Research team succeeds in ultra-fast switching of tiny light source

Ultrathin materials consisting of just a few atomic layers hold promise for applications in electronics and quantum technology. An international team led by the Technical University of Dresden has made remarkable progress in experiments carried out 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 within ultrathin luminescent particles. , which is essentially a two-dimensional material.

This result opens new perspectives not only for research but also for optical data processing and flexible detectors. The research is published in the journal Nature Photonics.

Two-dimensional semiconductors can exhibit fundamentally different properties compared to traditional bulk crystals. In particular, it facilitates the production of so-called exciton particles. When an electron, known to be negatively charged, absorbs energy and becomes excited within the material, it is removed from its original position. A mobile charge, or positively charged “hole”, is left behind.

Electrons and holes attract each other and form a bound state called an exciton, which is a type of electron pair. When another electron is nearby, it is attracted to it, forming a three-particle state known in scientific terms as a trion. Trion’s special feature is the combination of electrical charge and strong luminescence, which allows for simultaneous electronic and optical control.

For quite some time, the interaction between excitons and trions has been considered to be a switching process that is interesting in itself and may also be interesting for future applications. In fact, many laboratories have already successfully switched between the two states in a targeted manner, but the speed of the switch has so far been limited.

The research was led by Professor Alexei Chernikov of the Technical University of Dresden, and HZDR physicist Dr. Stefan Wijnaar succeeded in significantly accelerating this switch. This research was carried out within the framework of the Würzburg-Dresden Cluster of Excellence “Complexity and Topology in Quantum Materials, ct.qmat”. Researchers from Marburg, Rome, Stockholm and Tsukuba made important contributions to the project.

First catch, then separate

The experiment was carried out using special facilities at HZDR. FELBE free electron lasers emit powerful terahertz pulses in the frequency range between radio waves and near-infrared radiation. The researchers first generated excitons by irradiating an atomically thin layer of molybdenum diselenide with short laser pulses at extremely low temperatures. As soon as they were created, each exciton captured an electron from enough excitons already present in the material to become a trion.

“We then exposed the material to a terahertz pulse, and the trions turned back into excitons very quickly,” Winnaar explains. “We were able to show that because the excitons and trions emitted near-infrared radiation at different wavelengths.”

A decisive factor in the experiment was the frequency matching of the terahertz pulses to break the weak bond between excitons and electrons. Therefore, a pair consisting of only one electron and one hole has been recreated again. Soon after, this exciton captures another electron and becomes a trion again.

The dissociation into excitons occurred in record time. This bond was broken within a few picoseconds, or a trillionth of a second. “This can be produced on demand using terahertz radiation, almost 1000 times faster than what was previously possible with purely electronic methods,” emphasizes TU scientist Chernikov. .

New methods offer interesting prospects for research. The next step may be to extend the demonstrated process to a variety of complex electronic states and material platforms. Anomalous quantum states of matter resulting from strong interactions between many particles will therefore come within reach, as will applications at room temperature.

Outlook for data processing and sensor technology

The results could also be useful for future applications such as sensor technology and optical data processing.

“The idea is to adapt the effect to new types of modulators with fast switching,” explains Winnerl. “Combined with ultrathin crystals, this could be used to develop components that are extremely compact and can electronically control optically encoded information.”

Another area is the technically relevant detection and imaging applications of terahertz radiation.

“Based on a switching process demonstrated in atomically thin semiconductors, we will develop a detector that can operate in the terahertz range in the long term, is tunable over a wide frequency range, and can be realized as a terahertz camera with a large number of functions. “Pixel” suggests Chernikov. “In principle, even relatively low intensities should be sufficient to trigger the switching process.”

Converting trions to excitons causes a characteristic change in the wavelength of the emitted near-infrared light. Detecting this and converting it into an image is very easy and can be accomplished using existing cutting-edge technology.