| Nov 25, 2024 |
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(Nanowerk Spotlight) Skyrmions, known for their intricate spin textures, are topological quasiparticles initially studied in particle and solid-state physics. They hold great promise as next-generation information carriers for data storage. Recently, their photonic counterparts—optical skyrmions—have emerged as topologically structured light forms with significant potential to revolutionize optical information technologies.
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Current methods for generating optical skyrmions rely on large-scale spatial light modulators or complex configurations, limiting their practicality. While some efforts have leveraged evanescent or magnetic fields, these approaches are confined to optical near fields, making skyrmions challenging to detect and impractical for long-distance propagation. In free-space optics, achieving subwavelength skyrmion beams without bulky modulators has remained a challenge.
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In this work (Nature Communications, “Optical skyrmions from metafibers with subwavelength features”), we introduce a compact and flexible metafiber device that acts as a generator for optical skyrmions. This integrated platform enables the creation of skyrmions with customizable topological textures and subwavelength polarization properties.
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| Optical Skyrmions from meta-fiber-tips (Image: Courtesy of the researchers)
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Drawing inspiration from “lab-on-fiber” technology, we engineered metastructures on fiber tips to generate structured light fields with sophisticated topologies. The space-polarization nonseparability of customized vector beams is achieved near the fiber facet, enabling the excitation of optical skyrmions with designer-defined topologies.
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The metafiber design, depicted in Fig. 1, includes a polarization-maintaining single-mode fiber (PSF), an expansion section, and a metasurface layer on the fiber tip. By superimposing orthogonally polarized Bessel beams (zeroth- and first-order) carrying orbital angular momentum, the polarization Stokes vectors form a Stokes skyrmion with designer-controlled topologies. Adjusting the metasurface design allows for generating various skyrmion types, including Néel, Bloch, and anti-types.
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Experimental validation confirms the generation of skyrmions with topological numbers reaching up to 0.97, showcasing near-perfect skyrmion mapping on the Poincaré sphere. The generated skyrmions exhibit subwavelength polarization features, down to ~λ/5 (where λ is the wavelength), providing a practical solution for free-space propagating optical skyrmions with distinct topological textures.
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| Fig. 1. The schematic diagram of the skyrmions’ excitation from metafiber. The 3D intensity (I) and vector distribution of the excitation skyrmion are shown after exiting the metafiber. Sz: z-compoment of Stokes vector, PSF: polarization-maintaining single-mode fiber, λ: light wavelength. Inset: (a) The construction principle of skyrmion, composed by zeroth-order Bessel Beam and first-order Bessel Beam under orthogonal left circular polarization (LCP) and right circular polarization (RCP). (b-d) The properties of the excited skyrmion. (b) The elliptical polarization distribution with underneath electric field intensity. (c) Hue-colored Stokes polarization distribution. (d) Hue-lightness (HL)-colored unit Stokes Poincaré sphere. (Image: Nature Communications, CC BY)
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In summary, we demonstrate ultracompact metafiber devices as robust generators of optical Stokes skyrmions with tunable topological textures, subwavelength features, and diffraction-resilient propagation. This platform offers opportunities to explore spin skyrmions and reconfigurable metasurfaces powered by phase-change or 2D materials. By enabling diverse electromagnetic light field topologies in compact devices, this work lays the groundwork for advancing practical applications of optical skyrmions and designer structured light fields.
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Source: By Tiantian He, Tsinghua University
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