2D-Materials as an Efficient Light Source

Artistic representation of a two-photon source: The monolayer (below) emits exactly two photons of different frequencies under suitable conditions. (Source: JMU / K. Winkler)

Artistic representation of a two-photon source: The monolayer (below) emits exactly two photons of different frequencies under suitable conditions. (Source: JMU / K. Winkler)

Monolayers are at the heart of the research acti­vities. These materials have been surrounded by a virtual hype in the past ten years. This is because they show great promise to revolu­tionise many areas of physics. In physics, the term monolayer refers to solid materials of minimum thickness. Experts also speak of two-dimen­sional materials. In this form, they frequently exhibit unexpected pro­perties that make them interesting for research. The transition metal dichal­cogenides (TMDC) are particularly promising. They behave like semi­conductors and can be used to manufacture ultra-small and energy-efficient chips, for example.

Moreover, TMDCs are capable of generating light when supplied with energy. Christian Schneider, Sven Höfling and their research team from the Chair of Technical Physics of the Julius-Maxi­milians-Univer­sität Würzburg in Bavaria, Germany, have harnessed exactly this effect for their experiments. First, a monolayer was produced using a simple method. This usually involves a piece of sticky tape to peel a multi-layer film from a TMDC crystal in a first step. Using the same procedure, thinner and thinner layers can be stripped from this film. This process is repeated until the material on the tape is only one layer thick.

The researchers then cooled this monolayer down to a tempe­rature of just above absolute zero and excited it with a laser. This 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,” Schneider explains. “The light particles are generated in pairs so to speak.”

Such two-photon sources are interesting for the following reason: They can be used to transfer infor­mation 100 % tap-proof. For this purpose, the light particles are entangled with each other. The state of the first photon then has a direct impact on that of the second photon, regardless of the distance between the two. This fact can be used to encrypt communi­cation channels.

In a second study, the JMU scientists demonstrated another appli­cation option of the exotic mono­layers. For this purpose, they mounted a monolayer between two mirrors and again sti­mulated it with a laser. The radiation excited the TMDC plate to a level that it began to emit photons itself. These were reflected back to the plate by the mirrors where they excited atoms themselves to create new photons.

The light particles are cloned during this process. “Light and matter hybri­dise, forming new quasi particles in the process: the exciton polari­tons,” Schneider says. For the first time, it has now been possible to detect these polaritons at room tempera­ture in atomic mono­layers. In the medium run, this will open up interes­ting new applications. The photons have similar properties to laser light. But they are manu­factured in completely different ways: Ideally, the production of new light particles is self-sustaining after the initial excitation without requiring any additional energy supply. In a laser in contrast, the light-producing material has to be excited energe­tically from the outside on a permanent basis. This makes the new light source highly energy-effi­cient. Moreover, it is excellently suited to study certain quantum effects.  (Source: Univ. Wuerzburg)

Reference: N. Lundt et al.: Room-temperature Tamm-plasmon exciton-polaritons with a WSe2 monolayer, Nat. Comms. 7, 13328 (2016); DOI: 10.1038/ncomms13328

Link: Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Universität Würzburg, Würzburg, Germany

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