Photoexcitation in germanium probed by attosecond transient reflectivity spectroscopy

Light fields can sculpt the optical and electronic properties of solids by injecting charge carriers from the valence to the conduction band. The photoinjection process typically unfolds on time scales shorter than the period of the driving radiation, thus being - for visible and near-infrared light - below one femtosecond. Despite being pervasive, only a few experiments investigated the photoinduced charge excitation produced by an intense, ultrashort laser pulse with attosecond temporal resolution. Here we used attosecond transient reflection spectroscopy in combination with a two-fold, advanced theoretical approach to demonstrate that there is not a single photoexcitation mechanism responsible for charge injection in germanium. Instead, it stems from multi-photon absorption and band dressing, while intra-band motion has a hindering effect.