Graphene breaks light confinement limit

In a breakthrough for optical computing, researchers have used graphene to confine light to the smallest possible space, just one atom wide.

Confining light to small spaces is a fundamental challenge for computing using light rather than electric current. The smaller the space into which a pulse of light can be confined, the more compact a device using this technology, such as sensors or nanoscale lasers can be. Up to now, light has been confined to a space smaller than its own wavelength using the atomic lattices of metals to trap and guide photons. Now, researchers from the European Graphene Flagship, led by a team from ICFO (the Institute of Photonic Sciences in Barcelona), have found that this is yet another property that can be accessed using the single atom thick form of elemental carbon

“Graphene keeps surprising us: nobody thought that confining light to the one atom limit would be possible,” said research leader Prof Frank Koppens. In a paper published in Science, Koppens and colleagues at the University of Minho, Portugal, and MIT describe how they fabricated a composite nano-optical device starting with a layer of graphene, topped with an insulating hexagonal boron nitride layer, which was in turn topped with an array of metallic rods. Graphene is already known to have the ability to guide light in the form of plasmons, which are oscillations of electrons within the material structure that interact strongly with light.

“At first, we were looking for a new way to excite graphene plasmons,” said David Alcaraz Iranzo, lead author of the Science paper. “On the way, we found that the confinement was stronger than before and the additional losses minimal.” This is a step forward, because confinement using metal lattices has always led to a sharp drop in the energy of the light pulse. “So we decided to go to the one atom limit,” he continued, “with surprising results.”

The team sent infrared light through the device stack and observed that plasmon is propagated between the metal and the graphene. Reducing the thickness of hexagonal boron nitride layer to a single atom thick, they found to their surprise that the plasmons were still in an excited state – that is, the electrons were vibrating – and that this vibration could propagate freely along the single atom thick channel. This plasmon propagation could be switched on and off by applying an electric  voltage, which demonstrates the ability of the channel less than 1nm thick to control the light.

“The impressive results reported in this paper are testimony to the relevance for cutting-edge science of the Flagship work,” said Prof Andrea Ferrari, chair of the Graphene Flagship management panel. “Having read the ultimate limit of light confinement could lead to new devices with unprecedented small dimensions.”

Image: Artist’s impression of light trapped between graphene and insulating layer. Credit: ICFO