Skyrmions, originally conceived in nuclear physics, are topologically protected vortex-like configurations that have captivated scientists across various disciplines. These intricate structures, representing localized twists in a field, have been validated experimentally in magnetic materials and other complex matter phases over the past decade. The Stuttgart team has now succeeded in extending this concept to the manipulation of light fields, demonstrating that structured light interacting with precisely engineered metal surfaces can form stable skyrmion configurations conserved within larger skyrmion “bags.”

Central to this achievement was the fabrication of nanoscale patterns etched into a thin gold film with unprecedented precision. The team sculpted two intertwined hexagonal arrays of fine grooves onto the metal surface, each acting as a source for generating distinct skyrmion light fields. By controlling the moiré superlattice—interference patterns arising from the overlay of these twisted hexagonal lattices—the researchers could dictate the spatial arrangement and topological properties of the resulting plasmonic fields.

The experimental observations, led by doctoral researcher Julian Schwab, revealed that when two such skyrmion light fields merged, they gave rise to complex skyrmion bag states. These hierarchical structures consist of multiple skyrmions nested within a larger encompassing skyrmion, a configuration that until now existed primarily in theoretical constructs. Through careful tuning of the relative twist angle between the moiré lattices, Schwab and colleagues achieved deterministic control over the number and arrangement of individual skyrmions within each bag, effectively “sculpting” light fields with new symmetries and topologies.

This exquisite manipulation of plasmonic waves is far more than a scientific curiosity; it challenges prevailing boundaries in optical physics by generating light structures that do not naturally occur in free space. The potential applications of such controlled light topology are vast. For instance, the ability to engineer skyrmion bags could drastically enhance resolution beyond the diffraction limit in optical microscopy, enabling scientists to visualize nanoscopic details of biological and material samples with unmatched clarity.

Moreover, plasmons—collective oscillations of electrons coupled with photons—play a crucial role in this research. The structured gold surface supports surface plasmon polaritons, which confine light to scales far below its wavelength. By exploiting the unique interplay between plasmonic excitation and topological field configurations, the team effectively bridges the gap between light’s wave nature and particle-like topological robustness, offering new platforms to explore ultrafast nano-optics and information processing.

The implications of these findings reach into fundamental physics as well. Skyrmions, due to their topological protection, are immune to certain perturbations and defects, making them promising candidates for stable information carriers. By translating skyrmion physics into the optical domain, researchers may one day realize robust optical storage or logic devices that utilize the twisted nature of light, pushing quantum technologies closer to realization.

This endeavor was a highly interdisciplinary collaboration spanning institutions and expertise. Besides the University of Stuttgart’s experimental efforts, theoretical insights were contributed by the Technion in Haifa, which helped model and predict the behavior of these plasmonic skyrmion bags. Additionally, the University of Duisburg-Essen partnered in verifying the experimental conditions, ensuring the reproducibility and precision of the nanoscale patterning techniques employed.

While the current experiments harness gold as the plasmonic substrate, questions remain about the optimal materials to maximize the stability and efficiency of skyrmion light fields. Harald Giessen, head of the research group, envisions that future work will explore alternative plasmonic metals and novel two-dimensional materials to finetune these effects. Such advancements are critical for translating this fundamental physics insight into practical technologies.

On the theoretical front, this research deepens our understanding of moiré superlattices, a topic of intense study due to its relevance in exotic quantum phases such as superconductivity and correlated insulators. The plasmonic moiré lattices fashioned here represent an optical analog to these electronic systems, revealing new possibilities to control light-matter interactions through engineered symmetry-breaking and topological order.

Furthermore, the exquisite tunability demonstrated—varying the twist angle to adjust the skyrmion count within each bag—echoes themes emerging in the study of twisted bilayer graphene and other van der Waals heterostructures. It underscores a broader trend in physics: topology and moiré engineering as universal tools to unlock new states of matter and light.

The timing of this breakthrough could not be more exciting. As ultrafast laser technologies and nanoscale fabrication techniques continue to evolve, the capacity to create and manipulate complex light fields at will offers a new playground for experimentalists and theorists alike. The Fourth Physics Institute’s leadership in this domain solidifies their position at the forefront of ultrafast nano-optics research.

With the potential to overcome diffraction limits and create robust optical structures immune to disturbances, skyrmion bags of light may soon find applications not only in sophisticated microscopy but also in optical communication, quantum computing, and sensing technologies. While still in the early stages, this discovery heralds a shift towards topologically engineered photonics, enriching the toolkit scientists have to control light-matter interactions on the smallest scales.

In summary, the creation and control of skyrmion bags of light in plasmonic moiré superlattices represents a monumental stride forward in our ability to tailor complex light fields. By synergizing concepts from topology, plasmonics, and moiré physics, the team at the University of Stuttgart has opened promising avenues for both basic science and transformative technological applications. This fusion of theoretical elegance and experimental precision marks a milestone in the quest to harness light’s full potential in unprecedented ways.

Subject of Research:
Ultrafast nano-optics, plasmonic moiré superlattices, topological light fields

Article Title:
Skyrmion bags of light in plasmonic moiré superlattices

News Publication Date:
22-Apr-2025

Web References:
https://dx.doi.org/10.1038/s41567-025-02873-1
https://www.uni-stuttgart.de/universitaet/aktuelles/meldungen/Physiker-entdecken-versteckte-Symmetrie-exotischer-Kristalle/

References:
Julian Schwab, Alexander Neuhaus, Pascal Dreher, Shai Tsesses, Kobi Cohen, Florian Mangold, Anant Mantha, Bettina Frank, Guy Bartal, Frank-J. Meyer zu Heringdorf, Timothy J. Davis & Harald Giessen: Skyrmion bags of light in plasmonic moiré superlattices. Nature Physics, DOI: 10.1038/s41567-025-02873-1.

Image Credits:
University of Stuttgart / 4th Physics Institute

Keywords

Skyrmions, plasmonics, topological photonics, moiré superlattices, ultrafast nano-optics, gold nano-patterning, light field manipulation, diffraction limit, surface plasmon polaritons, nanoscale optics, optical microscopy, topological light structures

The research was mainly carried out by Tuomas Ollikainen, a PhD student at Aalto university who split his time between carrying out experimental work in Amherst in Massachusetts, and analyzing the data and developing his theories at Aalto.

‘We hadn’t been able to study the dynamics of these sorts of three-dimensional structures experimentally before, so this is the first step to this direction.’ says Ollikainen. ‘The fact that the knot decays is surprising, since topological structures like quantum knots are typically exceptionally stable. It’s also exciting for the field because our observation that a three-dimensional quantum defect decays into a one-dimensional defect hasn’t been seen before in these quantum gas systems’

Controlling quantum gasses

The researchers hope their new study opens up new avenues in experimental research. One of the key breakthroughs in the study was being able to have better control over the state of the quantum gas, which allowed them to detect changes in its structure, like the decay of the knots and the formation of the vortex.

‘Of course one can simulate these things but actually making quantum knots is not that easy. By being able to control the environment better we can explore different effects and get to understand more about these exciting quantum systems.’ tells Ollikainen.

‘When we tied quantum knots in 2016, it was the first realization of three-dimensionally winding topological structures. That was like breathing air another planet for the first time. Amazing.’ says Prof. Mikko Möttönen, head of Quantum Computing and Devices group where Ollikainen works.

‘I know that many researchers have paid attention to our work and got inspiration to try this out in completely different type of systems. It would be great to see this technology being used some day in a practical application, which may well happen. Our latest results show that while quantum knots in atomic gases are exciting, you need to be quick to use them before they untie themselves. Thus the first applications are likely to be found in other systems.’ Möttönen continues.