Ultrafast transport of electrons in semiconductors lies at the heart of high-speed electronics, electro-optics and
fundamental solid-state physics. Intense phase-locked terahertz (THz) pulses at photon energies far below electronic
interband resonances may serve as a precisely adjustable alternating bias, strongly exceeding d.c. breakdown voltages.
Here, we exploit the near-field enhancement in gold metamaterial structures on undoped bulk GaAs, driven by few-cycle
THz transients centered at 1 THz, to bias the semiconductor substrate with field amplitudes exceeding 12 MV/cm. Such
fields correspond to a potential drop of the bandgap energy over a distance of only two unit cells. In this extremely
off-resonant scenario characterized by a Keldysh parameter of γK ≈ 0.02, massive interband Zener tunneling injects a
sizeable carrier density exceeding 10<sup>19</sup> cm<sup>-3</sup>, and strong photoluminescence results. At a center frequency of 30 THz,
THz transients with peak fields of 72 MV/cm analogously excite carriers in a bulk, semiconducting GaSe crystal,
without metamaterial. Here, in contrast, we are able to drive coherent interband polarization and furthermore dynamical
Bloch oscillations of electrons in the conduction band, on femtosecond time scales. The dynamics entail the generation
of absolutely phase-stable high-harmonic transients containing spectral components up to the 22<sup>nd</sup> order of the
fundamental frequency, spanning 12.7 optical octaves throughout the entire terahertz-to-visible domain between 0.1 and
675 THz. Our experiments establish a new field of light-wave electronics exploring coherent charge transport at optical
clock rates and bring picosecond-scale electric circuitry at the interface of THz optics and electronics into reach.
A microscopic theory for the terahertz response of a semiconductor quantum well under coherent conditions is
presented. It is shown that excitonic effects influence the intersubband absorption under certain conditions. For
high-quality samples, one should be able to resolve both band-to-band and excitonic intersubband transitions
in an terahertz absorption measurement. Due to the competition of intersubband transitions and classical field-induced
carrier accelerations, an unexpected Fano feature is observed in the terahertz spectra. This result is in
excellent agreement with recent measurements.
The terahertz response of a two-dimensional electron gas (2DEG) is investigated theoretically. The developed
microscopic model shows that the terahertz absorption sensitively detects Coulomb-induced many-body correlations
within the entire 2DEG system. In particular, the resulting response follows from a nontrivial competition
between the ponderomotive and the Coulomb-correlation contributions. The result is in good agreement with
recent experiments while the response cannot be explained with a simple Drude-model analysis.