Understanding the ultrafast mechanisms that occur within materials when they are hit by extremely short light pulses is one of the great challenges of contemporary physics. A new study, led by the Politecnico di Milano and published in Nature Photonics, reveals a crucial yet previously underexplored aspect: the role of virtual charges in insulating materials.

The research, carried out at the Attosecond Research Center (ARC) of Politecnico di Milano and conducted in collaboration with the University of Tsukuba, the Max Planck Institute for the Structure and Dynamics of Matter, and the CNR-IFN, employed light pulses lasting only a few attoseconds (billionths of a billionth of a second) to investigate the behavior of single-crystal diamond.

Using an advanced technique called transient reflection spectroscopy, combined with cutting-edge numerical simulations, the researchers were able to isolate the effect of so-called virtual vertical transitions between electronic bands. These phenomena, associated with charge carriers that exist only during interaction with light, proved fundamental to understanding the optical response of materials under extreme conditions.