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Understanding what happens inside a material when it is hit by ultrashort light pulses is one of the great challenges of matter physics and modern photonics. A new study in Nature Photonics and led by Politecnico di Milano reveals a hitherto neglected but essential aspect, precisely the contribution of virtual charges, charge carriers that exist only during interaction with light, but which profoundly influence the material's response.

The research, conducted in partnership with the University of Tsukuba, the Max Planck Institute for the Structure and Dynamics of Matter, and the Institute of Photonics and Nanotechnology (CNR-IFN) investigated the behavior of monocrystalline diamonds subjected to light pulses lasting a few attoseconds (billionths of a billionth of a second), using an advanced technique called attosecond-scale transient reflection spectroscopy.

By comparing experimental data with state-of-the-art numerical simulations, researchers were able to isolate the effect of so-called virtual vertical transitions between the electronic bands of the material. Such an outcome changes the perspective on how light interacts with solids, even in extreme conditions hitherto attributed only to the movement of actual charges.

"Our work shows that virtual carrier excitation, which develops in a few billionths of a billionth of a second, are indispensable to correctly predict the rapid optical response in solids," said Matteo Lucchini, professor at the Department of Â鶹ÒùÔºics, senior author of the study, and associate at CNR-IFN.

"These results mark a key step in the development of ultra-fast technologies in electronics," adds Rocío Borrego Varillas, researcher at CNR-IFN.

The progress achieved offers new insights into the creation of ultra-fast optical devices, such as switches and modulators capable of operating at petahertz frequencies, a thousand times faster than current electronic devices. This requires a deep understanding of both the behavior of actual charges, and of virtual charges, as demonstrated by this study.