MIT researchers turn graphene light on and off

June 15, 2016 // By Julien Happich
A team of international researchers from the US (MIT), Israel, Croatia, and Singapore, have discovered that under certain circumstances, a flow of electric current running through a sheet of graphene can exceed the speed of slowed-down light and produce a kind of optical “boom”: an intense, focused beam of light.

The work reported in the journal Nature Communications, shows this new way of converting electricity into visible radiation is highly controllable, fast, and efficient so it could open new graphene usages.

“Graphene has this ability to trap light, in modes we call surface plasmons,” explained MIT postdoc and the paper lead author Ido Kaminer. Plasmons are a kind of virtual particle that represents the oscillations of electrons on the surface. The speed of these plasmons through the graphene is “a few hundred times slower than light in free space,” he says.

This effect coincides with another of graphene’s exceptional characteristics: electrons pass through it at very high speeds, up to a million meters per second, or about 1/300 the speed of light in a vacuum. That meant that the two speeds were similar enough that significant interactions might occur between the two kinds of particles, if the material could be tuned to get the velocities to match. That combination of properties suggested the possibility of using graphene to produce the opposite effect: to produce light instead of trapping it, and the researchers' theoretical work shows that this can lead to a new way of generating light. With an analogy to the shockwave of sound when one breaks the sound barrier, when the electrons' approach the light speed in graphene, they somehow break the ‘light barrier’, yielding a shockwave of light trapped in two dimensions.

The illustration depicts the process of light emission from a sheet of graphene, represented as the blue lattice on the top surface of a carrier material. The light-coloured arrow moving upwards at the centre depicts a fast-moving electron (going faster than light) that generates a shock wave, ejecting plasmons in two directions (shown as red wavy lines). Courtesy of the researchers.

This microscale, fast, and tuneable plasmon-based approach to emitting light may take off in new fields of application such as on-chip photonics.

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