Unraveling How Light’s Rotation Moves Matter — A New Framework to Distinguish Spin and Orbital Effects —

A research group led by Assistant Professor Shun Hashiyada and Professor Yoshito Y. Tanaka at the Research Institute of Electronic Science (RIES), Hokkaido University, has developed a new theoretical framework that enables the independent evaluation of two sources of “rotational force of light” (optical torque): spin (rotational motion due to polarization) and orbital (rotational motion due to wavefront twisting) angular momentum.

Light is not only a straight-traveling wave—it also possesses intrinsic rotational properties. This rotation exerts a mechanical influence on matter, known as optical torque. The origin of this torque lies in optical angular momentum, a conserved physical quantity. Even when light transfers angular momentum to matter through interaction, the total amount is preserved, manifesting as rotational motion in the material. This transfer is governed by the conservation law for optical angular momentum.

Previous theories described spin and orbital angular momentum using only the transverse components of light (those perpendicular to the direction of propagation). These could not account for longitudinal components (those aligned with the direction of propagation) that emerge in the presence of matter, making it difficult to correctly describe conservation laws for spin and orbital angular momenta within material systems.

In this study, the researchers redefined optical angular momentum using the time derivatives of the electric and magnetic fields, allowing it to be decomposed into spin and orbital components while accounting for both transverse and longitudinal field contributions. This formulation enables the conservation laws for each component to be correctly described even in the presence of matter—something that previous theories, which relied solely on transverse fields, could not achieve.

This theoretical foundation opens the door to precise quantitative analysis of spin-orbit conversion, in which spin and orbital angular momentum are interchanged. It also paves the way for future advances in the optical manipulation of nanomaterials, the study of chiral structures, and high-precision light–matter interaction technologies.

This research was published in Physical Review Research on June 4, 2025.