Recent advancements in extremely thin materials made up of just a few atomic layers are paving the way for new applications in electronics and quantum technology. A global team led by TU Dresden has achieved significant results in an experiment conducted at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The researchers successfully induced a rapid switching process between electrically neutral and charged light-emitting particles in a nearly two-dimensional material. This breakthrough opens up exciting possibilities for research, optical data processing, and flexible detector technology. The findings are detailed in the journal Nature Photonics.
Two-dimensional semiconductors possess unique properties that differentiate them from traditional bulk crystals. One notable aspect is their capacity to create exciton particles more easily. When an electron, which carries a negative charge, is energized in the material, it becomes excited and moves away from its original location, creating a mobile positively charged “hole.” The electron and hole attract each other, forming a bound state known as an exciton, effectively creating an electronic pair. If another electron is close by, it can join the pair, resulting in a three-particle state known as a trion. Trions are notable because they combine electrical charge with strong light emission, enabling simultaneous electronic and optical control.
For a while, the interaction between excitons and trions has intrigued researchers as a potential switching mechanism with promising future applications. While several laboratories have successfully demonstrated switching between these two states, the speeds achieved until now have been limited.
However, an international team, led by Prof. Alexey Chernikov from TU Dresden and HZDR physicist Dr. Stephan Winnerl, has significantly enhanced the speed of this switching process. This research was part of the Würzburg-Dresden Cluster of Excellence, known as “Complexity and Topology in Quantum Materials, ct.qmat,” with key contributions from researchers in Marburg, Rome, Stockholm, and Tsukuba.
First capture, then separation
The experiments were conducted using a specialized facility at HZDR, where the FELBE free-electron laser generated intense terahertz pulses, a frequency that lies between radio waves and near-infrared radiation. The researchers first exposed a thin layer of molybdenum diselenide to short laser pulses at cryogenic temperatures, causing excitons to form. Once generated, each exciton captured an existing electron from the material, turning into a trion.
“When we directed terahertz pulses at the material, the trions reverted back to excitons extremely quickly,” explains Winnerl. “We could demonstrate this because excitons and trions emitted near-infrared radiation at different wavelengths.” The experiment’s key factor was matching terahertz pulse frequencies to disrupt the bond between the exciton and electron, thus recreating a pair of just one electron and one hole. Shortly after, this exciton managed to capture another electron, forming a trion again.
The transition back to excitons occurred at record-breaking speed, with the bond separating in mere picoseconds — trillionths of a second. “This speed is almost a thousand times faster than what was previously achieved using only electronic methods, and it can be produced on demand with terahertz radiation,” emphasizes TU scientist Chernikov.
This innovative method opens exciting research avenues. The next phase may involve expanding the demonstrated processes to a broader range of complex electronic states and material platforms, potentially leading to unusual quantum states from strong interactions between many particles, as well as applications that operate at room temperature.
Prospects for data processing and sensor technology
The findings might also lead to future applications in sensor technology and optical data processing. “We could potentially adapt this effect for new types of modulators that allow for rapid switching,” indicates Winnerl. “Combined with these ultra-thin crystals, it could help create components that are extremely compact while being able to electronically manage optically encoded information.”
Another application could involve detecting and imaging technologically significant terahertz radiation. “Based on the switching processes proven in atomically thin semiconductors, we might eventually develop terahertz detectors that are tunable across a wide frequency spectrum, possibly evolving into terahertz cameras with numerous pixels,” suggests Chernikov.
“In theory, even low intensity should suffice to trigger the switching process.” The transformation of trions back to excitons results in distinct changes in the near-infrared light wavelength emitted. Detecting these changes and converting them into images could be straightforward, leveraging existing state-of-the-art technology.
