Defects in a Crystal Trap Light in Femtoseconds express
An ultra-thin crystal made of tungsten and sulfur atoms resembles a perfect parking lot: every spot is taken, everything is neat. But sometimes an atom is missing — leaving a defect pit. If an exciton (a pair consisting of an electron and a 'hole' that carries light energy) falls into such a pit, it can get stuck and emit light differently than in a pristine area.
Until now, this trapping was only inferred from indirect signs: the signal from excitons at defects was too weak. Now, scientists took a crystal with a large number of such pits and shone ultra-short laser pulses on it. It turned out that excitons fall into the pits in 300 femtoseconds, as if a car found a free parking spot in a billionth of a second. Moreover, excitons at defects have shorter lifetimes than in the pristine crystal, so some of them manage to 'jump' back. Furthermore, if the laser has lower energy, excitons at defects can receive an extra kick and escape, like a car driving out of a pit onto a smooth road.
Such 'pits' in two-dimensional crystals are already used in prototypes of quantum computers and ultra-sensitive sensors. Understanding how excitons enter and exit these pits will help make such devices faster and more reliable.
Original abstract
as written by the authors arXiv:2603.18230
Defects in two-dimensional transition metal dichalcogenides (TMDCs) broadly affect their optical and electronic properties. Directly capturing the ultrafast processes of exciton trapping and defect-bound exciton formation is crucial for understanding and advancing defect-mediated optoelectronics and quantum technologies. However, the weak transient optical absorption of defect-bound excitons has limited their experimental observation to date. Here, we report the direct observation of the ultrafast dynamics of defect-bound excitons in monolayer WS$$_2$$ crystals with a high density of mono-sulfur vacancies (V$$_S$$) and W-site defect complexes (S$$_W$$V$$_S$$) resulting from synthesis by alkali metal halide-assisted chemical vapor deposition. The dynamics of excitons bound to these defects, along with their coherent interactions with free excitons, are elucidated using ultrafast optical spectroscopy. Using above band-edge photoexcitation, we find that both free and defect-bound excitons simultaneously form within 300 fs from hot carrier relaxation. The defect-bound excitons exhibit shorter lifetimes than free excitons, leading to a population difference of the corresponding excitonic states and free exciton trapping within a 1--100 ps window. Band-edge photoexcitation of free and defect-bound exciton states reveals ultrafast interconversion within ~150 fs (comparable to our temporal resolution), indicating possible coherent coupling between these states. We further demonstrate efficient up-conversion of defect-bound excitons to free excitons with photon energies up to ~300 meV below the free exciton resonance. These findings provide insights into the ultrafast dynamics of defect-bound excitons in TMDCs and their coupling with free excitons, which are relevant to defect-engineered optoelectronic, quantum photonic, and valleytronic applications.
A femtosecond is one millionth of a billionth of a second. In 300 femtoseconds, light travels only 0.09 millimeters — less than the thickness of a human hair.