The terahertz band is located between microwaves and infrared in the electromagnetic spectrum and has the capacity to can transmit much more data than existing radio-based wireless systems. Unlike radio waves, however, the terahertz band relies on line-of-sight and does not work well when physical obstacles are blocking its path. The new metasurface device could help overcome this limitation by bouncing incoming terahertz waves in any desirable direction.
"A terahertz beam would be like a laser pointer, whereas today's radio wave transmitters are like light bulbs that send light everywhere,” said Kaushik Sengupta, an associate professor of electrical engineering at Princeton and a lead author of the new study, which is published in Nature Electronics. “A programmable metasurface is one that produces any possible fields; it's the ultimate projector."
According to the paper, the metasurface features hundreds of programmable terahertz elements, each less than 100 micrometres in diameter and just 3.4 micrometres tall, made of layers of copper and coupled with active electronics that collectively resonate with the structure. This allows the geometry of the device to be reconfigured up to several billions of times per second. Based on the desired application, a single incoming terahertz beam can be split into several dynamic, directable beams that can maintain line-of-sight with receivers.
"The tiles are like Lego blocks and are all programmable," said Sengupta.
As a proof of concept, the Princeton team tested tile arrays measuring two-by-two with 576 programmable elements and demonstrated beam control by projecting invisible terahertz holograms. According to the researchers, these elements are scalable across larger arrays, and one way to incorporate them into our environments would be as a type of smart wallpaper that could bounce the terahertz waves where required.
"This new work demonstrates a fascinating approach which, unlike most previous efforts, is scalable into the terahertz range," said Daniel Mittleman, a professor of engineering at Brown University who was not involved in the study.
"The key takeaway is that we are now getting a handle on practical methods for actively controlling the wave front, beam size, beam direction, and other features of terahertz beams."
Additional applications for the technology include gesture recognition and imaging, as well as industrial automation and autonomous vehicles, where terahertz band sensors and cameras could improve the awareness and reaction speed of self-driving cars.