Scientists from the Okinawa Institute of Science and Technology (OIST) have recently crossed an important barrier. For the first time in history, they directly observed dark excitons in atomically thin materials, making a world-first accomplishment in this burgeoning field. This prestigious international award comes in recognition of their pioneering discovery, which promises to set the stage for progress in classical and quantum information technologies.
Dark excitons are special quasiparticles that have proven to be promising information carriers for applications in different fields, owing to their intrinsic properties. In particular, they are less likely to see any interaction with light which causes them to degrade their quantum properties. This third property is what allows us to preserve quantum states for longer periods. This preservation is key to their future use in cutting-edge technology.
The research team specifically studied transition metal dichalcogenides (TMDs). It’s important to note that these materials have been in development for more than a decade. To visualize the transient states, they employed a state-of-the-art time- and angle-resolved photoemission spectroscopy (TR-ARPES) microscope. This microscope employs their proprietary tabletop extreme ultraviolet (XUV) source, which enables them to image electrons and excitons on femtosecond timescales.
The Role of Dark Excitons in Information Technology
Professor Keshav Dani, leader of OIST’s Femtosecond Spectroscopy Unit. He drew special attention to their ability to use dark excitons as information carriers. He stated,
“Dark excitons have great potential as information carriers, because they are inherently less likely to interact with light, and hence less prone to degradation of their quantum properties. However, this invisibility also makes them very challenging to study and manipulate.”
Indigo Dark excitons have special properties that preserve them for longer times. In fact, this feature can be incredibly useful for absorbing and digesting information. The researchers identified two distinct species of dark excitons: momentum-dark and spin-dark. These classifications are based on the contradicting characteristics of electrons and holes inside the excitons.
This difference in properties allows these excitons to survive for tens of nanoseconds. It profoundly isolates them from interactions with the natural environment. This isolation, while a challenge for fabrication and control, is advantageous for keeping their quantum states, which is key for their use in tomorrow’s technology.
“There are two ‘species’ of dark excitons: momentum-dark and spin-dark, depending on where the properties of electron and hole are in conflict.”
This experimental setup is OIST’s cutting edge TR-ARPES microscope. This cutting-edge technology has made it possible for researchers to directly probe and map the dynamics of dark excitons. This new creation technology gives scientists a previously unattainable means to control the behavior of excitons. They accomplish this by creating ultrabright excitons in specific valleys of TMD semiconductors.
Advanced Experimental Techniques at OIST
Bright excitons are engineered in one valley using a circularly polarized light. This fast selective generation is enabled by the unique atomic symmetry of TMDs. Luminescent bright excitons quickly relax into a cascade of lower-energy dark-exciton states that may be able to store valley information.
The conversion between bright and dark excitons takes place in just picoseconds. Yet, questions remain on the specific dark exciton types at play and their efficacy to robustly preserve valley information.
“Thanks to the sophisticated TR-ARPES setup at OIST, we have directly accessed and mapped how and what dark excitons keep long-lived valley information. Future developments to read out the dark excitons valley properties will unlock broad dark valleytronic applications across information systems.”
In particular, the finding of dark excitons provides fresh avenues for deep exploration. These discoveries will profoundly influence spintronics and valleytronics, disciplines that use electron spin and the special properties of crystal structures of materials to store information.
Xing Zhu, a researcher involved in the study, elaborated on the role of electron dynamics in electronics:
“The unique atomic symmetry of TMDs means that when exposed to a state of light with a circular polarization, one can selectively create bright excitons only in a specific valley. This is the fundamental principle of valleytronics.”
By manipulating the spin states and momentum characteristics of electrons within TMDs, researchers can develop innovative methods for data processing and storage.
Future Implications and Research Directions
The science moving quickly. By capturing these special characteristics of dark excitons, we might be able to spark huge advances in information technology. The team at OIST is well-positioned to continue pursuing these opportunities, opening up exciting new applications in industrial, agricultural, medical and many other fields.
Xing Zhu, a researcher involved in the study, elaborated on the role of electron dynamics in electronics:
“In the general field of electronics, one manipulates electron charge to process information.”
By manipulating the spin states and momentum characteristics of electrons within TMDs, researchers can develop innovative methods for data processing and storage.
As research progresses, the ability to read out dark excitons and leverage their unique properties could lead to revolutionary developments in information systems. The team at OIST is poised to explore these possibilities further, potentially unlocking new applications across various sectors.