Physicists have discovered that two-dimensional crystals placed between 'mirrors' can be made to emit laser light at room temperature. Image: University of Oldenburg/QMat.
Physicists have discovered that two-dimensional crystals placed between 'mirrors' can be made to emit laser light at room temperature. Image: University of Oldenburg/QMat.

In a paper in Nature Communications, an international team led by physicists Hangyon Shan, Christian Schneider and Carlos Anton-Solanas from the University of Oldenburg in Germany reports that crystals consisting of just three atomic layers can emit laser-like light at room temperature. Up to now, scientists have only been able to produce such effects at temperatures just above absolute zero.

This two-dimensional material thus has the potential to be used as a light source in miniaturized circuits and also in future quantum applications. "The transition from these cryogenic temperatures to room temperature makes these two-dimensional materials much more interesting for applications," said Schneider, who heads the Quantum Materials research group at the University of Oldenburg.

The physicists used the 2D material tungsten diselenide for their experiments. Tungsten diselenide belongs to a class of semiconductors that consist of a transition metal with one of the elements sulphur, selenium or tellurium.

"The monolayer crystals of these semiconductors interact very strongly with light and have been considered as a potential basis for micro- and nanolasers for some time," explained Anton-Solanas. Only last May, the same team reported in a paper in Nature Materials that a layer of the related semiconductor material molybdenum diselenide generated laser light at cryogenic temperatures.

Now the physicists have hit the next milestone and created the same effect at room temperature. The laser emission comes from hybrid particles composed of matter and light, known as exciton-polaritons, which are the result of coupling between light particles (photons) and excited electrons.

The excited electrons form when electrons in the ground state are pushed up to a higher energy state, which can be done with laser light. After a fraction of a second, the excited electrons emit a light particle. When this particle is trapped between two mirrors, it can in turn excite another electron – and the cycle continues until a light particle escapes the trap. The exciton-polaritons created in this coupling process combine the properties of both electrons and photons.

A particularly interesting aspect is that if sufficient exciton-polaritons are generated they cease to behave like individual particles and merge into a macroscopic quantum state. A sudden increase in light emission from the sample indicates such a transformation has taken place. Like the light from a laser, this radiation has only a single wavelength, meaning it is monochromatic. It also radiates in a specific direction and is able to display 'interference' phenomena, a property known as 'coherence' in physics.

To demonstrate this effect for tungsten diselenide, the physicists first fabricated samples of the semiconductor that were less than 1nm thick and placed them between special mirrors. They then stimulated the crystals with laser light and studied the resulting emissions using various techniques.

This produced strong evidence that the radiation had to come from hybrid particles with properties of both light and matter, which allowed the physicists to conclude that exciton-polaritons had indeed formed in the semiconductor. In addition, the physicists found evidence that these particles had merged into a common macroscopic quantum state.

"Our results strengthen the hope that two-dimensional materials can be suitable as a platform for new nanolasers that can also function at room temperature – a goal that various groups around the world have been pursuing for around 10 years," Schneider explained.

In May of this year, another team of researchers also found evidence of coherent laser emissions from exciton-polaritons in monolayer crystals at room temperature. "This reinforces our belief that our results are correct," said Anton-Solanas. In addition, the strong interaction between light and two-dimensional materials has special properties that make these materials interesting for circuits that control electric currents with light.

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