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. Recently, the increased capabilities in generating pulsed optical fields that are rigidly transported in linear media without diffraction or dispersion has opened the path to realisation of 3D optical skyrmionic structures. Here, the authors demonstrate 3D-localized optical merons by imprinting polarization textures onto the momentum-energy space of ultrafast light pulses.