Sub-cycle quantum motion in solids under strong terahertz fields

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

Terahertz-Driven Nonlinear Spin Response of Antiferromagnetic Nickel Oxide

Terahertz magnetic fields with amplitudes of up to 0.4 Tesla drive magnon resonances in nickel oxide while the induced dynamics is recorded by femtosecond magneto-optical probing. We observe distinct spin-mediated optical nonlinearities, including oscillations at the second harmonic of the 1 THz magnon mode. The latter originate from coherent dynamics of the longitudinal component of the antiferromagnetic order parameter, which are probed by magneto-optical effects of second order in the spin deflection. These observations allow us to dynamically disentangle electronic from lattice-related contributions to magnetic linear birefringence and dichroism-information so far only accessible by ultrafast THz spin control. The nonlinearities discussed here foreshadow physics that will become essential in future subcycle spin switching

Extremely non-perturbative terahertz nonlinearities in GaAs metamaterials

erahertz near fields of gold metamaterials resonant at a frequency of 0.88 THz allow us to enter an extreme limit of nonperturbative ultrafast terahertz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasistatic interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of 1019  cm−3. This process causes bright luminescence at energies up to 0.5 eV above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude-modulation of transmitted few-cycle terahertz transients. Our results pave the way towards highly nonlinear terahertz optics and optoelectronic nanocircuitry with subpicosecond switching times

Coherent cyclotron motion beyond Kohn’s theorem

In solids, the high density of charged particles makes many-body interactions a pervasive principle governing optics and electronics(1-12). However, Walter Kohn found in 1961 that the cyclotron resonance of Landau-quantized electrons is independent of the seemingly inescapable Coulomb interaction between electrons(2). Although this surprising theorem has been exploited in sophisticated quantum phenomena(13-15), such as ultrastrong light-matter coupling(16), superradiance(17) and coherent control(18), the complete absence of nonlinearities excludes many intriguing possibilities, such as quantum-logic protocols(19). Here, we use intense terahertz pulses to drive the cyclotron response of a two-dimensional electron gas beyond the protective limits of Kohn's theorem. Anharmonic Landau ladder climbing and distinct terahertz four-and six-wave mixing signatures occur, which our theory links to dynamic Coulomb effects between electrons and the positively charged ion background. This new context for Kohn's theorem unveils previously inaccessible internal degrees of freedom of Landau electrons, opening up new realms of ultrafast quantum control for electrons