Propagation-invariant spatiotemporal vortices

Diffraction, inherently linked to angular spectrum and affected by spatiotemporal coupling effects, imposes a fundamental limit on wave packets propagation. This limitation is particularly acute for spatiotemporal vortex beams, where diffraction distorts the beam profile and splits topological charges. While spatiotemporal coupling engineering has enabled diffraction-free (or propagation-invariant) wave packet propagation, achieving stable vortex propagation remains a significant challenge due to inherent constraints of regular wave dispersion. Here, we overcome this challenge by tailoring the wave system's dispersion to achieve both propagation-invariant behavior and stable vortex structures. We present the first experimental demonstration of propagation-invariant spatiotemporal vortex beams that maintain their spatiotemporal profiles and topological charges over extended propagation distances. The ability of these wave packets to stably carry diverse topological charges without splitting or deformation provides critical insights into spatiotemporal vortex dynamics and unlocks new possibilities for applications across optics, acoustics, and beyond.