Harnessing the Power of Excitons and Trions for Quantum Technologies

An international team of researchers from Helmholtz-Zentrum Dresden-Rossendorf has successfully demonstrated a rapid switching process between neutral and charged particles in a 2D material using terahertz pulses. This breakthrough could lead to advancements in optical data processing and the development of flexible detectors. The study was published in Nature Photonics.

Two-dimensional semiconductors exhibit fundamentally different characteristics compared to traditional bulk crystals, particularly in the formation of excitons—bound states between negatively charged electrons and positively charged "holes." When an electron is excited by absorbing energy, it leaves behind a mobile charge, or hole, and the attraction between the two forms an exciton. When an electron interacts with another nearby electron, it forms a three-particle state known as a trion.

Trions exhibit intense light emission and carry an electrical charge, enabling simultaneous optical and electronic control. The interaction between excitons and trions has long been seen as a promising switching mechanism with potential applications. While previous studies have managed to switch between the two states, the process had been slow.

A breakthrough was achieved by an international team led by Dr. Stephan Winnerl from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Professor Alexey Chernikov of Technische Universität Dresden. Their work has significantly sped up this switching process. The project, supported by the Würzburg-Dresden Cluster of Excellence “Complexity and Topology in Quantum Materials,” also included contributions from researchers in Tsukuba, Marburg, Rome, and Stockholm.

First Catch, Then Separation

The HZDR provided a specialized facility for the experiments, utilizing the powerful terahertz pulses from the FELBE free-electron laser, which operates in the frequency range between radio waves and near-infrared light.

Initially, the researchers generated excitons by illuminating an atomically thin layer of molybdenum diselenide with brief laser pulses at cryogenic temperatures. Once formed, each exciton quickly absorbed an electron from the material, where sufficient electrons were present, converting into trions.

When we then shot terahertz pulses at the material, the trions formed back into excitons extremely quickly. We were able to show it because excitons and trions emitted near-infrared radiation at different wavelengths.

Stephan Winnerl, Physicist, Helmholtz-Zentrum Dresden-Rossendorf 

The key to the experiment was the terahertz pulses' precise frequency, which broke the weak bond between the exciton and electron, allowing the formation of a pair consisting of just one electron and one hole. This exciton then quickly absorbed another electron, reverting back into a trion.

The exciton separation occurred at an unprecedented speed—within just a few picoseconds (trillionths of a second).

This is almost a thousand times faster than previously possible with purely electronic methods and can be generated on demand with terahertz radiation.

Alexey Chernikov, Professor, Technische Universität Dresden

This innovative method opens up new research possibilities. The next steps could involve applying these techniques to various complex electronic states and materials, potentially enabling room-temperature applications and exploring quantum states of matter that arise from strong multi-particle interactions.

Prospects for Data Processing and Sensor Technology

The findings could also prove valuable for future applications in sensor technology and optical data processing.

It would be conceivable to adapt the effect for new types of modulators with rapid switching. 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.

Stephan Winnerl, Physicist, Helmholtz-Zentrum Dresden-Rossendorf 

Additionally, the research may have applications in the imaging and detection of terahertz radiation, which holds technological significance.

“Based on the demonstrated switching processes in atomically thin semiconductors, it may be possible in the long term to develop detectors that work in the terahertz range, are adjustable in a wide frequency range, and could be realized as terahertz cameras featuring a large number of pixels,” suggested Chernikov.

“In principle, even a comparatively low intensity should be sufficient to trigger the switching process.” The wavelength of near-infrared light that is released changes characteristically when trions are converted to excitons. It would be rather easy to detect this and turn it into photos utilizing state-of-the-art technology that is now available.

Journal Reference:

Venanzi, T., et al. (2024) Ultrafast switching of trions in 2D materials by terahertz photons. Nature Photonics. doi.org/10.1038/s41566-024-01512-0.