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Single-layer ‘super material’ semiconductor shows promise as novel light source


[Image above] Artistic representation of a two-photon source: The monolayer (below) emits exactly two photons of different frequencies under suitable conditions. They are depicted in red and green in the picture. Credit: Karol Winkler

When it comes to next-generation electronic devices and energy sources, thinner is better.

That’s why graphene continues to be pegged by researchers as one of the materials that will revolutionize this sector. It’s graphene’s single-layer thickness, toughness, and supreme mechanical and thermal properties that make it ideal for developing electronic, optoelectronic, and electromechanical devices and sensors.

But manufacturing graphene in large quantities isn’t necessarily easy. Historically, it’s been an expensive, labor-intensive material to produce for commercial applications—so researchers continue to uncover new and innovative solutions to produce this high-potential material in a way that’s scalable.

But thin, 2-D materials like graphene have so much potential.

And like graphene, 2-D transition metal dichalcogenides (or TMDCs) are particularly promising. Researchers at the University of Würzburg in Germany say TMDCs are actually capable of generating light when supplied with energy.

“They behave like semiconductors and can be used to manufacture ultra-small and energy-efficient chips, for example,” according to a university press release.

The team used sticky tape to peel a multi-layer film from a TMDC crystal, the release explains. Using the same procedure, they stripped thinner and thinner layers from the film, repeating the process until the material on the tape is reduced to single-layer thickness.

The researchers cooled the monolayer to just above absolute zero and stimulated it with a laser, which causes the monolayer to emit single photons under specific conditions.

“We were now able to show that a specific type of excitement produces not one but exactly two photons,” Christian Schneider, study co-author, explains. “The light particles are generated in pairs so to speak.”

The most interesting thing about the two-photon sources, the team says, is that they can be used to transfer information 100% tap-proof—crucial for encrypting secure communication channels.

“The light particles are entangled with each other—a quantum mechanical process in which their state is interwoven. The state of the first photon then has a direct impact on that of the second photon, regardless of the distance between the two,” the release explains.

The team conducted a second study in which they mounted a monolayer between two mirrors and stimulated it with a laser—and this caused the TMDC plate to emit its own photons. The mirrors then reflected back those photons to the TMDC plate, which excited more atoms to create more new photons.

“We call this process strong coupling,” Schneider explains. In other words, the light particles are cloned. “Light and matter hybridize, forming new quasi-particles in the process: the exciton polaritons.”

The researchers say this is the first time it’s been possible to detect these polaritons at room temperature in atomic monolayers. And that opens to the door to some interesting new potential applications.

The “cloned” photons emit light much like lasers—but unlike lasers, the cloned photons are self-sufficient and don’t require any additional energy beyond the initial burst needed to jumpstart the cloning process. And that could lead to a new highly-energy efficient light source, the team says.

The studies, forthcoming in Nature Communications, are “Room temperature Tamm-Plasmon Exciton-Polaritons with a WSe2 monolayer” (DOI: 10.1038/ncomms13328); and “Cascaded emission of single photons from the biexciton in monolayered WSe2” (DOI: 10.1038/ncomms13409).