When we think of light striking a material, it’s easy to imagine photons simply bouncing off its surface or being absorbed by its atoms. But what actually happens inside matter when it is hit by unimaginably short bursts of light—pulses lasting mere attoseconds, a billionth of a billionth of a second—is far more complex and mysterious.
A groundbreaking study published in Nature Photonics has opened a new chapter in our understanding of this phenomenon. Led by researchers from the Politecnico di Milano, in collaboration with the University of Tsukuba, the Max Planck Institute for the Structure and Dynamics of Matter, and Italy’s Institute of Photonics and Nanotechnology (CNR-IFN), the work uncovers an overlooked but essential aspect of light–matter interaction: the role of virtual charges.
These are not particles that exist in the ordinary sense. Instead, they are fleeting, almost ghostlike charge carriers that appear only during the interaction with light—yet their effect on the material’s behavior is profound.
The Experiment That Changed Perspectives
To peer into this hidden world, the team turned to monocrystalline diamond, one of the purest and most ordered solids known. By bombarding the diamond with attosecond-scale pulses of light and measuring its response through a technique called transient reflection spectroscopy, they were able to capture processes unfolding at timescales previously beyond reach.
This wasn’t just about recording signals. The researchers compared their results with advanced numerical simulations, stripping away layers of complexity until they could isolate the contribution of these elusive “virtual vertical transitions” between the material’s electronic bands.
What they found was astonishing. Even though these virtual charges vanish as soon as the light pulse is gone, during those fleeting instants they play a decisive role in shaping how the material responds. In fact, without accounting for their influence, predictions about ultrafast optical behavior in solids fall short.
The Power of the Invisible
Traditionally, physicists have explained the optical response of materials by looking at the movement of real charges—electrons that jump between energy states or flow as currents. The new study shows that this picture is incomplete. Virtual carriers, born and extinguished within billionths of a billionth of a second, must also be included.
As Matteo Lucchini, professor at the Politecnico di Milano and senior author of the study, explains: “Our work shows that virtual carrier excitation, which develops in a few billionths of a billionth of a second, is indispensable to correctly predict the rapid optical response in solids.”
In other words, these ghostlike carriers may be temporary, but their impact is permanent in the sense that they redefine how we think about light and matter.
Toward Petahertz Technologies
This breakthrough is more than an intellectual triumph—it carries enormous implications for technology. Modern electronics operate at gigahertz (billions of cycles per second). Cutting-edge optical devices push into the terahertz (trillions of cycles per second). But the phenomena uncovered here hint at a future of petahertz frequencies—a thousand times faster still.
Imagine switches, modulators, and processors that operate at such staggering speeds. These devices would transform computing, telecommunications, and even our understanding of what information processing can mean. But to design them, engineers must grasp both the behavior of actual charges and the fleeting dynamics of virtual ones.
As Rocío Borrego Varillas, researcher at CNR-IFN and co-author of the study, notes: “These results mark a key step in the development of ultra-fast technologies in electronics.”
The Dawn of a New Photonic Era
Physics is often about peeling back layers of reality that were previously invisible. With this discovery, a veil has been lifted on the secret role of virtual charges in ultrafast processes. The insight not only enriches our fundamental understanding of matter but also sets the stage for technologies that once belonged to science fiction.
Every leap in human progress has come from seeing the world in a new way: Newton’s laws of motion, Maxwell’s unification of electricity and magnetism, Einstein’s relativity, and the birth of quantum mechanics. The revelation of virtual charges is another step in that tradition—a reminder that even in the blink of an eye, or rather in a fraction of a blink measured in attoseconds, whole new universes of behavior await discovery.
The future of photonics, and perhaps the future of technology itself, may rest on our ability to harness the power of what was once thought unreal: the dance of virtual charges inside matter, guiding us into a world where speed approaches the fundamental limits of nature.
More information: Gian Luca Dolso et al, Attosecond virtual charge dynamics in dielectrics, Nature Photonics (2025). DOI: 10.1038/s41566-025-01700-6
