In this thesis, non-perturbative charge carrier dynamics in crystalline solids have been explored in a novel coherent high-field regime bridging nonlinear optics and sub-cycle lightwave electronics.
A newly developed high-field laser source delivers phase-stable, ultrashort waveforms in the far- to mid-infrared spectral regime with extremely high field strengths, which serve as a particularly well-defined contactless bias field for the study of non-perturbative charge carrier dynamics and nonlinear spin control in solids. To this end, a fundamentally new approach for ultrafast electric spin injection via tailored near-fields in a three-dimensional optical antenna has been introduced. First operational prototypes set the stage for time-resolved studies of spin-polarized tunnel injection into technologically relevant semiconductor heterostructures. Combining phase-locked waveforms featuring peak electric fields on the order of 100MV/cm with octave-spanning, 8-fs-long optical pulses facilitates lightwave electronics at multi-THz clock rates with sub-cycle time resolution: The strong transients have been employed to drive coherent interband excitation across the fundamental band gap in undoped gallium selenide. Simultaneously, the carriers are accelerated within their respective energy bands through the whole Brillouin zone, giving rise to dynamical Bloch oscillations. This highly anharmonic quantum motion results in the emission of a record-bandwidth, phase-stable high-order harmonic spectrum which covers more than 12 optical octaves.
Yet more importantly, the terahertz-driven high-harmonic emission has been temporally resolved in intensity and relative phase and in precise correlation with the driving waveform. A novel cross-correlation scheme with synchronized electro-optic sampling clocks the underlying dynamics with an accuracy of only a fraction of 1/20 of a single driving field cycle: The high-order harmonics are emitted as a unipolar pulse train of ultrashort and nearly unchirped bursts, which emerge exactly at the driving field crests. As explained by a quantum-mechanical many-body theory, these findings reveal a novel strong-field quantum interference between several, off-resonantly driven interband polarization pathways, including even electronic transitions well below the Fermi level.
A sophisticated examination of non-perturbative high-order harmonic generation along different crystallographic directions in gallium selenide has brought a surprising polarization behaviour of emitted harmonics to light. A phenomenological model based on the properties of frequency combs reconciles the spectrally, temporally and polarization-resolved findings and enables a comparison of the unraveled properties of solid-state high-order harmonic generation to straightforward symmetry arguments known from perturbative nonlinear optics
