Tunable Subwavelength Optical Skyrmions Embedded in Bottle Beams via Metasurfaces

Optical skyrmions, as topologically structured optical fields, hold significant potential for advanced optical tweezers and high-density optical storage. However, most recently proposed schemes for the generation of optical skyrmion are diffraction-limited or confined in near field, with bulky optical setups. Here, we propose and experimentally realize subwavelength optical skyrmions confined within a 3D dark focus. Our approach is built upon a chip-scale spin-decoupled metasurface, enabling simultaneous generation and manipulation of skyrmions. Through superoscillation in the focal region, this metasurface facilitates the formation of optical skyrmions beyond the diffraction limit. The skyrmion size reaches 0.06 λ in theory and 0.15 λ for experimental verification, both significantly smaller than the diffraction limit in this situation. Remarkably, these subwavelength skyrmions are generated approximately 1000 λ from the metasurface plane, under a relatively low numerical aperture of 0.3, underscoring the practicality of our metasurface-based strategy. Moreover, the skyrmion size can be flexibly tuned by modulating the intensity profile of the incident wave, greatly enhancing the adaptability of the scheme for practical applications. This work establishes a feasible platform for embedding ultra-compact optical skyrmions into 3D dark traps and opens new avenues toward multifunctional cold-atom manipulation and topological optical information technologies.