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Topological spin textures, spatially organized patterns linked to the intrinsic angular momentum of particles, have proved to be highly advantageous for the development of spintronics and quantum technologies. One of the most studied among these textures are skyrmionic textures, which are two-dimensional and stable patterns of spin orientation. Recently, the study of skyrmionic textures has gained significant attention in the field of optics and photonics, revealing novel physical properties and promising potential applications.

In the context of optics and photonics, physicists have so far primarily studied skyrmionic textures in real space. However, they could also be studied in so-called momentum space, where light is described based on their in-plane momentum.

Researchers at Fudan University and Nanyang Technological University recently demonstrated the realization of meron spin textures in momentum space, employing micro-structured materials that control the propagation of light. Their paper, in Â鶹ÒùÔºical Review Letters, could open new exciting possibilities for studying the interactions between topology and light, while also potentially contributing to the development of optical devices and quantum technologies.

"This work originates from our long-standing explorations about the hidden properties of bound states in the continuum (BICs)," Lei Shi, co-senior author of the paper, told Â鶹ÒùÔº. "BICs are important topological singularities, enabling ultra-high quality factors and carrying polarization vortex configurations in momentum space. These properties have made BICs ideal platforms for studying topological light field manipulation."

As part of their earlier works, Shi and his colleagues have extensively explored the topological configurations of BICs. BICs are unusual physical states that do not radiate and remain bound, even if they exist in open systems and in a range of energies at which waves typically leak or spread out.

"In our previous studies, we investigated the topological configurations of BICs across various research areas, including , , and even the ," said Shi. "These studies have initially established our understanding of how BICs modulate light fields."

Over the past few years, the researchers were also inspired by remarkable advances in the . These are structured patterns of the physical vectors in light fields that closely mirror the topological structure of vortex-like spin textures known as magnetic skyrmions.

"Skyrmionic light fields, as an important class of structured light fields, have garnered significant interest due to their novel properties and potential applications," said Shi. "Research on skyrmionic light fields is still in its early stages, and many aspects remain unexplored. Among these, the generation of skyrmionic light fields is particularly critical, as it forms the basis for further studies and applications."

Earlier research suggests that the generation of skyrmionic light fields is closely related to optical vortices. The recent study by Shi and his colleagues builds on this important insight and on their previous attempts to manipulate light fields via BICs.

"We started to explore whether the unique properties of BICs could be utilized for the generation of skyrmionic light fields," said Shi. "On the one hand, we aimed to investigate the relationship between the topological polarization configurations of BICs and skyrmionic light fields, as well as the underlying physical mechanisms driving this connection. On the other hand, we sought to develop a compact and simple method for generating skyrmionic light fields, which could pave the way for their broader applications in the future."

Notably, most previously proposed methods for generating skyrmionic light fields are complex and require bulky systems. Thus, easy methods are still in high demand for practical applications. A further objective of this recent study, therefore, was to devise new and scalable light field generation strategies.

"One of the key challenges in advancing their practical application lies in the development of compact, miniaturized generators, which is also a cutting-edge research focus," said Jiajun Wang, co-corresponding author of the paper, told Â鶹ÒùÔº.

To generate meron spin textures in momentum space, Shi, Wang and their colleagues first designed a photonic crystal slab. This is a micro-structured material that was fabricated by etching periodic holes into a free-standing dielectric film.

"By focusing a monochromatic circularly polarized laser beam onto the photonic crystal slab, we can realize the generation of optical meron textures (one type of skyrmionic light field)," explained Shi.

"Using a home-made Fourier-optics-based measurement system, we measured the far-field polarization distributions of the transmitted light and obtained corresponding Stokes parameters. Here, the normalized Stokes vectors are the pseudo-spins of the studied structured light."

By analyzing the measurements they collected, the researchers were able to characterize momentum-space spin textures within the transmitted light. This ultimately validated the potential of their approach for generating skyrmionics light fields.

"We proposed a novel method to generate Stokes meron spin textures in momentum space, and revealed the connection among BICs, momentum-space polarization vortex, phase vortex and skyrmionic light field," said Shi. "Our method not only addresses critical challenges in skyrmionic light field generation but also establishes a robust framework for their future explorations and applications."

"BICs exist in photonic crystal slabs—periodic dielectric structures without a real-space center," added Wang. "Compared to traditional metasurfaces, our method offers the obvious advantage of requiring no alignment."

The new approach for generating skyrmionic light fields devised by Shi, Wang and their colleagues could soon be employed by other research groups to investigate topologically structured light. In the future, it could contribute to the practical application of skyrmionics light fields for optical information processing, for collecting optical measurements or for the development of quantum optical devices.

"We now plan to continue exploring the hidden effects of the momentum-space topology enabled by BICs," added Shi. "Furthermore, we aim to further investigate the novel effects and potential applications of the generated skyrmionic light fields, such as its propagation properties and stability under disturbance."

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