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Researchers at the Cavendish Laboratory, University of Cambridge have demonstrated a new way to control radiation in the terahertz range—an often-overlooked part of the electromagnetic spectrum—with unprecedented dynamic range and speed. The findings could open the door to advanced technologies in communications, imaging, and sensing and mark major progress in the development of practical devices that operate in the terahertz range.

The study is in the journal Light: Science & Applications.

The terahertz range sits between microwaves and infrared light on the electromagnetic spectrum. Despite the potential of terahertz waves in many fields, for example in walk-through security scanners in airports and for skin cancer detection, terahertz waves are hard to manipulate efficiently. That's because terahertz wavelengths are tens of thousands of times smaller compared to radio wavelengths, and traditional methods don't work well at this scale.

But being able to manipulate terahertz waves is very important, particularly for communications, where a data signal must be encoded onto a wave to transmit information.

"Think of how you listen to an old analog radio, which works at much larger wavelengths: you turn the dial to tune into your desired station. Inside the radio, you're adjusting a capacitor so that the radio picks up the frequency of the station you want," explained Dr. Wladislaw Michailow, who led the research at the Cavendish Laboratory and is a Junior Research Fellow at Trinity College. "This tuning concept is very useful in many devices, but because terahertz wavelengths are so small, we had to come up with a new concept to realize tuning in the terahertz range."

Capacitors are components that store and release electric energy. By tweaking how much charge each capacitor can hold—a property called capacitance—the frequency of devices, such as detectors or modulators, can be tuned. As the wavelength becomes smaller, the dimensions of the capacitors must be scaled down commensurately, but making them small enough to reach the terahertz range would be impossible in this traditional way.

In the terahertz region, researchers have realized modulators using metamaterials. Metamaterials use the same principle that contributes, e.g., to the vibrant colors of butterflies in nature, but the underlying physics works in the terahertz range just as well.

Metamaterials are arrays of tiny resonators, smaller than the wavelength of the radiation, that are designed to resonate at a certain frequency. By embedding a conductive material like the two-dimensional material graphene into them, the optical response of such materials can be tuned—that's how modulators can be realized.

Usually, graphene is used as a variable resistor in such devices; nanoscale gaps within the resonators are shorted with graphene. This dampens the resonance, and as a result, changes the strength of the transmitted radiation.

"But this approach isn't very efficient, as it simply causes the resonance to collapse. That's like putting a sock on a flute, instead of playing the flute," said Wladislaw. "Rather than suppressing the resonance, we created ultra-thin, tunable capacitors from graphene. This allows us instead to shift the resonance the way we want—like playing a melody on a flute."

In their research, the scientists created ultra-small patches made from graphene and placed them inside each tiny structure or resonator of the array in the metamaterial. These graphene patches are incredibly small, less than a micron wide (that's a thousandth of a millimeter), and serve as tunable capacitors working on the nanoscale.

The researchers also designed the devices to reflect signals from its back surface, which made the performance even better.

"This way we were able to achieve a modulation depth of more than four orders of magnitude," said Dr. Ruqiao Xia, who built and measured the devices during her Ph.D. at the Cavendish Laboratory. "This is one of the highest values ever reported in the terahertz range."

Moreover, the demonstrated devices are also fast. Generally, it is easy to realize large modulation with slow speeds, or small modulation with large speeds, but not together. These new devices achieve an unprecedented intensity modulation depth (>99.99%) in combination with a speed of already 30 MHz.

"The performance of our devices significantly exceeds that of many comparable modulator technologies, and thanks to the use of metamaterials, we can adapt the design for use across the entire terahertz range," elaborated Ruqiao.

Beyond the immediate performance improvements, the team believes their design could influence many future technologies.

"By changing the design of the nanoscale gap in any metamaterial relying on a resonator, you can significantly influence the optical response and hence improve modulation efficiency," explained Wladislaw. "The approach we've taken here could be applied to many other types of metamaterial-based modulators."

Terahertz technologies are still in their early stages, but their potential is growing quickly.

"Terahertz waves can have many applications in material spectroscopy, security screening, pharmaceutics, medicine and terahertz communications. The aspect we focus on in our current project, Teracom, is the development of future communication systems," said Prof David Ritchie, Head of the Semiconductor Â鶹ÒùÔºics Group at the Cavendish Laboratory. "These results are a big step forward towards the realization of next generation communication systems, beyond the era of 5G and 6G."

The team collaborated with colleagues at the Department of Engineering at Cambridge University, Queen Mary University of London, and the University of Augsburg, Germany, on this work.