Field-driven attosecond charge dynamics in germanium

The possibility to excite and control charges in matter on ultrafast timescales is a key requisite to overcome the current limits of information transfer and data processing. The major route towards this milestone is based on the employment of short light pulses to manipulate the electro-optical properties of a solid. Nevertheless, the elusive physical mechanisms that unfold on extreme timescales are often complex and entangled, hindering their correct identification and possible exploitation. Here we investigate light-driven excitation in monocrystalline germanium by using attosecond transient reflection spectroscopy. We show that the complex regime established during light-matter interaction cannot be treated with simplified models but requires a detailed analysis in time and reciprocal space to address diverse phenomena such as tunnelling, band dressing, intra-band motion and multiphoton injection. Although single-photon absorption activates and develops earlier during excitation, two-photon processes, tunnelling and other field-driven phenomena reach their maximum effect soon after the peak of the pump pulse. Going against past observations, our results suggest that field-driven phenomena-namely intra-band transitions-can hinder charge injection, confirming that it is impossible to establish the next generation of petahertz information technology without a deep understanding of the diverse physical mechanism behind light-matter interaction.Attosecond transient reflectivity spectroscopy, in combination with extensive time-dependent density functional theory calculations, is used to study field-driven carrier injection in germanium in the time window of few femtoseconds around pulse overlap, paving a route towards achieving full optical control over charge carriers in semiconductors.