Ultrafast space-time optical merons in momentum-energy space.

Skyrmions, topologically non-trivial localized spin structures, are fertile ground for exploring emergent phenomena in condensed matter physics and next-generation magnetic-memory technologies. Although magnetics and optics readily lend themselves to two-dimensional realizations of spin texture, only recently have breakthroughs brought forth three-dimensional (3D) magnetic skyrmions, whereas their optical counterparts have eluded observation to date because their realization requires precise control over the spatiotemporal spectrum. Here, we demonstrate freely propagating 3D-localized optical skyrmionic structures with a non-trivial topological profile by imprinting meron polarization texture on open and closed spectral surfaces in the momentum-energy space of an ultrafast optical wave packet. Precise control over the spatiotemporal polarization texture of light - a key requisite for synthesizing 3D optical merons - is the product of synergy between novel methodologies in the modulation of light jointly in space and time, digital holography, and large-area birefringent metasurfaces. Our work advances the fields of polarization optics and topological photonics and may inspire new developments in imaging, metrology, optical communications, and quantum technologies.