Complex topological skyrmions from a “needle”
image: (a) Schematics of (a1) the GRIN lens and (a2) the GRIN-lens array, with radial and azimuthal distributions of the fast and slow axes at their cross section, the radius of which can be less than 500 µm, and (a3) a picture of a real GRIN lens. (b),(c) Diverse polarization vectorial light fields corresponding to the different complex photonic quasiparticles generated can be controlled by cascades of GRIN lenses, represented by (b) polarization-ellipse distributions and (c) Stokes-vector distributions, which include (b1),(c1) a skyrmion of topological number of Ns = 2, (b2),(c2) a skyrmionium of Ns = 0, (b3),(c3) a quadruskyrmion of Ns = 4 comprising four elementary skyrmions of Ns = 1, and (b2),(c2) a quadrumeron of Ns =2 comprising four elementary merons of Ns = 1/2.
Credit: by Yijie Shen, Chao He, Zipei Song, Binguo Chen, Honghui He, Yifei Ma, Julian A.J. Fells, Steve J. Elston, Stephen M. Morris, Martin J. Booth, and Andrew Forbes.
Skyrmions are topologically protected quasiparticles with sophisticated spin textures, widely studied in condensed-matter systems, magnets and recently in photonics, which predicted great potential in ultra-high-capacity information storage, due to their diversified and stable topological spin textures in ultrasmall particle-like region. Motivated by the demand of ultra-capacity information carriers, it is an emerging research direction to create and control more complex quasiparticles with higher-order extended topological textures in addition to the fundamental skyrmions, such as the transformable meron lattices and skyrmion bundles in chiral magnets, skyrmion bags with large topological charges and heliknotons with complex knots in liquid cystals, to name a few. However, these abovementioned topological textures all exist as steady states in materials, maybe perfect for solid-state information storage but impossible for long-range dynamic information transfer. The recent studies of optical skyrmions (Nature Photonics 18, 15–25 (2024). https://doi.org/10.1038/s41566-023-01325-7) may solve this big problem. Importantly, the topological spin textures can be created in higher-dimensional structured light fields (Light Sci Appl 11, 205 (2022). https://doi.org/10.1038/s41377-022-00897-3), and open new direction of topologically stable long-range large-capacity optical communications to revolutionize our information society. Therefore, the emergence of new forms of optical quasiparticle with extended topological structures and orders are always highly desired and promising the expansion of fundamental and applied physical frontier.
In a recent paper published in Physical Review Applied (Phys. Rev. Appl. 21 (2024) https://doi.org/10.1103/PhysRevApplied.21.024025), an internationally collaborated group proposed a new family of quasiparticles, termed multiskyrmions, which possess multipole-like extended configurations with increasingly complex new topologies and controlled by multiple topological orders, beyond the limit of normal skyrmions. In addition, we present the experimental generation and flexible control of large-range of on-demand topological states by a photonic technique. Moreover, the standard skyrmions are usually produced by exotic structured materials such as chiral magnetics, plasmonic systems. While, we show that our new quasiparticle alphabet can be constructed from simple GRIN lenses, lending to immediate and widespread implementation, and in more compact systems.
Moreover, the photonic quasiparticles in GRIN lenses can be coupled to free-space optical systems, realizing a long-range transport of controlled topologies. Based on this advantage, we propose a practical quasiparticle-based information transfer protocol of ultra-capacity encryption, where the multiple topological numbers of diversified quasiparticles are used to encode and transfer information with robust topologies against environmental perturbations. In addition, the capacity and channels in this scheme can be flexibly spanned by arranging the GRIN-lens/quasiparticle array, beating the current optical communication methods.
"We believe this work is a milestone. As skyrmions or quasiparticles in magnets already raised the revolution of ultra-capacity steady data storage, whilst, our work start to face the challenge and open new research direction of skyrmion-based informatics from steady storage to dynamic transport. Our method lends integrated and programmable solutions of complex particle textures, with impacts on both photonic and general condensed-matter systems for revolutionizing topological informatics and logic devices." the scientists forecast.