Scientists use graphene to squeeze light to just one atom thick paving the way for tiny switches and sensors

Vaughn Highfield April 23, 2018

Graphene is regarded as one of the ultimate materials we know of. Not only is it stronger than steel, it can be just one atom thick, meaning we can create strong lightweight structures out of the stuff and could soon use it to build tiny, ultra-slim electronics.

Researchers at the Institute of Photonic Sciences (ICFO) in Barcelona, along with a team at the Graphene Flagship, have used the wonder material to squeeze light down to just a single atom thick. Their study, published in Science, explains how they used graphene to confine the light, heralding in a new age of ultra-small optical sensors, detectors and switches.

Currently, computer chip sizes are struggling to shrink down in size, going against Moore’s Law – a theory that outlined the exponential shrinkage of devices that’s fuelled the chipmaking industry for years. The research team’s findings could lead to the creation of ultra-small switches that could facilitate the continuation of Moore’s law once again.

The ICFO team’s breakthrough is significant because previous attempts to use metals to shrink light have generally failed to ever compress a lightwave into such a small form factor. Other experiments that managed to push light below its diffraction limit would always result in a loss of energy. Graphene overcomes this.

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“Graphene keeps surprising us: nobody thought that confining light to the one-atom limit would be possible,” exclaims Professor Frank Koppens who led the research at ICFO. “It will open a completely new set of applications, such as optical communications and sensing at a scale below one nanometer.”

As with any good scientific discovery, it appears the research teams actually stumbled upon graphene’s light funnelling properties by accident.

“At first we were looking for a new way to excite graphene plasmons,” explains David Alcaraz Iranzo, lead author on the paper from ICFO. “On the way, we found that the confinement was stronger than before and the additional losses minimal. So we decided to go to the one atom limit with surprising results.”

The team of researchers used stacks of heterostructures – two-dimensional materials – to build up a nano-optical device. They then used a graphene monolayer, which acts as a semi-metal, and stacked onto it a hexagonal boron nitride monolayer to act as an insulator. On top of this, they deposited an array of metallic rods. The decision to use graphene was simply because of its light-guiding properties due to its oscillating electrons known as plasmons.

If that’s all a little lost on you, don’t worry. Essentially, the research was designed to see how plasmons propagated in between metal and graphene. The accidental breakthrough came when the researchers decided they would reduce the gap between metal and graphene as much as possible to see if energy would be lost if they confined the light.

To their surprise, by using a hexagonal boron nitride monolayer as a spacer, graphene’s plasmons were still excited and able to propagate freely even when confined to a channel of just one atom thick. The team then managed to control this propagation by applying an electrical voltage, demonstrating the control of light guided in channels smaller than one nanometer.

“The impressive results reported in this paper are a testimony to the relevance for cutting-edge science of the Flagship work,” explained Professor Andrea C Ferrari, science and technology officer at Graphene Flagship. “Having reached the ultimate limit of light confinement could lead to new devices with unprecedented small dimensions.”

It’ll certainly be a while yet before we start seeing the findings of this research actually trickle down into consumer goods – partly due to the prohibitive cost of manufacturing graphene at scale. However, this research has undoubtedly moved us one step closer to the ultra-slim and lightweight devices of the future.

[Image: ICFO]