13 Jun 2018
Reducing wavelength of light could allow it to be selectively absorbed/emitted by semiconductor, promising better solar cells, tunable emitters.MIT and Israel’s Technion, could enable more efficient solar cells collecting a wider range of wavelengths, and new types of lasers and LEDs with fully tunable emission ranges, say the researchers.
The fundamental principle behind the new approach is a way to get the momentum of photons to more closely match that of electrons (which is normally many orders of magnitude greater). Because of the huge disparity in momentum, these particles usually interact weakly.
The scientists comment, “bringing these momenta closer together enables much greater control over their interactions, which promises new kinds of basic research, based on these processes, as well as many new applications.” The new findings are published in Nature Photonics.
“While silicon is a hugely important substance as the basis for most present-day electronics, it is not well-suited for applications that involve light, such as LEDs and solar cells — even though it is currently the principal material used for solar cells despite its low efficiency,” commented Ido Kaminer of Technion.
“Improving the interactions of light with an important electronics material such as silicon could be an important milestone toward integrating photonics with electronic semiconductor chips.”
Most people looking into this problem have focused on the silicon itself, Kaminer added, but this approach of the MIT-Technion partnership is very different: “We are trying to change the light instead of changing the silicon. People often try to design the matter to control light-matter interactions, but they do not usually think about adapting the light itself.”
One way to achieve this is by slowing down, or shrinking, the light enough to drastically lower the momentum of its individual photons, to take them closer to that of the electrons. In their theoretical study, the researchers showed that light could be slowed by a factor of a thousand by passing it through a multilayered thin-film material overlaid with a layer of graphene.
The layered material, made of gallium arsenide and indium gallium arsenide layers, alters the behavior of photons passing through it in a highly controllable way. This enables the researchers to control the frequency of emissions from the material by as much as 20 to 30 percent, commented Technion’s Yaniv Kurman, the paper’s lead author.
The interaction of a photon with a pair of oppositely charged particles — such as an electron and its corresponding “hole” — produces a quasiparticle called a plasmon, or a plasmon-polariton, which is a kind of oscillation that takes place in an exotic material such as the two-dimensional layered devices used in this research.
Such materials “support electromagnetic oscillations on its surface, really tightly confined” within the material, said MIT’s Nicholas Rivera. “This process effectively shrinks the wavelengths of light by orders of magnitude, bringing it down almost to the atomic scale.”
“Because of that shrinkage, the light can then be absorbed by the semiconductor, or emitted by it,” he added. In the graphene-based material, these properties can actually be controlled directly by simply varying a voltage applied to the graphene layer. In that way, “we can totally control the properties of the light, not just measure it,” said Kurman.
'New kinds of solar cells, lasers, LEDs'
The researchers further comment that, in principle, this approach could lead to new kinds of solar cells capable of absorbing a wider range of light wavelengths, which would make the devices more efficient at converting sunlight to electricity. It could also lead to light-producing devices, such as lasers and LEDs, that could be tuned electronically to produce a wide range of colors.
“The work is very general,” Kurman said, “so our results should apply to many more cases than the specific ones used in this study. We could use several other semiconductor materials, and some other light-matter polaritons.”
While this work was not conducted with silicon, it should be possible to apply the same principles to silicon-based devices, the team says. “By closing the momentum gap, we could introduce silicon into the world of plasmon-based devices,” Kurman said.
Frank Koppens, a professor of physics at the the Institute of Photonic Sciences in Barcelona, who was not involved in this research, says “the quality of this work is very high, and quite an out-of-the-box result. It is highly significant, as it is a clear break with the conventional view on emitter-light interactions.” Since the work so far is theoretical, he added, “the main question will be if this effect is visible in experiments. I’m convinced it will be shown soon, though.”
Koppens added, “one can envision many applications, such as more efficient light emitters, solar cells, photodetectors and so on. All integrated on a chip. It’s also a new way to control the wavelength of a light emitter, and I’m sure there will be applications that we didn’t even think of.” The work was supported by MIT’s MISTI Israel program.