The team, headed by Phaedon Avouris of IBM's TJ Watson Research Center in New York and Zhiqiang Li of the National High Magnetic Field Laboratory in Florida, studied graphene that had been patterned into a periodic array of microdisks. The structure absorbs light by confining it into regions that are hundreds of times smaller than the wavelength of the light by exploiting plasmons that occur within the individual microdisks. Plasmons are quantized collective oscillations of electrons – and they interact strongly with light.

Graphene appears to be emerging as a very promising plasmonic material thanks to the material's unusual electronic properties, which result in its electrons moving extremely fast and behaving like relativistic "Dirac" particles with virtually no rest mass. Graphene absorbs light particularly well in the terahertz and infrared parts of the electromagnetic spectrum – something that could lead to novel applications in photonics and quantum optics. "What is more, unlike plasmons in metals, the plasmons in graphene should
be strongly affected by an external magnetic field – all because graphene electrons behave like massless fermions," explained project leader Hugen Yan.

Longer-lived edge plasmons

The researchers obtained their results by measuring the light transmission spectrum of the graphene disk arrays, using a Fourier transform IR spectrometer together with a silicon bolometer, while a magnetic field was applied perpendicular to the disks. To their surprise, they found that plasmons at the edges of the nanostructures appear to last longer when a magnetic field is applied. This is counter-intuitive, Yan tells, because there are potentially more defects in the vicinity of an edge plasmon that should, conversely, reduce its lifetime.

According to the team, the applied magnetic field may be suppressing electron backscattering at the edges of the microdisks, so allowing the plasmons to last longer in the samples. The result is doubly unexpected given that the phenomenon has never been observed before in conventional 2D electron gas systems in the terahertz range.

And that is not all: the lifetimes of the edge plasmons can also be tuned by varying the magnitude of the applied magnetic field, with larger fields encouraging longer-lived plasmons.

"A long plasmon lifetime is a big advantage for applications in chemical and biological sensing, as well as in electric field enhancement, and could lead to a variety of magneto-optical applications in the future," said Yan.

The IBM team is now planning to study magnetoplasmons in other graphene microstructures, such as graphene rings, dots, ribbons and elliptical disks.

The current work is detailed in Nano Letters.