Extremely thin crystals as laser light sources

Crystals consisting of only three layers of atoms can emit light equal to that of a laser at room temperature. Thus, these novel materials can potentially be of use as light sources in miniaturized circuits, or in future quantum applications. This is reported by an international team of researchers from the Universities of Oldenburg, Jena, and Würzburg, as well as from the Arizona State University (USA), the Westlake University (China), the Fraunhofer Institute for Applied Optics and Precision Engineering IOF, and the National Institute of Materials Science in Tsukuba (Japan) in the journal Nature Communications. Until now, comparable effects could only be achieved in vacuum and at temperatures just above zero degrees. "The transition from these cryogenic temperatures to room temperature implies much more interesting application prospects for these two-dimensional materials," as Prof. Dr. Christian Schneider from the University of Oldenburg states. 

For these investigations, the team used the compound tungsten diselenide. This substance belongs to a class of semiconductors which consist of a transition metal and one of the following elements: sulfur, selenium, or tellurium. "The single-layer crystals of these semiconductors show strong light interaction and been regarded as a possible basis for micro- and nanolasers have for some time," expounds Dr. Anton-Solanas from the Oldenburg group. As recently as May, the same team reported in  Nature Materials that a layer of the material molybdenum diselenide produces laser light at low temperatures.

Tailor-made optical resonators from Jena 

"With the help of tailor-made optical resonators from Jena, we have now reached the next milestone and created the same effect at room temperature," explains Dr. Falk Eilenberger, head of a junior research group for photonics in 2D materials at the Institute of Applied Physics of the University of Jena. Laser emission is based on physical objects that simultaneously consist of matter and light – so-called exciton polaritons. This is a coupling between light particles and excited electrons. These objects are formed when electrons in solids are placed in a state of higher energy, by laser light for example. After fractions of a second, they emit a light particle again. When caught between two mirrors, this, in turn, can excite a new electron – a cycle which can continue until a particle of light escapes the trap. The resulting exciton polaritons combine various interesting properties of electrons and light particles (photons). 

Particularly fascinating: If the number of exciton polaritons becomes large enough, they will no longer behave as individual particles, but merge into a macroscopic quantum state. This transformation can be detected in a sample by means of suddenly increasing light emission. The generated radiation, like the light of a laser, has only a single wavelength, it is monochrome, so to speak. It also propagates in a certain direction and is able to form so-called interferences, a property called coherence in physics. 

Two-dimensional materials as a platform for novel nano lasers 

To prove this effect for tungsten diselenide, the team first produced less than one nanometer thick samples of the semiconductor and placed them between suitable mirrors, produced at the Fraunhofer IOF in Jena. Then, the physicists stimulated the crystals with laser light and investigated the resulting emissions using various methods. What they observed strongly indicated that the radiation must originate in objects, which contain properties of both light and matter. From this they gather that exciton polaritones had indeed formed in the semiconductor. In addition, the researchers found signs that these particles had merged into a common macroscopic quantum state. 

"Our results support the hope that two-dimensional materials will be suitable as a platform for novel nano lasers that can function at room temperature – a goal that various groups worldwide have been pursuing for around ten years," as Schneider explains. In May of this year, another team had also discovered evidence of coherent laser emissions of exciton polaritons in single-layer crystals at room temperature. "This confirms that our results are correct," says Anton-Solanas. Moreover, the strong interaction between light and two-dimensional materials has special properties that make them interesting for circuits, in which light could control electric currents.