Construction of optical spatiotemporal skyrmions

A skyrmion is a topological quasiparticle with nontrivial topology and a three-dimensional vector field configuration. Its shape is topologically protected—like a knot in a rope that cannot be undone without cutting. Owing to this stability, skyrmions can exist in diverse systems, including ultracold atomic gases, liquid crystals, magnetic materials, and even water waves. Their nanoscale size and robustness make them promising candidates for high-density data storage, information transfer, and novel computing technologies.

 

In recent years, scientists have translated the concept of skyrmions into optics, creating optical skyrmions by precisely controlling light’s phase, polarization, and orbital angular momentum. These structures remain stable even under perturbations, offering exciting possibilities for optical communication and photonic computing. However, previous research has focused almost entirely on spatial-domain skyrmions [Fig. 1(a)], while vectorial topological quasiparticles that evolve in time—spatiotemporal skyrmions—have been scarcely explored. Such structures not only possess spatial topological twisting but also exhibit complex temporal evolution, opening new horizons for high-dimensional optical topology.

 

In this work, we propose and experimentally realize, for the first time, optical skyrmions in the spatiotemporal domain. The approach relies on transverse orbital angular momentum (OAM) and vectorial shaping of pulsed light, expanding light-field control from a purely spatial framework to a full spatiotemporal paradigm. By temporally and spatially aligning spatiotemporal Gaussian-mode pulses with orthogonally polarized spatiotemporal vortex-mode pulses, we constructed the fundamental topology of a spatiotemporal skyrmion [Fig. 1(b)]. These textures span all possible polarization states in the spatiotemporal domain. Due to uniform vector shaping along the y-axis, the skyrmion exhibits no helical twisting perpendicular to its plane (the x–t plane), forming an ideal skyrmion tube with identical slices along its length.

 

The spatiotemporal skyrmions revealed fascinating evolution during propagation. In a dispersion-matched medium, their structure is maintained by a precise balance between medium dispersion and spatial diffraction—much like two people pulling a rope in perfect equilibrium. In this regime, the beam size changes gradually with propagation distance, while a phenomenon known as the Gouy phase shift causes the skyrmion’s helicity to rotate uniformly along the propagation axis. Initially, the skyrmion is of the Néel type (arrows pointing inward or outward, middle column in Fig. 2), then transitions into a mixed type midway (first and third columns), and eventually becomes a Bloch type (arrows rotating in circles) far from the source. In contrast, free-space propagation can stretch and deform the “light knot,” altering its topology. These results shed light on the complex evolution of light in space-time and offer new strategies for controlling optical topological structures.

 

From space to space-time, we have, for the first time, constructed and directly observed optical spatiotemporal skyrmions within ultrafast light wave packets—extending light’s topological structures beyond the three-dimensional spatial limit into the four-dimensional spatiotemporal realm. This breakthrough not only enables new possibilities for light–matter interactions, ultrafast optical communication, and topological information storage, but also opens the door to exploring higher-dimensional optical topological states. In the future, these spatiotemporal “light knots” may become the highways of information transfer, milestones in optical topology, and keys to unlocking unexplored worlds of structured light.

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