23 May 2014 ICFO in Science

Making waves in graphene

Focusing and bending of light with graphene Researchers from CIC nanoGUNE, in collaboration with ICFO and Graphenea – members of the EU Graphene Flagship, have developed an antenna-based platform for launching and controlling light propagating along graphene. The experiments show that the dramatically squeezed graphene-guided light can be focused and bent, following the fundamental principles of conventional optics. The work, published in Science, thus opens new opportunities for smaller and faster photonic devices and circuits.

Propagating light needs at least the space of half its wavelength, which is much larger than state-of-the-art electronic building blocks in our computers. For that reason, there has been a search for a solution to squeezing light and have it propagate through nanoscale materials. The wavelength of light captured by a graphene layer can be strongly shortened by a factor of 10 to 100 compared to light propagating in free space. As a consequence, this light propagating along the graphene layer - called graphene plasmon - requires much less space.

The researchers have shown that a nanoscale metal rod on graphene (acting as an antenna for light) can capture infrared light and transform it into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable. The excitation of graphene plasmons is purely optical, where the phase and wavefronts of these plasmons can be directly controlled by tailoring the geometry of the antennas. For the focusing experiment, the researchers curved the antenna and were able to have graphene plasmons focus away from the antenna, similar to the light beam that is concentrated with a focusing glass or a concave mirror. They were also able to observe that graphene plasmons refract (bend) when they pass through a prism-shaped graphene bilayer, analogous to the bending of a light beam passing through a glass prism, verifying that this material is the thinnest refracting optical prism ever achieved so far.

Future developments based on this study could lead to extremely miniaturized optical circuits and devices that could be useful for sensing and computing, among other applications.