Researchers at the Massachusetts Institute of Technology (MIT; US) discovered a phenomenon in a sheet of graphene analogical to a jet airplane traveling faster than the sound waves, which creates a “sonic boom”: Here, a flow of electric current can exceed the speed of slowed-down light and produces a kind of “optical boom” — an intense, focused beam of light.
Creating the optical boom
“The key to this process is the ability of novel materials like graphene and other 2D materials to trap light and guide it while slowing it down by more than two orders of magnitude,” explains Ido Kaminer, postdoctoral fellow in the MIT Department of Physics and assistant professor of electrical engineering at the Israel Institute of Technology, Technion. This slows the speed of light — the phase velocity — inside such materials to the speed of the charge carriers. “Matching the velocities enables super strong light-matter interactions, and we are still discovering the amazing implications of such super strong interactions since it goes beyond our intuition for how light interacts with matter,” says the researcher.
The effect of this "optical boom" is the result of light moving slower than the charge carrier that emits it. Kaminer notes that the first observation of an "optical boom" is actually a few decades old and was achieved in 1934 by Pavel Äerenkov, who later was awarded the Nobel Prize for this discovery. “However, in the 80+ years that passed since then, the ‘optical boom effect,’ or as we teach it to student, the ‘Äerenkov effect,’ was mostly irrelevant to modern technology.” Why? The charge particles necessary for the emission of light had to go to relativistic (very high) speeds to compete with the speed of light — something the expert says we cannot achieve in micro- and nanodevices. “It is only now, thanks to modern materials like graphene, that we can slow down the phase velocity of light enough to bring it closer to the speed of the charge carriers in small devices.”
Graphene — the enabler technology
Graphene is the enabler technology here, the mediator that makes it possible to trap light and guide it with very small speeds, more than two orders of magnitude slower than light in free space. “In the last year or two we are also seeing something even more exciting: that graphene is not unique in any way, and there are other materials that can slow down light by even larger factors,” Kaminer says. “Graphene is only the most famous in this group of modern materials.”
Quantum theory and real-life parameters
“We have developed the quantum theory needed to describe the conversion process and used the recent condensed matter theory to describe the properties of the graphene,” Kaminer says in response to the question what problem this research work at hand has solved. He adds, “We have also investigated real life parameters and suggested potential implementations.”
The expert himself was even surprised at the outcome of this study. “Even though conventional sonic boom and optical boom processes are ‘classic’ in the sense that they do not require quantum physics, the effect we found here needed the quantum corrections in order for it to be possible. The light was not slow enough — but almost — and then the quantum correction made the additional leap.”
“There is a wide interest in new kinds of light sources that can be integrated on-chip, to become part of the silicon industry,” Kaminer says. “This will allow us to integrate the photonics and optics technology into our modern silicon technology.”
The work ahead
What is next for Kaminer and his team? “A lot,” he says. “We are studying other ways to create new light sources in hard-to-reach spectral ranges like the terahertz and the ultraviolet.” What is more, they have also developed the quantum theory of the interaction of light and matter in two dimensional materials like graphene.
“And what I personally find the most exciting is the open scientific questions that are so far not explained and we may be able to solve through our work,” the researcher shares, adding that graphene devices, for instance, have shown to emit light through processes that are not completely understood. He says he does not know whether the quantum Äerenkov effect in graphene is partially responsible for that. “New experiments are the bottleneck here, and there is a new technique that uses ultrafast electron microscope that may eventually help us solve many of these mysteries.”
The paper “Efficient plasmonic emission by the quantum Äerenkov effect from hot carriers in graphene,” published in Nature, details the work.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today