The nonlinear optical response of quantum well excitons is investigated experimentally using polarization resolved four wave mixing, optical-pump optical-probe, and optical-pump Terahertz-probe spectroscopy. The four-wave mixing data reveal clear signatures of coherent biexcitons which concur with straight-forward polarization selection rules at the Γ point. The type-I samples show the well-established time-domain beating signatures in the transients as well as the corresponding spectral signatures clearly. The latter are also present in type-II samples; however, the smaller exciton and biexciton binding energies in these structures infer longer beating times which, in turn, are accompanied by faster dephasing of the type-II exciton coherences. Furthermore, the THz absorption following spectrally narrow, picosecond excitation at energies in the vicinity of the 1s exciton resonance are discussed. Here, the optical signatures yield the well-established redshifts and blueshifts for the appropriate polarization geometries in type-I quantum well samples also termed “AC Stark Effect”. The THz probe reveals intriguing spectral features which can be ascribed to coherent negative absorption following an excitation into a virtual state for an excitation below the 1s exciton resonance. Furthermore, the scattering and ionization of excitons is discussed for several excitation geometries yielding control rules for elastic and inelastic quasiparticle collisions.
The nonlinear optical response of quantum well excitons excited by optical fields is analyzed by numerical solutions of the semiconductor Bloch equations. Differential absorption spectra are computed for resonant pumping at the exciton resonance and the dependence of the absorption changes on the polarization directions of the pump and probe pulses is investigated. Coherent biexcitonic many-body correlations are included in our approach up to third-order in the optical fields. Results are presented for spatially-direct type-I and spatiallyindirect type-II quantum well systems. Due to the spatial inhomogeneity, in type-II structures a finite coupling between excitons of opposite spins exists already on the Hartree-Fock level and contributes to the absorption changes for the case of opposite circularly polarized pump and probe pulses.
The semiconductor Bloch equations provide a very versatile and microscopic approach to compute and analyze optical and electronic properties of semiconductors. Here, we focus on high harmonic generation arising from the driving of crystalline systems with very strong optical and Terahertz pulses. Implementing a proper gauge allows us to solve the semiconductor Bloch equations in the length gauge. The length gauge turns out to be advantageous since it converges for a smaller number of bands than the velocity gauge and, in addition, enables a unique distinction between inter- and intraband contributions. Besides odd harmonics polarized parallel to the incoming field our approach also describes even harmonics which originate from the Berry curvature and are polarized perpendicular to the incident field. Next, we demonstrate that the electron and hole collision/recombination dynamics is mainly responsible for the anisotropy of the interband high harmonic generation. Our findings connect the electron/hole backward scattering to van Hove singularities and the forward scattering with critical lines in the band structure and we show that this dynamics can be controlled by properly designed two-color fields. Furthermore, we consider excitonic effects within a two-band model and show that they can strongly enhance the high harmonic emission intensity for suitably chosen incident pulses. When an odd-order harmonic corresponds to the energy of the 1s exciton this harmonic is several orders of magnitude larger than the emission from non-interacting electrons and holes.
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
semiconductor systems.
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