Crystals consisting of only three layers of atoms can emit light at room temperature that is similar to a laser. These novel materials thus have the potential to be used as light sources in miniaturised electronic switches or even in future quantum applications. This was reported by an international team of researchers from the Universities of Oldenburg, University of Jena, University of Würzburg, Arizona State University (USA), Westlake University (China), the Fraunhofer Institute for Applied Optics and Precision Engineering IOF and the National Institute for Materials Science in Tsukuba (Japan) led by the Oldenburg physicists Dr Hangyon Shan, Prof. Dr Christian Schneider and Dr Carlos Anton-Solanas in the journal Nature Communications. Until now, comparable effects could only be produced in a vacuum and at temperatures close to absolute zero. "The transition from these cryogenic temperatures to room temperature means that these two-dimensional materials become much more interesting for applications," says Schneider, who heads the Quantum Materials working group at the University of Oldenburg.
The team used the compound tungsten diselenide for its experiments. This compound belongs to a class of semiconductors that consist of a transition metal and 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," Anton-Solanas explained. Only last May, the same team reported in the journal Nature Materials that a layer of the semiconductor material molybdenum diselenide generated laser light at cryogenic temperatures.
Particles composed of matter and light
"With the help of customised optical resonators from Jena, the researchers have now reached the next milestone and produced the same effect at room temperature," explains Dr. Falk Eilenberger, who heads the junior research group Photonics in 2D Materials at the IAP of Friedrich Schiller University. The laser emission comes from hybrid particles composed of matter and light, known as exciton-polaritons. They are the result of coupling between light particles and excited electrons. The excited electrons form when electrons in the ground state are elevated into a higher energy state, for example by laser light. After a fraction of a second, they 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 interesting properties of both electrons and light particles (photons).
A particularly interesting aspect: If the number of exciton polaritons becomes large enough, they no longer behave as individual particles but merge into a macroscopic quantum state. A sudden increase in light emission from the sample indicates that this transformation has taken place. Like the light from a laser, the resulting radiation has only a single wavelength –so it is monochromatic. It also radiates in a specific direction and is able to display "interference" phenomena, a property known as "coherence" in physics.
Samples less then a nanometre thick
To demonstrate this effect for tungsten diselenide, the team first produced samples of the semiconductor that were less than a billionth of a metre (one nanometre) thick and placed them between special mirrors, which were produced at the Fraunhofer IOF in Jena. The physicists then stimulated the crystals with laser light and studied the resulting emissions using various techniques. They found strong evidence that the radiation had to come from hybrid particles that have properties of both light and matter. This allowed them to conclude that exciton-polaritons had indeed formed in the semiconductor. In addition, the researchers 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 ten 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 switches in which light could control electric currents.
Hangyong Shan et al: „Spatial coherence of room-temperature monolayer WSe2 exciton-polaritons in a trap”, Nat Commun 12, 6406 (2021). https://doi.org/10.1038/s41467-021-26715-9