Nanocrystals Enable a New Generation of Visible-Light Communication

A faster conversion from blue light to red and green light leads to a 20-fold increase in data transmission speeds

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New nanocrystal speeds up the modulation from blue laser light to red or green light for visible-light communication
A nanocrystal-based material converts blue laser emission to white light for combined illumination and data communication.
Photo-illustration: KAUST

Something called visible-light communication (VLC) is shaping up to be a viable alternative to Wi-Fi. Wi-Fi sends data between electronic devices through the use of radio bands, while VLC uses visible light emitted by light-emitting diodes (LEDs). In practical terms, this involves LED light fixtures turning on and off faster than our eyes can resolve, and in so doing, transmitting binary data. VLC technology has also been shown to have some attractive applications in automobiles.

The key to getting VLCs to work is the speed at which an LED can be switched on and off. But not all parts of an LED operate at the same speed. The LEDs used in such a VLC system typically produce a white light. When a white LED is combined with another diode that emits blue light, that blue light can be combined with phosphors to convert the blue light to red and green light. It is this conversion of turning blue light into red and green that is a stumbling block because it doesn’t occur as fast as the switching on and off of the light.

Now researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have developed a new nanocrystalline material that responds to switching 40 times as fast as the previous method of converting blue light to red and green light. This should enable transmission data rates of 2 gigabits per second. This is a marked improvement over today’s LED communications systems, which are capable of only 100 megabits per second, according to the researchers.

In research described in the journal ACS Photonics, the KAUST researchers developed a cheap, solution-based process for producing perovskite nanocrystals that were approximately eight nanometers in size. In that same solution, the researchers included a conventional nitride phosphor. When blue laser light strikes the nanocrystal, it emits a green light; when it hits the nitride phosphor, red light is produced.

In measurements taken using a type of high-speed spectroscopy, the researchers discovered that the optical processes of turning blue light to green or red light occurred within seven nanoseconds. This speed made it possible to modulate the optical emission so that they could use the frequency of 491 megahertz, leading ultimately to the 20-fold increase in transmission data rates that they reported.

“The rapid response is partly due to the size of the crystals,” said Osman Bakr, an associate professor at KAUST, in a press release. Because the space in the crystal is so confined, it makes it more likely that the electron will recombine with a hole and emit a photon, explained Bakr.

Perhaps the key attractive feature of the nanocrystals is that they provide the same quality of white light as seen in today’s LED technology—they just manage to make the color changes much more rapidly.

Boon Ooi, the KAUST professor who led the research, added: “We believe that white light generated using semiconductor lasers will one day replace the LED white-light bulbs for energy-efficient lighting.”

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