Period-averaged electromagnetic spin angular momentum is a well-established quantity for monochromatic fields, governing phenomena such as light-matter interactions with chiral particles and spin-orbit coupling effects. In contrast, the spin angular momentum of nonmonochromatic fields remains unexplored. Here, we extend the concept of optical spin to the domain of nonmonochromatic electromagnetic fields. Through this formulation, we uncover the precessional dynamics of electromagnetic spin in specific polychromatic configurations, including the superposition of circularly and linearly polarized plane waves propagating orthogonally at different frequencies, as well as fields generated by a precessing magnetic dipole. We discover that the dynamics of the electromagnetic spin in these cases obeys a Landau-Lifshitz-like equation, establishing a profound parallel between the dynamics of magnetization and photonic spin.

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• Magneto-optical spectra
• Optics & lasers
• Photonics