Jun 15 2016
Airplanes exceeding the speed of sound create a shockwave that generates a popular “boom” of sound. Recently, a team of researchers from MIT and elsewhere found a similar method in a graphene sheet, where under a specific circumstance, a flow of electric current could surpass the speed of slowed-down light to generate a sort of optical “boom”, which really is a strong, focused beam of light.
This illustration depicts the process of light emission from a sheet of graphene, which is represented as the blue lattice on the top surface of a carrier material. The light-colored arrow moving upwards at the center depicts a fast-moving electron. Because the electron is moving faster than light itself, it generates a shock wave, which spews out plasmons, shown as red squiggly lines, in two directions. (Photo credit: Courtesy of the researchers)
The researchers state that this completely new method of transforming electricity into visible radiation is extremely controllable, efficient and rapid, and could result in it being adapted into a wide range of new applications.
The research has been published in the Nature Communications journal, in a paper authored by two MIT professors — Marin Soljačić, professor of physics; and John Joannopoulos, the Francis Wright Davis Professor of physics — as well as postdoc Ido Kaminer, and six others in Israel, Croatia, and Singapore.
The new discovery occurred due to an intriguing observation. The researchers established that when light hits a sheet of graphene, which is a 2D form of carbon material, it could slow down by a factor of a few hundred. That spectacular slowdown delivered an appealing coincidence. The decreased speed of photons traveling via the graphene sheet was extremely close to the speed of electrons as they traveled via the same material.
Graphene has this ability to trap light, in modes we call surface plasmons. [The speed of these plasmons via the graphene is] a few hundred times slower than light in free space.
Ido Kaminer, Postdoc, MIT
Plasmons are a type of virtual particle that indicates the electron oscillations on the surface.
This outcome dovetailed with another of superior characteristics of graphene. Here, the electrons were able to pass through it at extremely high speeds, up to a million meters per second, or approximately 1/300 the speed of light in a vacuum. This indicated that the two speeds were adequately alike and major interactions could happen between the two types of particles, but only if the material could be adjusted to obtain matching velocities.
That mixture of properties — slowing down light and facilitating electrons to travel very quick — is “one of the unusual properties of graphene,” says Soljačić. That indicated the possibility of using graphene to create the reverse effect: to generate light instead of trapping it.
Our theoretical work shows that this can lead to a new way of generating light. This conversion is made possible because the electronic speed can approach the light speed in graphene, breaking the ‘light barrier.’
Marin Soljačić, Professor of Physics, MIT
Just as breaking the sound barrier generates a shockwave of sound, he says, “In the case of graphene, this leads to the emission of a shockwave of light, trapped in two dimensions.”
The occurrence harnessed by the researchers is termed as the Čerenkov effect. It was initially illustrated 80 years ago by Soviet physicist Pavel Čerenkov. Typically linked with astronomical occurrence and harnessed as a way of identifying ultrafast cosmic particles as they race via the universe, and also to identify particles resulting from high-energy collisions in particle accelerators, the effect so far was not thought to be relevant to Earthbound technology since it only functioned when objects were traveling close to the speed of light.
However, the team states that the opportunity to harness this effect in a realistic form is possible by the slowing of light within a graphene sheet.
There are several different methods of converting electricity into light, ranging from the heated tungsten filaments that Thomas Edison developed over 100 years ago, to fluorescent tubes, to the LEDs that supply power to numerous display screens, and are gaining support for use in domestic lighting area. However, the new plasmon-based method may gradually be part of more compact, more efficient, swifter, and more tunable choices for particular applications, the researchers explain.
Maybe this method is the way to create efficient and controllable plasmons on a scale that is well-suited with existing microchip technology. Such graphene-based systems could probably be key on-chip components for the expansion of novel, light-based circuits, which are predicted to be a new direction in the development of computing technology aimed at ever-smaller and more efficient gadgets.
If you want to do all sorts of signal processing problems on a chip, you want to have a very fast signal, and also to be able to work on very small scales.
Ido Kaminer, Postdoc, MIT
Computer chips have previously decreased the scale of electronics to the range where the technology is bumping into certain primary physical limits, so “you need to go into a different regime of electromagnetism,” he says.
Using light instead of flowing electrons as the foundation for moving and storing data has the probability to push the operating speeds “six orders of magnitude higher than what is used in electronics,” Kaminer says - simply put, in theory up to a million times swifter.
He adds, one hurdle that the researchers faced while attempting to extend optically based chips, is that while electricity can be effortlessly limited within wires, light is more challenging as it spreads out. However, within a layer of graphene, under the appropriate conditions, the beams can be excellently confined.
“There’s a lot of excitement about graphene, because it could be easily integrated with other electronics [enabling its potential use as an on-chip light source.]
Marin Soljačić, Professor of Physics, MIT
So far, the work is theoretical, he says, so the next step will be to create working versions of the system to prove the concept. “I have confidence that it should be doable within one to two years,” he says. The subsequent step would then be to improve the system for maximum efficiency.
[This finding] is a truly innovative concept that has the potential to be the key toward solving the long-standing problem of achieving highly efficient and ultrafast electrical-to-optical signal conversion at the nanoscale. The novel instance of Čerenkov emission discovered by the authors of this work opens up whole new prospects for the study of the Čerenkov effect in nanoscale systems, without the need of sophisticated experimental set-ups. I look forward to seeing the significant impact and implications that these findings will surely have at the interface between physics and nanotechnology.
Jorge Bravo-Abad, Assistant Professor, Autonomous University of Madrid
The study was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office, through the Institute for Soldier Nanotechnologies at MIT. The team included researchers Yichen Shen, Ognjen Ilic, and Josue Lopez at MIT; Yaniv Katan at Technion, in Haifa, Israel; Hrvoje Buljan at the University of Zagreb in Croatia; and Liang Jie Wong at the Singapore Institute of Manufacturing Technology.