When powerful infrared light pulses hit a solid material, they can trigger ultra-fast effects, so fast they happen in attoseconds (a billionth of a billionth of a second). These effects enable scientists to adjust the material’s optical and electrical properties reversibly, opening the door to the super-speed control of new functionalities in future devices.
However, to fully harness this potential, researchers require a profound understanding of how light interacts with the material’s electrons. That’s tricky, because the behavior involves complex, overlapping dynamics, like real and virtual particles moving between and within energy bands. These tangled processes make it challenging to decode how the material responds to the light field.
A new study, published in Nature Photonics and led by Politecnico di Milano, reveals a previously neglected but essential aspect: the contribution of virtual charges, charge carriers that exist only during interaction with light, yet profoundly influence the material’s response.
In a global collaboration, scientists from the University of Tsukuba, the Max Planck Institute, and Italy’s CNR-IFN investigated how monocrystalline diamonds respond to ultra-short bursts of light, lasting just a few attoseconds, the fastest timescale ever measured. Using a cutting-edge method called attosecond-scale transient reflection spectroscopy, they captured how the diamond’s electrons react in real time, offering a glimpse into quantum behavior at lightning speed.
By combining real-world experiments with advanced computer simulations, scientists have pinpointed a subtle effect known as virtual vertical transitions, a kind of quantum leap between energy bands that doesn’t involve actual particles moving. This discovery alters our understanding of how light interacts with solid materials, particularly under extreme conditions. Until now, such effects were thought to come only from the movement of real electric charges, but this shows that even virtual quantum pathways can play a decisive role.
Matteo Lucchini, professor at the Department of Physics, senior author of the study, and associate at CNR-Ifn said, “Our work shows that virtual carrier excitation, which develops in a few billionths of a second, is indispensable to predict the rapid optical response in solids correctly.”
“These results mark a key step in the development of ultra-fast technologies in electronics,” adds Rocío Borrego Varillas, researcher at CNR-IFN.
This breakthrough brings us closer to building ultra-fast optical devices, such as switches and modulators, that can operate at petahertz speeds —thousands of times faster than today’s electronics. To make that leap, scientists need to understand not just how real electric charges move, but also how virtual charges behave in the quantum realm. This study shows both are key to unlocking the next generation of lightning-speed technology.
Journal Reference:
- Gian Luca Dolso, Shunsuke A. Sato, Giacomo Inzani, Nicola Di Palo, Bruno Moio, Rocío Borrego-Varillas, Mauro Nisoli, Matteo Lucchini. Attosecond virtual charge dynamics in dielectrics. Nature Photonics, 2025; 19 (9): 999 DOI: 10.1038/s41566-025-01700-6