This is an artistic representation of the WSe2 monolayer, which emits exactly two photons of different frequencies under suitable conditions. Image: Karol Winkler.
This is an artistic representation of the WSe2 monolayer, which emits exactly two photons of different frequencies under suitable conditions. Image: Karol Winkler.

Monolayers, or two-dimensional (2D) materials, have received a great deal of interest over the past 10 years, as they show great promise for revolutionizing many areas of physics. The term monolayer refers to solid materials with minimum thickness: occasionally, as with graphene, a monolayer is only one-atom thick, but in crystals they can comprise three or more atomic layers.

Monolayers frequently exhibit unexpected properties that make them interesting for research, with the so-called transition metal dichalcogenides (TMDC) proving particularly promising. These 2D materials behave like semiconductors and so can be used for manufacturing ultra-small and energy-efficient computer chips, for example.

Moreover, TMDCs are also capable of generating light when supplied with energy. As reported in a paper in Nature Communications, Christian Schneider, Sven Höfling and their research team from the Chair of Technical Physics at the Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, Germany, have now found ways to harness this effect.

First, they produced a monolayer of a TMDC known as tungsten diselenide (WSe2) by using a piece of sticky tape to peel a multi-layer film from a bulk WSe2 crystal; using the same procedure, they then stripped thinner and thinner layers from this film. They repeated this process until the material on the tape comprised a single layer.

The researchers then cooled this WSe2 monolayer down to just above absolute zero and excited it with a laser, causing the monolayer to emit single protons 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."

Light sources able to produce pairs of photons at a time are of interest because they potentially offer a completely secure way to transmit information, without the risk of interception. This involves entangling the two photons with each other – a quantum mechanical process in which their states are 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. This fact can be used to encrypt communication channels.

In a second study, the JMU scientists demonstrated another potential application of these exotic monolayers. This involved mounting a WSe2 monolayer between two mirrors and then stimulating it with a laser, exciting the monolayer to a level where it begins emitting photons itself. These photons are reflected back to the monolayer by the mirrors, exciting more atoms to emit more photons.

"We call this process strong coupling," Schneider explains, and it essentially results in the photons being cloned. "Light and matter hybridize, forming new quasi particles in the process: the exciton polaritons," Schneider continues. For the first time, it has now been possible to detect these polaritons at room temperature in atomic monolayers.

In the medium term, this work could lead to a range of interesting applications. The ‘cloned’ photons have similar properties to laser light, but they are generated in a completely different way. Ideally, the production of new light particles is self-sustaining after the initial excitation, without requiring any additional energy. In a laser, by contrast, the light-producing material has to be continuously excited by an external source of energy. This makes the new light source highly energy-efficient, as well as ideally suited to studying certain quantum effects.

This story is adapted from material from the Julius-Maximilians-Universität Würzburg, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.