Attosecond virtual charge dynamics in dielectrics

The interaction of intense infrared pulses with a solid target can initiate light-field-driven phenomena that enable the reversible manipulation of their electro-optical properties on an attosecond timescale. This interaction regime therefore offers a unique opportunity to induce and control new functionalities with very high speed. However, the efficient exploitation of coherent light–matter states for future applications requires a detailed understanding of the underlying physical processes. This task is complicated by the complex and intertwined nature of inter- and intraband dynamics of real and virtual carriers underlying field-driven phenomena in solids. Here we used attosecond transient reflection spectroscopy to investigate ultrafast virtual electron dynamics in a prototype dielectric (monocrystalline diamond) over a broad photon energy range not previously accessed. Independent calibration of the pump–probe delay axis allowed direct comparison with numerical calculations, revealing that virtual interband transitions affect the timing and adiabaticity of the crystal response, even in a regime believed to be dominated by intraband motion. By demonstrating that virtual interband transitions are indispensable for an accurate description of strong-field-induced phenomena in solids, our results constitute a relevant step towards understanding transient nonlinear optical processes, a cornerstone for the future development of information processing and petahertz electronics.