You use Wi-Fi everyday, but have you heard of its cousin, Li-Fi? U.S. Navy-funded researchers are developing a form of visible light-based communication that transmits data using fast-blinking LEDs rather than Wi-Fi’s familiar radio waves. A paper published earlier this year in Nature Nanotechnology could help Li-Fi take a step forward. Its authors have fabricated an artificial material with a quirky response to light that may eventually speed up light-based data transmission. Li-Fi works a bit like Morse code, with LED blinks corresponding to the zeros and ones of computer language. An LED light transmits flashes of light to a light detector lodged in a computing device, which translates the signal into digital data. The blinks are so rapid that the human eye cannot detect them. The faster the LED blinks, the faster it’s possible to transmit data. Liu’s new light-manipulating material could boost the blink frequency of LEDs by one or two orders of magnitude. This would translate to a significant increase in data transmission speeds for the modified LEDs.
“You can start with a very cheap LED and improve the speed by 50 times,” said Zhaowei Liu, an optical engineer at the University of California, San Diego, and an author of the new paper.
It would also be possible to start with an advanced, fast-blinking LED and boost its signal by a similar factor. Li-Fi could thrive under specialized uses. For instance, the study was in part funded by the U.S. Office of Naval research. The Navy is interested in using Li-Fi to improve submarine communications, since radio waves travel poorly underwater and current acoustic communications are slow. Li-Fi could also come in hand at petrochemical plants or on airplanes, where Wi-Fi causes interference with electronics.
But the primary application of Li-Fi may be broader. The U.S. Federal Communications Commission has warned of crowding in wireless communications as the radio frequency spectrum gets too full. Li-Fi could ease the crowding. The visual light spectrum is 10,000 times larger than the radio frequency spectrum, providing plenty of space for new data transmission channels. And visible light does not interfere with radio waves, so Wi-Fi and Li-Fi could coexist. Devices could switch back and forth between them, the same way a hybrid car switches between electricity and gas. Li-Fi could even be incorporated into existing lighting infrastructure on streets and in buildings.
Li-Fi has already achieved blistering speeds in the lab. The fastest Li-Fi transmission from a single LED published to date was at a rate of 3.5 gigabits per second over a distance of 5 centimeters, achieved by Harald Haas and colleagues at the University of Edinburgh earlier this year. That’s still slower than the record Wi-Fi transmission speed, which clocked in at 100 gigabits per second, but shows promise. Over longer distances, using LEDs originally intended for lighting, and in otherwise more realistic conditions, Li-Fi is slower than the speeds achieved in the lab. Anagnostis Paraskevopoulos and colleagues at the Heinrich Hertz Institute in Germany, for instance, managed to achieve data transmission rates up to 500 megabits per second over distances of one to two meters and transmission rates up to 100 megabits per second over 20 meters.
Liu and colleagues plan to boost blink rate and data transmission by incorporating artificial substance called a hyperbolic metamaterial into LEDs. To create their novel material, the researchers alternated 10-nanometer-thick layers of silica with silver, each about 10,000 times thinner than a strand of hair. They arranged multiple 305-nanometer-tall stacks of these alternating layers on a sheet of glass. They inscribed each layer of silica and silver with a pattern of trenches and then coated their stacks in a transparent plastic mixed with rhodamine dye molecules. Rhodamine dye fluoresces when it absorbs light. The researchers excited the dye molecules using a laser and then measured their brightness and blink rate as they fluoresced, demonstrating that they had greatly enhanced the dye molecules’ light emission.
“This is the work that shows there’s a big potential for these hyperbolic metamaterials,” said Zubin Jacob, an electrical engineer at the University of Alberta.
Hyperbolic metamaterials possess unusual properties because they are patterned at a scale smaller than the wavelength of visible light, which is around 400 to 700 nanometers. When light hits a substance, it creates something called plasmonic resonance, a phenomenon in which electrons oscillate collectively within a material. Metamaterials are able to achieve patterns of plasmonic resonance not seen in naturally occurring substances. When plasmonic resonance aligns with fluorescent emission, it’s possible to amplify the emission, forming the basis for the enhanced brightness and blink speed the researchers achieved. Liu warned that his team still needs to incorporate their new metamaterial into LEDs. But Haas cautiously praised Liu’s results, saying that they could help solve a challenge in the Li-Fi industry if they deliver. Off-the-shelf bulbs are optimized for visible light, not for communications, and so it’s only possible to modulate their intensity comparatively slowly. Liu and colleagues’ blink rate-boosting materials could be a boon.
“These devices are perhaps able to provide a step towards the results we would like to achieve,” Haas said.