A new way to control light could boost future wireless tech

"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."

The team reported the work in Optica, Optica Publishing Group's journal for high-impact research. In the study, Zhang and his collaborators describe how they used a nonlinear metasurface to achieve the first experimental demonstration of skyrmions that can be actively switched between electric and magnetic configurations within toroidal terahertz light pulses. Metasurfaces are extremely thin materials patterned at the nanoscale, allowing them to manipulate light in ways conventional optical components 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 light structures

Terahertz waves are drawing increasing interest for next-generation communication and sensing technologies. This research is part of a larger effort to develop terahertz light sources that do more than emit pulses, with an emphasis on shaping those pulses for practical use.

One especially promising structure is the toroidal vortex of light, which forms a ring where the electromagnetic field curves back on itself into a stable, donut-like shape. These vortices offer additional ways to encode information, but most existing systems can produce only a single type of pattern and usually lack the ability to switch between modes.

To address this limitation, the researchers designed an integrated device capable of toggling between electric and magnetic toroidal vortex patterns in free-space terahertz pulses. The approach relies on a specially engineered nonlinear metasurface made from precisely arranged metallic nanostructures.

When near-infrared femtosecond laser pulses with different polarization patterns strike the metasurface, the device generates distinct terahertz toroidal pulses. Depending on the polarization, the resulting vortex carries either an electric-mode or magnetic-mode skyrmion texture. The mechanism works much like selecting different keys to produce different outcomes, with one light pattern activating the electric mode and another activating 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 conducted 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."

Measuring and validating skyrmion switching

To test how well the system worked, the team built an ultrafast terahertz measurement setup that allowed them to observe the light pulse as it traveled through space. Instead of relying on a single measurement, they scanned the pulse across multiple positions and time points to reconstruct how the electromagnetic field evolved.

These measurements revealed the defining features of the toroidal light pulses and clearly distinguished between the two skyrmion modes. The researchers also used fidelity measurements to evaluate performance, confirming reliable switching behavior along with high purity of each mode.

Looking ahead, the team plans to refine the technology for communication-focused applications. Future work will focus on improving long-term stability, repeatability, and efficiency, while also making the system smaller and more robust. They also aim to expand the approach beyond two modes by adding additional controllable states, which would allow for more complex and flexible information encoding.