We investigate the ultrafast photoinjection process initiated by a few-femtosecond optical pulse in monocrystalline undoped germanium with attosecond transient reflectivity spectroscopy. By comparison with theoretical calculations, we decouple several distinct but concurring physical phenomena that are found to exhibit different timing within the pump envelope. As a result of their complex interplay, we found that intra-band motion hinders charge injection, in contrast with what has previously been observed in other semiconductors.
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.
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