In a world-first, researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) have directly observed the elusive “dark excitons” in atomically thin semiconductors, providing a crucial breakthrough in the emerging field of valleytronics. Their results, published in Nature Communications, open new pathways for both classical and quantum information technologies.
Professor Keshav Dani, head of OIST’s Femtosecond Spectroscopy Unit, explained the challenge: dark excitons, by their very nature, do not emit light and have remained hidden from direct study. Yet they hold immense promise as stable information carriers, less prone to environmental interference than today’s fragile quantum bits (qubits).
From Electronics to Valleytronics
For decades, electronics has revolved around the manipulation of electron charge, while spintronics pushed the frontier by using electron spin. Valleytronics adds another layer. Here, the atomic crystal structure of ultrathin semiconductors—particularly transition metal dichalcogenides (TMDs)—creates distinct momentum states known as “valleys.” These valleys can store and process information, much like a new digital alphabet for the quantum age.
Xing Zhu, co-first author and PhD student at OIST, described it simply: “In electronics we move charge. In spintronics we use spin. In valleytronics, we exploit valleys in the momentum landscape. Dark excitons are a natural fit, because they can store valley information for longer timescales, resisting disruption from heat or noise.”
Bright vs Dark Excitons
When light excites electrons in TMDs, they jump to a higher energy band, leaving behind positively charged “holes.” Electrons and holes pair to form quasiparticles called excitons. If their spins and valleys align, they recombine quickly, emitting light as “bright excitons.”
Dark excitons, however, arise when spins or valley positions misalign. These “forbidden pairs” cannot recombine immediately, which prevents them from radiating light. The result: dark excitons persist for nanoseconds—an eternity in quantum physics—while remaining shielded from environmental noise.
Dr. David Bacon, co-first author and now at University College London, classified them into two species: momentum-dark and spin-dark. Momentum-dark excitons form when electrons scatter into mismatched valleys, while spin-dark excitons appear when spins are misaligned. This mismatch not only blocks recombination but also extends their lifetimes.
Breakthrough With Ultrafast Imaging
The OIST team’s success lies in overcoming the invisibility of dark excitons. Using a proprietary, table-top TR-ARPES (time- and angle-resolved photoemission spectroscopy) microscope equipped with an extreme ultraviolet light source, they tracked excitons at femtosecond timescales. One femtosecond equals a quadrillionth of a second.
This setup allowed them, for the first time, to measure momentum, spin, and population of excitons simultaneously. Within a trillionth of a second after excitation, bright excitons transformed into momentum-dark excitons. Soon after, spin-dark excitons took over, surviving on nanosecond timescales—thousands of times longer than bright excitons.
Dr. Julien Madéo, a member of the unit, noted: “We’ve mapped how dark excitons form, persist, and carry valley information. This marks the birth of dark valleytronics as a field.”
Why Dark Valleytronics Matters
Today’s qubits, whether built from superconductors or trapped ions, demand extreme cooling near absolute zero and remain highly unstable. Dark excitons, by contrast, show natural resilience against heat and environmental noise. This could reduce reliance on massive cooling systems, lowering costs and energy consumption in future quantum devices.
Potential applications include:
- Quantum computing: Dark excitons may form a new class of qubits that operate under milder conditions.
- Secure communications: Valley-specific excitons could encode information in ways resistant to interception.
- Energy-efficient chips: Classical valleytronic devices might outperform traditional transistors by harnessing both charge and valley degrees of freedom.
Dr. Vivek Pareek, co-first author and now at the California Institute of Technology, emphasized the open questions: “We know dark excitons can preserve valley information. The challenge is learning how to read out and manipulate that information reliably. That’s where the next phase of research will go.”
The Bigger Picture
This discovery builds on a decade of progress in 2D materials research. Since graphene’s rise in 2004, TMDs such as molybdenum disulfide and tungsten diselenide have emerged as star candidates. Their hexagonal atomic symmetry gives rise to mirrored valleys, enabling selective control with circularly polarized light.
By using such light, researchers can target excitons in specific valleys. Bright excitons convert into dark ones, which then hold valley information for longer. Mapping this process was the missing piece, now provided by the OIST team.
Looking Ahead
The findings are not an immediate blueprint for commercial quantum computers, but they represent a crucial leap. By establishing how dark excitons evolve and survive, the work sets the stage for practical dark valleytronic devices.
Professor Dani summarized the outlook: “This is about laying foundations. With dark excitons, we see a pathway to robust, scalable quantum information systems that are less demanding and more stable. It’s an exciting frontier that merges ultrafast optics, quantum physics, and materials science.”
For the broader scientific community, the breakthrough signals a turning point: quantum technologies may not remain confined to giant cryogenic labs but could one day rely on the subtle valleys within atomically thin crystals. What was once hidden in the dark may now illuminate the future of information.
