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, laying the foundation for new breakthroughs in both classical and quantum information technologies. Their findings have been published in Nature Communications . Professor Keshav Dani, head of the unit, highlights the significance: "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 PhD 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.
Defining landscapes of energy with bright and dark excitons
Over the past decade, progress has been made in the development of a class of atomically thin semiconducting materials known as TMDs (transition metal dichalcogenides). As with all semiconductors, atoms in TMDs are aligned in a crystal lattice, which confines electrons to a specific level (or band) of energy, such as the valence band. When exposed to light, the negatively charged electrons are excited to a higher energy state – the conduction band – leaving behind a positively charged hole in the valence band. The electrons and holes are bound together by electrostatic attraction, forming hydrogen-like quasiparticles called excitons. If certain quantum properties of the electron and hole match, i.e. they have the same spin configuration and they inhabit the same 'valley' in momentum space (the energy minima that electrons and holes can occupy in the atomic crystal structure) the two recombine within a picosecond (1ps = 10−12 second), emitting light in the process. These are 'bright' excitons.
However, if the quantum properties of the electron and hole do not match up, the electron and hole are forbidden from recombining on their own and do not emit light. These are characterized as 'dark' excitons. "There are two 'species' of dark excitons," explains Dr. David Bacon, co-first author who is now at University College London, "momentum-dark and spin-dark, depending on where the properties of electron and hole are in conflict. The mismatch in properties not only prevents immediate recombination, allowing them to exist up to several nanoseconds (1ns = 10−9 second – a much more useful timescale), but also makes dark excitons more isolated from environmental interactions."
"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. However, bright excitons rapidly turn into numerous dark excitons that can potentially preserve the valley information. Which species of dark excitons are involved and to what degree they can sustain the valley information is unclear, but this is a key step in the pursuit of valleytronic applications," explains Dr. Vivek Pareek, co-first author and OIST graduate who is now a Presidential Postdoctoral Fellow at the California Institute of Technology.
Observing electrons at the femtosecond scale
Using the world-leading TR-ARPES (time- and angle resolved photoemission spectroscopy) setup at OIST, which includes a proprietary, table-top XUV (extreme ultraviolet) source, the team has managed to track the characteristics of all excitons after the creation of bright excitons in a specific valley in a TMD semiconductor over time by simultaneously quantifying momentum, spin state, and population levels of electrons and holes – these properties have never been simultaneously quantified before.
Their findings show that within a picosecond, some bright excitons are scattered by phonons (quantized crystal lattice vibrations) into different momentum valleys, rendering them momentum-dark. Later, spin-dark excitons dominate, where electrons have flipped spin within the same valley, persisting on nanosecond scales.
With this, the team has overcome the fundamental challenge of how to access and track dark excitons, laying the foundation for dark valleytronics as a field. Dr. Julien Madéo of the unit summarizes: "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."
