Graphene offers the ability to ‘squeeze’ light for faster photonic devices
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Researchers from CIC nanoGUNE, ICFO and Graphenea say they have developed a platform technology that allows light to be trapped and controlled using graphene. The experiments, based on optical antennas, show that graphene guided light can be focused and bent according to the fundamental principles of conventional optics. The team believes this will open new opportunities for smaller and faster photonic devices and circuits.
"Although light is very fast, it needs too much space," said Professor Rainer Hillenbrand of nanoGUNE. "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."
According to the researchers, graphene can 'squeeze' the wavelength of light captured by a factor of at least 10 compared to light propagating in free space. As a consequence, they say, this light propagating along the graphene layer – a graphene plasmon – requires much less space.
However, transforming light efficiently into graphene plasmons and manipulating them has been a major challenge. The research team – all members of the EU Graphene Flagship – says it has shown that the antenna concept of radio wave technology could be a solution.
In its work, the team has demonstrated that a nanoscale metal rod on graphene can capture infrared light and transform it into graphene plasmons.
Pablo Alonso-González, a nanoGUNE researcher, added: "The excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by tailoring the antennas geometrically. This is essential to develop applications based on focusing and guiding of light."
Based on theoretical studies by nanoGUNE research fellow Alexey Nikitin, nanoGUNE's nanodevices group fabricated gold nanoantennas on graphene. The nano optics group then used a near field microscope to image how infrared graphene plasmons are launched and propagate along the graphene layer. The researchers found that waves on graphene propagate away from the antenna, like waves on a water surface.
In further tests, the researchers tried to focus and refract the waves and found the graphene plasmons behaved in a similar manner to a light beam that is concentrated with a lens or concave mirror.
It was also found that graphene plasmons refract when they pass through a prism shaped graphene bilayer. The graphene plasmons are said to be refracted because the conductivity in the two atom thick prism is larger than in the surrounding one atom thick layer. In the future, the researchers believe, conductivity changes could be generated electronically, allowing for efficient control of refraction.