Graphene squeezes light to one atom

Researchers at the Institute of Photonic Sciences in Barcelona, Spain have managed to reach the ultimate level of light confinement by confining light down to a space one atom, the smallest possible thereby paving way for ultra-small optical switches, detectors and sensors.

Light as a means of communication channel is proven to be the fastest and while there is a lot of potential of using light in computations, there is still a lot of research that needs to be done to bring light as a means of communication in ultra-sensitive sensors or on-chip nanoscale lasers.

New techniques searching for ways to confine light into extremely tiny spaces, much smaller than current ones, have been on the rise. Researchers had previously found that metals can compress light below the wavelength-scale (diffraction limit), but more confinement would always come at the cost of more energy loss. This fundamental issue has now been overcome.

Researchers at the Institute of Photonic Sciences and colleagues used stacks of two-dimensional materials, called heterostructures, to build up a new nano-optical device. They took 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.

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, i.e. without additional energy losses.

Strikingly, they saw 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 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. 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.