Researchers have developed an ultrafast light-emitting device that can flip on and off 90 billion times a second and could form the basis of optical computing.

At its most basic level, your smart phone’s battery is powering billions of transistors using electrons to flip on and off billions of times per second. But if microchips could use photons instead of electrons to process and transmit data, computers could operate even faster.

But first engineers must build a light source that can be turned on and off that rapidly. While lasers can fit this requirement, they are too energy-hungry and unwieldy to integrate into computer chips.

Duke University researchers are now one step closer to such a light source. In a new study, a team from the Pratt School of Engineering pushed semiconductor quantum dots to emit light at more than 90 billion gigahertz. This so-called plasmonic device could one day be used in optical computing chips or for optical communication between traditional electronic microchips.

The study was published online on July 27 in Nature Communications.

“This is something that the scientific community has wanted to do for a long time,” said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. “We can now start to think about making fast-switching devices based on this research, so there’s a lot of excitement about this demonstration.”

The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon.

The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots — spheres of semiconducting material just six nanometers wide — that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz.

“There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons,” said Gleb Akselrod, a postdoctoral research in Mikkelsen’s laboratory. “Now we have made an important step towards solving these problems.”

“The eventual goal is to integrate our technology into a device that can be excited either optically or electrically,” said Thang Hoang, also a postdoctoral researcher in Mikkelsen’s laboratory. “That’s something that I think everyone, including funding agencies, is pushing pretty hard for.”

The group is now working to use the plasmonic structure to create a single photon source — a necessity for extremely secure quantum communications — by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible.

Aside from its potential technological impacts, the research demonstrates that well-known materials need not be limited by their intrinsic properties.

“By tailoring the environment around a material, like we’ve done here with semiconductors, we can create new designer materials with almost any optical properties we desire,” said Mikkelsen. “And that’s an emerging area that’s fascinating to think about.”

Scientists studying thin layers of phosphorus have found surprising properties that could open the door to ultrathin and ultralight solar cells and LEDs.

The team used sticky tape to create single-atom thick layers, termed phosphorene, in the same simple way as the Nobel-prize winning discovery of graphene.

Unlike graphene, phosphorene is a semiconductor, like silicon, which is the basis of current electronics technology.

“Because phosphorene is so thin and light, it creates possibilities for making lots of interesting devices, such as LEDs or solar cells,” said lead researcher Dr Yuerui (Larry) Lu, from The Australian National University (ANU).

“It shows very promising light emission properties.”

The team created phosphorene by repeatedly using sticky tape to peel thinner and thinner layers of crystals from the black crystalline form of phosphorus.

As well as creating much thinner and lighter semiconductors than silicon, phosphorene has light emission properties that vary widely with the thickness of the layers, which enables much more flexibility for manufacturing.

“This property has never been reported before in any other material,” said Dr Lu, from ANU College of Engineering and Computer Science, whose study is published in the Nature serial journal Light: Science and Applications.

“By changing the number of layers we can tightly control the band gap, which determines the material’s properties, such as the colour of LED it would make.

“You can see quite clearly under the microscope the different colours of the sample, which tells you how many layers are there,” said Dr Lu.

Dr Lu’s team found the optical gap for monolayer phosphorene was 1.75 electron volts, corresponding to red light of a wavelength of 700 nanometers. As more layers were added, the optical gap decreased. For instance, for five layers, the optical gap value was 0.8 electron volts, a infrared wavelength of 1550 nanometres. For very thick layers, the value was around 0.3 electron volts, a mid-infrared wavelength of around 3.5 microns.

The behaviour of phosphorene in thin layers is superior to silicon, said Dr Lu.

“Phosphorene’s surface states are minimised, unlike silicon, whose surface states are serious and prevent it being used in such a thin state.”