Attosecond charge carrier dynamics in germanium.

The ultrafast optical response of a solid is mainly dictated by charges in the band structure. To achieve control over such properties, the photoexcitation process must be investigated with attosecond temporal resolution. We report the first observation of intra- and inter-band dynamics of charge carriers in undoped monocrystalline germanium. In this pump-probe experiment, an intense few-femtosecond infrared pulse injects charges from the valence to the conduction band. A quasi-isolated extreme-ultraviolet (XUV) attosecond pulse then induces transitions from the 3d core states, probing the valence and conduction band dynamics. Pump-induced variations in the reflectivity of the sample are measured as a function of the XUV photon energy and the pump-probe delay. Two complementary theoretical approaches allow for an $ab initio$ simulation of the experiment and to disentangle the various contributions to the ultrafast optical response. The analysis of the carrier population at high symmetry points in the photoexcited bands reveals the complex light-induced charge redistribution mechanism, in terms of real and virtual carriers, because of intra-band motion and inter-band transitions.