagnetic momentum density, g, include the optical spin momentum? We answer this long-standing question in the affirmative by evaluating, explicitly, the torque exerted on a particle by a beam of light carrying angular momentum.