Ultrafast Photon Spin‐Waves Generated From Photonic Crystal Cavities

Ultrafast manipulation of polarization provides a powerful approach to controlling light–matter interactions on femtosecond timescales. Here, we demonstrate the generation of photon spin‐waves—spatiotemporal oscillations of the spin angular momentum of light—arising from the beating of non‐degenerate orthogonally polarized modes in photonic crystal cavities. To capture this phenomenon, we develop a polarization‐dependent quasi‐normal‐mode (QNM) theory to describe the temporal polarization dynamics of pulsed excitation in anisotropic nanocavities, predicting ultrafast oscillations of the Stokes parameters. Numerical simulations confirm that slight structural perturbations lift the degeneracy, producing sub‐picosecond spin oscillations that are tunable by cavity geometry. Remarkably, the oscillating spin is coupled to the far field, giving rise to propagating photon spin‐waves at the tails of transmitted pulses. We further verify the generality of this mechanism by investigating photon spin‐wave generation in square‐lattice photonic crystals. Our work reveals a compact and versatile platform for engineering ultrafast spin dynamics of light, with promising applications in chiral light–matter interaction, ultrafast magnetism via the inverse Faraday effect (IFE), photonic spin angular momentum information processing, and ultrafast spin‐lasers.