Research team succeeds in switching tiny light sources ultra-rapidly

Extremely thin materials, consisting of just a few atomic layers, promise applications in electronics and quantum technologies. An international team led by TU Dresden has made remarkable progress with an experiment at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR): the experts were able to induce an extremely fast switching process between electrically neutral and charged luminescent particles in a ultra-thin frame. , effectively two-dimensional material.

The result opens new perspectives for research as well as for optical data processing and flexible detectors. The research is presented in the journal Natural photonics.

Two-dimensional semiconductors can exhibit fundamentally different properties than more conventional bulk crystals. In particular, it is easier to generate what are called exciton particles: if an electron, known to be negatively charged, is excited in the material by absorbing energy, it is moved away from its position d ‘origin. It leaves behind a mobile charge – a positively charged “hole”.

Electrons and holes attract each other and together form a bound state called an exciton, a kind of electronic pair. If another electron is nearby, it is attracted to it to form a three-particle state, called a trion in scientific jargon. The special feature of the trion is the combination of an electrical charge with strong light emission, which allows simultaneous electronic and optical control.

For some time, the interaction between the exciton and the trion has been considered a switching process that is both intriguing in its own right and likely to be of interest for future applications. In fact, many laboratories have already succeeded in switching between the two states in a targeted manner, but so far with limited switching speeds.

The study was led by Professor Alexey Chernikov from TU Dresden and HZDR physicist Dr. Stephan Winnerl has now been able to significantly accelerate this switching. The work was carried out within the framework of the Würzburg-Dresden center of excellence “Complexity and topology of quantum materials, ct.qmat”. Researchers from Marburg, Rome, Stockholm and Tsukuba made important contributions to the project.

First the capture, then the separation

The experiments took place in a special facility at the HZDR. The FELBE free electron laser delivers intense terahertz pulses, a frequency range between radio waves and near-infrared radiation. The researchers first illuminated an atomically thin layer of molybdenum diselenide at cryogenic temperatures with short laser pulses, thereby generating excitons. As soon as they were created, each exciton captured an electron among those already present in sufficient numbers in matter, and thus became trions.

“When we then fired terahertz pulses at the material, the trions converted back to excitations extremely quickly,” says Winnerl. “We were able to show this because the excitons and trions emitted near-infrared radiation at different wavelengths.”

The deciding factor in the experiment was the frequency adaptation of the terahertz pulses to break the weak bond between the exciton and the electron. A pair composed of a single electron and a hole has therefore been recreated. Shortly after, this exciton captures another electron and becomes a trion again.

The exciton separation took place at record speed. The link was broken in a matter of picoseconds, or trillionths of a second. “This is almost a thousand times faster than was previously possible with purely electronic methods and can be generated on demand with terahertz radiation,” emphasizes Chernikov, a scientist at TU.

The new method offers interesting research perspectives. The next step could be to extend the demonstrated processes to a variety of complex electronic states and hardware platforms. Unusual quantum states of matter, resulting from the strong interaction between numerous particles, would thus be within our reach, as would applications at room temperature.

Perspectives for data processing and sensor technology

The results could also become useful for future applications, for example in sensor technology or optical data processing.

“It would be possible to adapt the effect to new types of fast-switching modulators,” says Winnerl. “In combination with the ultra-thin crystals, this could be used to develop components that are both extremely compact and capable of electronically controlling optically encoded information.”

Another area would be applications in the detection and imaging of technologically relevant terahertz radiation.

“Based on the switching processes demonstrated in atomically thin semiconductors, it might be possible in the long term to develop detectors operating in the terahertz range, tunable in a wide frequency range and capable of being realized as terahertz cameras with a large number of pixels,” suggests Chernikov. “In principle, even a relatively low intensity should be enough to trigger the switching process.”

The conversion of trions to excitons results in characteristic changes in the wavelength of the emitted near-infrared light. Detecting this and converting it into images would be quite simple and could be achieved using already existing cutting-edge technology.