WASHINGTON — Researchers have created an optical device that can generate both electric and magnetic vortex-ring-like light patterns. These structured light vortices, known as skyrmions, are highly stable and resistant to disturbances, making them promising for reliably encoding information in wireless applications.
"Our device not only generates more than one vortex pattern in free-space-propagating terahertz pulses but can also be used to switch, on demand, between two modes using the same integrated platform," said corresponding author Xueqian Zhang from Tianjin University . "Such controllability is essential for real applications, where reliable selection and reproduction of a desired stated are crucial for practical information encoding."
In Optica , Optica Publishing Group 's journal for high-impact research, Zhang and colleagues describe how they used a nonlinear metasurface to achieve the first experimental demonstration of skyrmions that can be switched between electric and magnetic modes in toroidal terahertz light pulses. Metasurfaces are ultra-thin materials with nano-scale patterns that enable them to bend, focus or filter light in ways that conventional devices cannot.
"Our results move the concept of switchable free-space skyrmions toward a controllable tool for robust information encoding," said co-corresponding author Yijie Shen from Nanyang Technological University . "This work could inspire more resilient approaches to terahertz wireless communication and light-based information processing. This type of control could also enable light-based circuits that generate, switch and route different signal states in a controlled way."
Programmable terahertz vortices
Terahertz waves are increasingly being looked at for next-generation communications and sensing. The new work grew out of the research team's broader effort to develop terahertz light sources that don't just emit terahertz pulses but can also shape these pulses in useful ways.
Toroidal vortices of light — which have a ring-shaped loop where the electromagnetic field curls around itself, forming a stable, donut-like structure — are attractive because they can add extra ways to encode information. However, most existing devices can make only one kind of toroidal vortex pattern, often in a limited way, and don't have a built-in capability to switch between different modes.
To make an integrated device that can be used to switch between electric and magnetic toroidal vortex light patterns in free-space terahertz pulses, the researchers used a specially designed nonlinear metasurface made from carefully patterned metallic nanostructures.
When near-infrared femtosecond laser pulses with different polarization shapes are directed onto the metasurface, it generates different terahertz toroidal light pulses that carry either an electric-mode or a magnetic-mode vortex texture. The process is much like using different keys to open different doors: One light pattern turns on the electric mode while another pattern turns on the magnetic mode.
"The core innovation lies in the nonlinear metasurface that converts shaped near-infrared femtosecond laser pulses into tailored terahertz toroidal light pulses," said first author Li Niu from Tianjin University, who performed the experiments.
Project leader Jiaguang Han from Tianjin University added, "By employing simple optical elements such as wave plates and vortex retarders to control the polarization pattern of the input laser, we are able to create a compact device that can actively switch between two distinct topological light states."
Tracking skyrmion modes
To evaluate the device performance, the research team built an ultrafast terahertz measurement setup that let them "watch" the generated terahertz pulse as it moves through space. Rather than relying on a single snapshot, they scanned the pulse at different positions and times to reconstruct how the field pattern evolves.
The measured field patterns clearly exhibited the characteristic profiles of the toroidal pulse and the two distinct skyrmion modes. Performance was further quantified using fidelity measures, which confirmed both reliable switching capability and high mode purity.
Looking ahead, the researchers aim to advance the concept toward communication-relevant applications. This will entail enhancing long-term stability, repeatability and overall efficiency, while also making the entire system more compact and robust. Furthermore, they intend to extend the system beyond two states by incorporating additional controllable modes, thereby enabling richer encoding schemes.
Paper: L. Niu, X. Feng, X. Zhang, W. Yu, Q. Wang, Y. Lang, Q. Xu, X. Chen, J. Ma, H. Qiu, Y. Shen, W. Zhang, J. Han, "Electric-Magnetic-Switchable Free-Space Skyrmions in Toroidal Light Pulses via a Nonlinear Metasurface" 13, (2025).
DOI: 10.1364/OPTICA.578501 .
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