Researchers at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain, have hit a breakthrough in coming ever closer to the successful development of new graphene technologies.
Published in Science, the ICFO scientists - along with other members of the Graphene Flagship - said they have "reached the ultimate level of light confinement" thanks to graphene, and were able to confine light down to a space one atom, the smallest possible.
This is important because light can function as an ultra-fast communication channel, for example between different sections of a computer chip, but it can also be used for ultra-sensitive sensors or on-chip nanoscale lasers.
The researchers said the development could pave the way to ultra-small optical switches, detectors and sensors.
"Graphene keeps surprising us: nobody thought that confining light to the one-atom limit would be possible. It will open a completely new set of applications, such as optical communications and sensing at a scale below one nanometer," said Professor Frank Koppens at ICFO, who led the research.
This team of researchers built up a new nano-optical device by taking a graphene monolayer (which acts as a semi-metal), and stacked onto it a hexagonal boron nitride (hBN) monolayer (an insulator), and on top of this deposited an array of metallic rods. They used graphene because it can guide light in the form of plasmons, which are oscillations of the electrons, interacting strongly with light.
"At first we were looking for a new way to excite graphene plasmons," added David Alcaraz Iranzo, the lead author 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."
They found that by sending infra-red light through their devices, the researchers observed how the plasmons propagated in between the metal and the graphene. To reach the smallest space conceivable, they decided to reduce the gap between the metal and graphene as much as possible to see if the confinement of light remained efficient.
They found that even when a monolayer of hBN was used as a spacer, the plasmons were still excited, and could propagate freely while being confined to a channel of just one atom thick. They then managed to switch this plasmon propagation on and off, simply by applying an electrical voltage, demonstrating the control of light guided in channels smaller than one nanometer.
"This enables new opto-electronic devices that are just one nanometer thick, such as ultra-small optical switches, detectors and sensors,2 the researchers added. "Due to the paradigm shift in optical field confinement, extreme light-matter interactions can now be explored that were not accessible before."
The atom-scale toolbox of two-dimensional materials has now also proven applicable for many types of new devices where both light and electrons can be controlled even down to the scale of a nanometer.
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