Shining a light on dark valleytronics: First direct observation of dark excitons in atomically thin materials

In a world-first, researchers from the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology (OIST) have directly observed the evolution of the elusive dark excitons in atomically thin materials, ...

Their findings have been published in Nature Communications . Professor Keshav Dani, head of the unit, says, "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. "Building on a previous breakthrough at OIST in 2020 , we have opened a route to the creation, observation, and manipulation of dark excitons."

"In the general field of electronics, one manipulates electron charge to process information," explains Xing Zhu, co-first author and Ph.D. student in the unit. "In the field of spintronics, we exploit the spin of electrons to carry information. Going further, in valleytronics, the crystal structure of unique materials enables us to encode information into distinct momentum states of the electrons, known as valleys." The ability to use the valley dimension of dark excitons to carry information positions them as promising candidates for quantum technologies. Dark excitons are by nature more resistant to environmental factors like thermal background than the current generation of qubits, potentially requiring less extreme cooling and making them less prone to decoherence, where the unique quantum state breaks down.

The TR-ARPES setup used in the research. Credit: Jeff Prine (OIST) Read more

The experimental setup at OIST, featuring the world-leading TR-ARPES (time- and angle resolved photoemission spectroscopy) microscope, which features a proprietary, tabletop XUV (extreme ultraviolet) source, capable of imaging the electrons and excitons at femtosecond timescales (1fs = one quadrillionth (10 -15 ) of a second). Credit: Jeff Prine & Andrew Scott (OIST) Read more

Graphical illustration of the results, showing how the population of different exciton emerge and evolve over time at a picosecond scale (1ps = 10 −12 second). Credit: Jack Featherstone (OIST), adapted from Zhu et al. (2025) Nature Communications 16 6385. Read more

The atomic structure of ultrathin semiconductors like TMDs is hexagonal, and this symmetry is reflected in momentum space, where the conduction (top) and valence (bottom) bands each have local energy minima and maxima at specific points (K), which can be visualized as valleys in a momentum landscape. Credit: Momentum landscape figure adapted Bussolotti et al., (2018) Nano Futures 2 032001. Insert adapted from Zhu et al., (2025) Nature Communications 16 6385 Read more