A new, tunable smart surface can transform a single pulse of light into multiple beams, each aimed in different directions. The proof-of-principle development opens the door to a range of innovations in communications, imaging, sensing, and medicine.

The research comes out of the Caltech lab of Harry Atwater, a professor of applied physics and materials science, and is possible due to a type of nano-engineered material called a metasurface. “These are artificially designed surfaces which basically consist of nanostructured patterns,” says Prachi Thureja, a graduate student in Atwater’s group. “So it’s an array of nanostructures, and each nanostructure essentially allows us to locally control the properties of light.”

The surface can be reconfigured up to millions of times per second to change how it is locally controlling light. That’s rapid enough to manipulate and redirect light for applications in optical data transmission such as optical space communications and Li-Fi, as well as lidar.

“[The metasurface] brings unprecedented freedom in controlling light,” says Alex M.H. Wong, an associate professor of electrical engineering at the City University of Hong Kong. “The ability to do this means one can migrate existing wireless technologies into the optical regime. Li-Fi and LIDAR serve as prime examples.”

Metasurfaces remove the need for lenses and mirrors

Manipulating and redirecting beams of light typically involves a range of conventional lenses and mirrors. These lenses and mirrors might be microscopic in size, but they’re still using optical properties of materials like Snell’s Law, which describes the progress of a wavefront through different materials and how that wavefront is redirected—or refracted—according to the properties of the material itself.

By contrast, the new work offers the prospect of electrically manipulating a material’s optical properties via a semiconducting material. Combined with nano-scaled mirror elements, the flat, microscopic devices can be made to behave like a lens, without requiring lengths of curved or bent glass. And the new metasurface’s optical properties can be switched millions of times per second using electrical signals.

“The difference with our device is by applying different voltages across the device, we can change the profile of light coming off of the mirror, even though physically it’s not moving,” says paper co-author Jared Sisler—also a graduate student in Atwater’s group. “And then we can steer the light like it’s an electrically reprogrammable mirror.”

The device itself, a chip that measures 120 micrometers on each side, achieves its light-manipulating capabilities with an embedded surface of tiny gold antennas in a semiconductor layer of indium tin oxide. Manipulating the voltages across the semiconductor alters the material’s capacity to bend light—also known as its index of refraction. Between the reflection of the gold mirror elements and the tunable refractive capacity of the semiconductor, a lot of rapidly-tunable light manipulation becomes possible.

“I think the whole idea of using a solid-state metasurface or optical device to steer light in space and also use that for encoding information—I mean, there’s nothing like that that exists right now,” Sisler says. “So I mean, technically, you can send more information if you can achieve higher modulation rates. But since it’s kind of a new domain, the performance of our device is more just to show the principle.”

Metasurfaces open up plenty of new possibilities

The principle, says Wong, suggests a wide array of future technologies on the back of what he says are likely near-term metasurface developments and discoveries.

“The metasurface [can] be flat, ultrathin, and lightweight while it attains the functions normally achieved by a series of carefully curved lenses,” Wong says. “Scientists are currently still unlocking the vast possibilities the metasurface has available to us.

“With improvements in nanofabrication, elements with small feature sizes much smaller than the wavelength are now reliably fabricable,” Wong continues. “Many functionalities of the metasurface are being routinely demonstrated, benefiting not just communication but also imaging, sensing, and medicine, among other fields... I know that in addition to interest from academia, various players from industry are also deeply interested and making sizable investments in pushing this technology toward commercialization.”

As the world’s 5G rollout continues with its predictable fits and starts, the cellular technology is also starting to move into a space already dominated by another wireless tech: Wi-Fi. Private 5G networks—in which a person or company sets up their own facility-wide cellular network—are today finding applications where Wi-Fi was once the only viable game in town. This month, the Newbury, England–based telecom company Vodafone is releasing a Raspberry Pi–based private 5G base station that it is now being aimed at developers, who might then jump-start a wave of private 5G innovation.

“The Raspberry Pi is the most affordable CPU[-based] computer that you can get,” says Santiago Tenorio, network architecture director at Vodafone. “Which means that what we build, in essence, has a similar bill of materials as a good quality Wi-Fi router.”

The company has teamed with the Surrey, England–based Lime Microsystems to release a crowd-funded range of private 5G base-station kits ranging in price from US $800 to $12,000.

“In a Raspberry Pi—in this case, a Raspberry Pi 4 is what we used—then you can be sure you can run that anywhere, because it’s the tiniest processor that you can have,” Tenorio says.

Santiago Tenorio holds one of Lime Microsystems’ private 5G base-station kits.Vodafone

Private 5G for Drones and Bakeries

There are a range of reasons, Tenorio says, why someone might want their own private 5G network. At the moment, the scenarios mostly concern companies and organizations—although individual expert users could still be drawn to, for instance, 5G’s relatively low latency and network flexibility.

Tenorio highlighted security and mobility as two big selling points for private 5G.

A commercial storefront business, for instance, might be attracted to the extra security protections that a SIM card can provide compared to password-based wireless network security. Because each SIM card contains its own unique identifier and encryption keys, thereby also enabling a network to be able to recognize and authorize each individual connection, Tenorio says private 5G network security is a considerable selling point.

Plus, Tenorio says, it’s simpler for customers to access. Envisioning a use case of a bakery with its own privately deployed 5G network, he says, “You don’t need a password. You don’t need a conversation [with a clerk behind a counter] or a QR code. You simply walk into the bakery, and you are into the bakery’s network.”

As to mobility, Tenorio suggests one emergency relief and rescue application that might rely on the presence of a nearby 5G station that causes devices in its range to ping.

Setting up a private 5G base station on a drone, Tenorio says, would enable that drone to fly over a disaster area and, via its airborne network, send a challenge signal to all devices in its coverage area to report in. Any device receiving that signal with a compatible SIM card then responds with its unique identification information.

“Then any phone would try to register,” Tenorio says. “And then you would know if there is someone [there].”

Not only that, but because the ping would be from a device with a SIM card, the private 5G rescue drone in the above scenario could potentially provide crucial information about each individual, just based on the device’s identifier alone. And that user-identifying feature of private 5G isn’t exactly irrelevant to a bakery owner—or to any other commercial customer—either, Tenorio says.

“If you are a bakery,” he says, “You could actually know who your customers are, because anyone walking into the bakery would register on your network and would leave their [International Mobile Subscriber Identity].”

Winning the Lag Race

According to Christian Wietfeld, professor of electrical engineering at the Technical University of Dortmund in Germany, private 5G networks also bring the possibility of less lag. His team has tested private 5G deployments—although, Wietfeld says that they haven’t yet tested the present Vodafone/Lime Microsystem base station—and have found private 5G to provide reliably better connectivity.

Wietfeld’s team will present their research at the IEEE International Symposium on Personal, Indoor and Mobile Radio Communications in September in Valencia, Spain. They found that private 5G can deliver connections up to 10 times as fast as connections in networks with high loads, compared to Wi-Fi (the IEEE 802.11 wireless standard).

“The additional cost and effort to operate a private 5G network pays off in lower downtimes of production and less delays in delivery of goods,” Wietfeld says. “Also, for safety-critical use cases such as on-campus teleoperated driving, private 5G networks provide the necessary reliability and predictability of performance.”

For Lime Networks, according to the company’s CEO and founder Ebrahim Bushehri, the challenge comes in developing a private 5G base station that maximized versatility and openness to whatever kinds of applications developers could envision—while still being reasonably inexpensive and retaining a low-power envelope.

“The solution had to be ultraportable and with an optional battery pack which could be mounted on drones and autonomous robots, for remote and tactical deployments, such as emergency-response scenarios and temporary events,” Bushehri says.

Meanwhile, the crowdfunding behind the device’s rollout, via the website Crowd Supply, allows both companies to keep tabs on the kinds of applications the developer community is envisioning for this technology, he says.

“Crowdfunding,” Bushehri says, “Is one of the key indicators of community interest and engagement. Hence the reason for launching the campaign on Crowd Supply to get feedback from early adopters.”

Skies over Tokyo are thick with air traffic these days amid an influx of international tourists. But one plane recently helped revive the dream of airborne Internet access for all. Researchers in Japan announced on 28 May that they have successfully tested 5G communications equipment in the 38 gigahertz band from an altitude of 4 kilometers.

The experiment was aimed at developing an aerial relay backhaul with millimeter-wave band links between ground stations and a simulated High-Altitude Platform Station (HAPS), a radio station aboard an uncrewed aircraft that stays aloft in the stratosphere for extended periods of time. A Cessna flying out of Chofu Airfield in western Tokyo was outfitted with a 38 GHz 5G base station and core network device, and three ground stations were equipped with lens antennas with automatic tracking.

With the Cessna as a relay station, the setup enabled communication between one ground station connected to the 5G terrestrial network and a terrestrial base station connected to a user terminal, according to a consortium of Japanese companies and the National Institute of Information and Communications Technology.

“We developed technology that enables communication using 5G [New Radio] by correctly directing 38 GHz beams toward three ground stations while adapting to the flight attitude, speed, direction, position, altitude, etc. during aircraft rotation,” said Shinichi Tanaka, a manager in broadcaster SKY Perfect JSAT’s Space Business Division. “We confirmed that the onboard system, designed for the stratosphere, has adequate communication and tracking performance even under the flight speed and attitude fluctuations of a Cessna aircraft, which are more severe than those of HAPS.”

The sharpest beam width of the ground station antenna is 0.8 degrees, and the trial demonstrated a tracking method that always captures the Cessna in this angular range, Tanaka added.

A Cessna [top left] carried a 38 GHz antenna [top right] during a flight, functioning as a 5G base station for receivers on the ground [bottom right]. The plane was able to connect to multiple ground stations at once [illustration, bottom left].NTT Docomo

Millimeter wave bands, such as the 38 GHz band, have the highest data capacity for 5G and are suited for crowded venues such as stadiums and shopping centers. When used outdoors, however, the signals can be attenuated by rain and other moisture in the atmosphere. To counter this, the consortium successfully tested an algorithm that automatically switches between multiple ground stations to compensate for moisture-weakened signals.

Unlike Google’s failed Loon effort, which focused on providing direct communication to user terminals, the HAPS trial is aimed at creating backhaul lines for base stations. Led by Japan’s Ministry of Internal Affairs and Communications, the experiment is designed to deliver high-speed, high-capacity communications both for the development of 5G and 6G networks as well as emergency response. The latter is critical in disaster-prone Japan—in January, communication lines around the Noto Peninsula on the Sea of Japan were severed following a magnitude-7 earthquake that caused over 1,500 casualties.

“This is the world’s first successful 5G communication experiment via the sky using the Q-band frequency,” said Hinata Kohara, a researcher with mobile carrier NTT Docomo’s 6G Network Innovation Department. “In addition, the use of 5G communication base stations and core network equipment on the aircraft for communication among multiple ground stations enables flexible and fast route switching of the ground [gateway] station for a feeder link, and is robust against propagation characteristics such as rainfall. Another key feature is the use of a full digital beamforming method for beam control, which uses multiple independent beams to improve frequency utilization efficiency.”

Doppler shift compensation was a challenge in the experiment, Kohara said, adding that the researchers will conduct further tests to find a solution with the aim of commercializing a HAPS service in 2026. Aside from SKY Perfect JSAT and NTT Docomo, the consortium includes Panasonic Holdings, known for its electronics equipment.

The HAPS push comes as NTT Docomo announced it has led another consortium in a $100 million investment in Airbus’ AALTO HAPS, operator of the Zephyr fixed-wing uncrewed aerial vehicle. The solar-powered wing can be used for 5G direct-to-device communications or Earth observation, and has set records including 64 days of stratospheric flight. According to Airbus, it has a reach of “up to 250 terrestrial towers in difficult mountainous terrain.” Docomo said the investment is aimed at commercializing Zephyr services in Japan, including coverage of rural areas and disaster zones, and around the world in 2026.