Yoshito Tanaka and Shun Hashiyada (Hokkaido University) have redefined optical angular momentum—the source of the “rotational force (optical torque)” that light exerts on matter—based on the time derivatives of electric and magnetic fields. By separating the redefined angular momentum into two components: spin (rotation of polarization) and orbital (rotation of wavefront), they have successfully derived, for the first time, conservation laws for spin and orbital angular momentum that hold in the presence of matter. This achievement enables quantitative analysis of “spin-orbit conversion,” where spin and orbital components are mutually transformed. It is expected to serve as a foundation for analysis of chiroptical response and for optical micromanipulation technologies.

https://doi.org/10.1103/PhysRevResearch.7.L022052