"Our device not only generates multiple vortex patterns in terahertz pulses that propagate through free space, but it can also seamlessly switch between two operational modes using a single integrated platform," explained Xueqian Zhang, the lead author from Tianjin University. "This level of control is vital for real-world applications, where the reliable selection and reproduction of specific states are essential for effective information encoding."
The findings were published in Optica, the journal by Optica Publishing Group that highlights significant research advancements. In this study, Zhang and his team revealed how they utilized a nonlinear metasurface to achieve the first experimental demonstration of skyrmions that can be actively toggled between electric and magnetic configurations within toroidal terahertz light pulses. Metasurfaces are ultra-thin materials designed at the nanoscale, enabling them to manipulate light in ways that traditional optical components cannot.
"Our results advance the concept of switchable free-space skyrmions towards a practical tool for robust information encoding," stated Yijie Shen, co-corresponding author from Nanyang Technological University. "This research could pave the way for more resilient methods in terahertz wireless communication and light-based information processing. Such control may also facilitate the development of light-based circuits that can generate, switch, and route various signal states in a precise manner."
Programmable Terahertz Light Structures
Terahertz waves are gaining traction as a key technology for next-generation communication and sensing applications. This research is part of a broader initiative to create terahertz light sources that not only emit pulses but also shape them for practical utility.
One particularly promising configuration is the toroidal vortex of light, which forms a stable, donut-shaped ring where the electromagnetic field curves back on itself. These vortices present additional opportunities for information encoding; however, most existing systems can only produce a single pattern type and typically lack the capability to switch modes.
To overcome this limitation, the researchers developed an integrated device that can alternate between electric and magnetic toroidal vortex patterns in free-space terahertz pulses. This innovative approach utilizes a specially engineered nonlinear metasurface composed of precisely arranged metallic nanostructures.
When near-infrared femtosecond laser pulses with varying polarization patterns interact with the metasurface, the device generates distinct terahertz toroidal pulses. Depending on the polarization, the resulting vortex embodies either an electric-mode or magnetic-mode skyrmion texture. This mechanism operates similarly to selecting different keys to achieve different outcomes, with one light pattern activating the electric mode and another activating the magnetic mode.
"The key innovation is the nonlinear metasurface that transforms shaped near-infrared femtosecond laser pulses into tailored terahertz toroidal light pulses," remarked Li Niu from Tianjin University, who conducted the experimental work.
Project leader Jiaguang Han from Tianjin University added, "By using simple optical elements like wave plates and vortex retarders to control the polarization pattern of the input laser, we have created a compact device that can actively switch between two distinct topological light states."
Measuring and Validating Skyrmion Switching
To evaluate the system's performance, the team constructed an ultrafast terahertz measurement setup that enabled them to observe the light pulse as it propagated through space. Instead of relying on a single measurement, they scanned the pulse across various positions and time points to reconstruct the evolution of the electromagnetic field.
These measurements unveiled the defining characteristics of the toroidal light pulses and clearly differentiated between the two skyrmion modes. The researchers also conducted fidelity measurements to assess performance, confirming reliable switching behavior along with high purity for each mode.
Looking forward, the team intends to refine the technology for communication-focused applications. Future efforts will concentrate on enhancing long-term stability, repeatability, and efficiency, while also miniaturizing and strengthening the system. They aim to extend the approach beyond two modes by introducing additional controllable states, facilitating more complex and flexible information encoding.
