Researchers have achieved a breakthrough in the ultrafast switching of tiny light sources, paving the way for advancements in electronics and quantum technologies. By precisely controlling the conversion between electrically neutral and charged luminescent particles in an ultra-thin material, they have opened up new possibilities for optical data processing and flexible detectors.
The art of Exciton-Trion — Part 1
Without the benefit of bulk crystals, two-dimensional semiconductors show a wealth of interesting properties. A key feature of these is their ability to produce ‘exciton’ particles – formed by coupling an electron with a “hole” (its positive counterpart).
If an additional electron is present nearby, it can be pulled onto the exciton to create a three-particle state called a ‘trion’. Trions are the unique combination of a long-lived electrical charge and efficient light emission, enabling both electronic and optical manipulation.
The interaction of excitons and trions has been a topic of interest for quite some time due to the prospect of flexible switching operations. But so far, switching speeds have been constrained. This speed of switching has now been increased significantly by the researchers, which prompts new scientific and application options.
Over the last few years, researchers have been able to show that it is possible to control magnetization on realistic timescales by using a combination of femtosecond and terahertz pulses.
The method was developed using a lab in the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), which has a free-electron laser called FELBE. The laser generates powerful terahertz pulses — a specific frequency range in the far-infrared part of the electromagnetic spectrum that lies between radio waves and near-infrared light.
In the experiment, researchers first shone brief laser pulses on a cryogenically cooled atomically thin layer of molybdenum diselenide to form this kind of excitons. But as these excitons formed, they immediately snatched up a second electron, turning them into trions.
The researchers then beamed terahertz pulses at the material, and what they saw was incredible. When we irradiated the material with the terahertz pulses, the trions immediately recombined into excitons,’ says HZDR physicist Dr. Stephan Winnerl. ‘You can tell this by looking at them, as excitons and trions emit near-infrared radiation at different wavelengths.’
The secret to this near-instantaneous switching was the perfect matching of the terahertz pulse frequency with the fragile bond holding the exciton and extra electron together. As a result, only the pair split within 100 picoseconds – nearly a thousand times faster than could be done electronically.
Conclusion
The scientists’ pioneering efforts in demonstrating ultrafast switching of nanoscale light sources with terahertz pulses, open up new horizons for future electronics and quantum technologies. Imagine what the exact control over exciton—trion interplay at unprecedented speed will enable it would allow better sensor technologies, optical data processing, or tunable terahertz detectors. The world of research is still in its early stages, but as these newfound capabilities are further investigated the types of applications that could be realized become increasingly exciting and could foretell a future with vast new computing power, flexible electronics, and new deeper understandings of quantum phenomena.
Key to Ultrafast Switching
The breakthrough came through the use of a specialized facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), which houses a free-electron laser called FELBE. This laser produces intense terahertz pulses – a unique frequency range that lies between radio waves and near-infrared radiation.
The researchers first illuminated an atomically thin layer of molybdenum diselenide with short laser pulses at cryogenic temperatures, generating the excitons. As these excitons were created, they quickly captured an additional electron, transforming into trions.
The team then directed the terahertz pulses at the material, and the results were remarkable. ‘When we shot the terahertz pulses at the material, the trials formed back into excitons extremely quickly,’ explains HZDR physicist Dr. Stephan Winnerl. ‘We were able to observe this because excitons and trions emit near-infrared radiation at different wavelengths.’
The key to this ultrafast switching was the precise matching of the terahertz pulse frequency to the weak bond between the exciton and the additional electron. This allowed for the rapid separation of the pair, recreating the exciton within a matter of picoseconds – nearly a thousand times faster than previously possible using purely electronic methods.
Conclusion
The researchers’ groundbreaking work in ultrafast switching of tiny light sources using terahertz pulses has opened up exciting new possibilities for the future of electronics and quantum technologies. The ability to precisely control the interplay between excitons and trions at record-breaking speeds could lead to advancements in sensor technology, optical data processing, and the development of versatile terahertz detectors. As researchers continue to explore these newfound capabilities, the potential for innovative applications continues to grow, promising a future of enhanced computational power, flexible electronics, and a deeper understanding of quantum phenomena.
