The device, described in the journal Light: Science & Applications, combines functions that are typically performed by separate hardware components.

The researchers demonstrated that the programmable surface can perform basic logical operations while simultaneously modulating terahertz signals, suggesting that future communication systems could integrate signal transmission and elementary data processing within a single device. Such an approach could prove useful in applications requiring extremely fast responses, including autonomous vehicles, industrial robots, drones and intelligent sensing systems.

As wireless networks evolve beyond current 5G technology, researchers are increasingly focusing on terahertz frequencies, which lie between microwaves and infrared radiation in the electromagnetic spectrum. Their high frequencies enable them to carry far larger amounts of information than today's microwave-based communication systems, while their short wavelengths also make them attractive for imaging and sensing because they can provide higher spatial resolution than conventional radar.

Future communication networks, however, are expected to do more than simply transmit growing volumes of data. Many emerging applications require devices not only to communicate but also to interpret information and react almost instantly. Sending every measurement to a separate processor before taking action introduces delays that may become unacceptable in time-critical applications.

One proposed solution is to perform some processing directly where the signal is generated or transmitted. Programmable metasurfaces have emerged as one possible way to achieve this.

A metasurface is an ultrathin structure composed of many microscopic elements, each smaller than the wavelength of the radiation it manipulates. Together, these elements can dynamically control electromagnetic waves by changing how they are transmitted or reflected. Unlike conventional materials with fixed properties, programmable metasurfaces can alter their behaviour in real time, opening possibilities for devices that actively manipulate signals rather than simply passing them along.

In the new study, researchers designed a programmable terahertz metasurface capable of performing two functions on the same platform: carrying out simple logical operations and encoding information into transmitted signals.

Instead of controlling every microscopic element individually, the researchers divided the surface into four independently controlled sections, known as subarrays. Each section can be switched electrically between different transmission states, allowing the overall surface to be reconfigured while keeping the system relatively simple.

The device is based on aluminium gallium nitride/gallium nitride (AlGaN/GaN) high-electron-mobility transistors, or HEMTs. Applying different voltages changes the electrical properties of an ultrathin electron channel within the semiconductor structure, enabling dynamic control over how much terahertz radiation passes through the metasurface.

Experiments showed that the device operated across a broad frequency range between 170 and 260 gigahertz. At 240 GHz, changing the control voltage increased wave transmission from approximately 30% to 78%, demonstrating that the metasurface could efficiently regulate the intensity of transmitted terahertz signals.

The researchers then used this capability to demonstrate that the metasurface could perform elementary computing tasks directly on electromagnetic waves.

Two of the programmable subarrays served as logical inputs representing binary values of zero and one. Depending on how the incoming wave was modified as it passed through the device, the output corresponded to the result of a logical operation. Using this approach, the team implemented the AND, OR and XNOR logic functions directly on the transmitted terahertz wave.

Rather than processing information only after it reaches conventional electronic circuits, the metasurface itself performs part of the computation while the wave is still propagating through the device. In effect, the communication component also becomes a simple computational element.

The study's second major achievement was the demonstration of four-level pulse amplitude modulation, or PAM-4, on the same programmable platform.

Pulse amplitude modulation is used to encode digital information by varying the strength of a signal. Conventional binary transmission uses two amplitude levels representing zeros and ones. PAM-4 employs four distinct amplitude levels, allowing each transmitted symbol to carry more information and increasing overall data throughput without requiring additional bandwidth.

The researchers showed that their programmable metasurface generated four clearly distinguishable terahertz signal levels, demonstrating that the same device can both process and encode information before transmission.

If modulation can be carried out directly by the metasurface interacting with the electromagnetic wave, future communication hardware could become more compact because functions currently assigned to separate antennas, modulators and signal-processing components could potentially be integrated into a single programmable platform.

To validate the concept under realistic operating conditions, the researchers built a quasi-optical transmission link operating at 220 GHz. In this experimental system, terahertz radiation was transmitted through the programmable metasurface before being detected by a receiver.

Within this test platform, the team demonstrated dynamic logical operations at frequencies of up to 200 megahertz, stable PAM-4 signal modulation and single-tone modulation bandwidths reaching 6 gigahertz. These results indicate that the device functions not merely as a laboratory prototype but as part of a working transmission system.

The work was carried out by an international research team. Among the participants was Taiichi Otsuji of Tohoku University in Sendai, Japan, who represented the ENSEMBLE3 Centre of Excellence in Warsaw.

ENSEMBLE3 is a Polish research centre specialising in nanophotonics, advanced materials and crystal-growth technologies. Its research focuses on developing materials with engineered electromagnetic properties for applications including photonics, optoelectronics, telecommunications, medicine, solar energy and security technologies.

The researchers caution that the technology still faces engineering challenges before practical deployment. Increasing the number of independently controlled sections within the metasurface causes the different signal levels to become less distinct, making them more difficult to separate reliably. Likewise, achieving greater modulation depth can reduce operating speed, illustrating the trade-offs that remain in optimising the system.

Despite these limitations, the study points toward a new approach to designing future communication and sensing hardware. Rather than building separate antennas, modulators and elementary computing units, engineers may be able to integrate multiple functions into a single programmable surface that manipulates electromagnetic waves as they pass through it.

Such multifunctional devices could become increasingly important as future 6G networks seek to support communications, sensing and real-time data processing simultaneously, allowing information to be filtered, encoded and partially analysed before it ever reaches conventional electronic processors.

Krzysztof Petelczyc (PAP)

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