Graphenea's research team is in the headlines again, with our latest graphene research featured in the prestigious international magazine Science. We have continued our collaboration with researchers at nanoGune, ICFO, and others, to advance the application of graphene to miniaturized optical circuitry.

Optical circuits and devices could make signal processing and computing much faster. "However, although light is very fast it needs too much space", explains Rainer Hillenbrand, Ikerbasque Professor at nanoGUNE and UPV/EHU. In fact, 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, a quest for squeezing light to propagate it through nanoscale materials arises.

For the past decade or so, researchers have intensively been exploring the use of surface plasmons as a means of squishing light into small spaces. Surface plasmons are light waves attached to the surface of a material, with a very low spatial profile. In particular, metals such as gold and silver show great plasmonic behavior and the geometry of metal nanoparticles can be tailored to reduce the volume that the light occupies. However, it has been challenging to find the means of manipulating surface plasmons on metals in a similar way that electrons are manipulated in conventional electronic circuits, which would be an essential ingredient to computer logic based on light.

Two years ago, our teams showed that graphene makes for a good plasmonic material. The wavelength of surface plasmons on graphene is reduced by a factor of 10 to 100 compared to the wavelength of light in air, allowing for a related compression of the volume that the light occupies. Now the researchers have shown, using Graphenea's graphene, that light from air can be efficiently coupled to graphene plasmons, and that those plasmons can be steered and focused by ultrathin optical elements made from graphene.

Image: Steering light with an ultrathin prism, courtesy of nanoGune.

For coupling light to graphene, we used metal nano-antennas, operating under much the same principles as radiowave antennas, but on the nanometer length scale. We showed that such an antenna can inject as much as 28 times more light into graphene than a non-resonant piece of metal of similar size. The antennas are fabricated directly on top of our CVD graphene.

We further show that a region of bilayer graphene acts as a nano-scale prism for the plasmon waves, bending light according to principles of conventional optics. That makes our bilayer prism the thinnest prism ever made. In free space, regular light refracts through a prism due to the difference in refractive index between the glass of the prism and its surroundings. On graphene, the small bilayer region actually refracts the plasmons due to a difference in conductivity. Conductivity is a parameter which can be modified with simple electronic means, opening up a pathway to on-demand real-time steering of light waves on graphene.

Our publication, out late last week, has already been highlighted on the esteemed portals phys.org, physicsworld.com, Science Daily, and AzoNano, receiving praise from the likes of Prof. Alexander Grigorenko at the University of Manchester. Graphenea continues our collaboration with world-leading scientific researchers to keep pushing the boundaries of graphene research and technology.