Phase-locked electromagnetic transients in the terahertz (THz) spectral domain have become a unique contact-free probe
of the femtosecond dynamics of low-energy excitations in semiconductors. Access to their nonlinear response, however,
has been limited by a shortage of sufficiently intense THz emitters. Here we introduce a novel high-field source for THz
transients featuring peak amplitudes of up to 108 MV/cm. This facility allows us to explore the non-perturbative
response of semiconductors to intense fields tailored with sub-cycle precision. In a first experiment intense transients
drive Rabi-oscillations between excitonic states in Cu2O, implying exciting perspectives for future THz quantum optics.
At electric fields beyond 10 MV/cm, we observe the breakdown of the power expansion of the nonlinear polarization in
bulk semiconductors. Furthermore, we employ the intense magnetic field components of our transients to coherently
control spin waves in antiferromagnetically ordered solids. Finally, intersubband cavity polaritons in semiconductor
microcavities are exploited to push light-matter coupling to an unprecedented ultrastrong and sub-cycle regime.
The exciton binding energy in GaAs-based quantum-well (QW) structures is in the range of ~10 meV, which falls in the
THz regime. We have conducted a time-resolved study to observe the resonant interactions of strong narrowband THz
pulses with coherent excitons in QWs, where the THz radiation is tuned near the 1s-2p intraexciton transition and the
THz pulse duration (~3 ps) is comparable with the exciton dephasing time. The system of interest contains ten highquality
12-nm-wide GaAs QWs separated by 16-nm-wide Al 0.3Ga 0.7As barriers. The strong and narrowband THz pulses
were generated by two linearly-chirped and orthogonally-polarized optical pulses via type-II difference-frequency
generation in a 1-mm ZnTe crystal. The peak amplitude of the THz fields reached ~10 kV/cm. The strong THz fields
coupled the 1s and 2p exciton states, producing nonstationary dressed states. An ultrafast optical probe was employed to
observe the time-evolution of the dressed states of the 1s exciton level. The experimental observations show clear signs
of strong coupling between THz light and excitons and subsequent ultrafast dynamics of excitonic quantum coherence.
As a consequence, we demonstrate frequency conversion between optical and THz pulses induced by nonlinear
interactions of the THz pulses with excitons in semiconductor QWs.
The optical response of semiconductor quantum wells is investigated theoretically to explain nonlinear transients
generated via intense terahertz (THz) fields. A microscopic description of THz-induced interaction processes is
developed while several numerical examples are presented to illustrate properties in a typical THz-pump and
optical-probe configuration. The results identify signatures of the ac-Stark effect, ponderomotive contributions,
and extreme-nonlinear dynamics.
The interaction of semiconductors with terahertz radiation is discussed. The main ingredients of a consistent
microscopic description are presented. The theory is evaluated to analyze direct terahertz emission features of