Ultrafast LEDs to allow for light-based communications

Researchers have set a new speed record after making fluorescent molecules emit photons of light 1,000 times faster than normal, an advance that paves the way for superfast light emitting diodes (LEDs).
In an LED, atoms can be forced to emit roughly 10 million photons in the blink of an eye. Modern telecommunications systems, however, operate nearly a thousand times faster, researchers said.
To make future light-based communications using LEDs practical, researchers must get photon-emitting materials up to speed.
In the new study, engineers from Duke University increased the photon emission rate of fluorescent molecules to record levels by sandwiching them between metal nanocubes and a gold film.

"One of the applications we're targeting with this research is ultrafast LEDs," said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke.
In the experiment, her group manufactured 75-nanometre silver nanocubes and trapped light between them, greatly increasing the light's intensity.

When fluorescent molecules are placed near intensified light, the molecules emit photons at a faster rate through an effect called Purcell enhancement.
The researchers found they could achieve a significant speed improvement by placing fluorescent molecules in a gap between the nanocubes and a thin film of gold.
To attain the greatest effect, Mikkelsen's team needed to tune the gap's resonant frequency to match the colour of light that the molecules respond to.

They used computer simulations to determine the exact size of the gap needed between the nanocubes and gold film to optimise the setup. That gap turned out to be just 20 atoms wide.
"We can select cubes with just the right size and make the gaps literally with nanometer precision," said Gleb Akselrod, first author on the study.
"When we have the cube size and gap perfectly calibrated to the molecule, that's when we see the record 1,000-fold increase in fluorescence speed," Akselrod said.
Researchers plan to design a system with individual fluorescent molecule placed precisely underneath a single nanocube.

According to Akselrod, they can achieve even higher fluorescence rates by standing the molecules up on edge at the corners of the cube.
"If we can precisely place molecules like this, it could be used in many more applications than just fast LEDs," said Akselrod.
"We could also make fast sources of single photons that could be used for quantum cryptography. This technology would allow secure communication that could not be hacked - at least not without breaking the laws of physics," Akselrod said.
The results appear in the journal Nature Photonics.