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We experimentally and theoretically investigate injection currents generated by femtosecond single-color circularly-polarized laser pulses in (110)-oriented GaAs quantum wells. The current measurements are performed by detecting the emitted Terahertz radiation at room temperature. The microscopic theory is based on a 14 x 14 k • p band-structure calculation in combination with the multi-subband semiconductor Bloch equations. For symmetric GaAs quantum wells grown in (110) direction, an oscillatory dependence of the injection currents on the exciting photon energy is obtained. The results of the microscopic theory are in good agreement with the measurements.

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We present first experiments that visualize the spatial pattern formed by optically excited carriers in a photoconductive switch. The luminescence emitted from an electrically biased GaAs/AlGaAs multiple quantum well is spatially resolved with a CCD camera after optical excitation with femtosecond laser pulses. Information about the spatial electron and hole distributions is extracted from the measured luminescence patterns using a simple analytical model. Our analysis demonstrates that the field-driven electron–hole dynamics substantially varies across the excitation spot. This result can be traced back to the density dependence of screening combined with the lateral variation of the carrier density. Our results prove the importance of spatial effects for the carrier dynamics in photoconductive switches and open up new avenues for further optimization of these devices as, e.g., THz emitters.

We present an experimental study of the influence of frequency chirp in 20-fs optical pulses on broadband semiconductor continuum nonlinearities. Differential-transmission (DT) measurements were performed in which either the exciting pump pulse or the readout probe pulse, or both pump and probe, were chirped. We demonstrate that in certain chirp configurations the spectrally integrated DT is enhanced on an ultrafast time scale compared with measurements with unchirped pulses. Therefore pulse chirping has the potential to improve and optimize all-optical ultrafast switching. Spectrally resolved DT measurements explain these findings. Positive and negative DT contributions are observed in different spectral ranges. The spectral position and the magnitude of these contributions change in time. Proper chirping of the pulses optimizes the readout of the positive contributions and maximizes the spectrally integrated DT. A simple quantitative model confirms these considerations.

We investigate the nonlinear optical properties of low-temperature-grown (LT) GaAs. It is shown that the strength of the absorptive nonlinearity is weak in as-grown undoped LT-GaAs with a fast recovery time of the nonlinear absorption. The reason for this behavior is revealed by a quantitative study of the nonlinear optical properties of the most important defect transition from the neutral As antisite to the conduction band. Based on this analysis, we demonstrate two methods for the improvement of the ultrafast nonlinearity of LT-grown semiconductors: post-growth annealing or Be doping of LT-GaAs yields a material with a strong ultrafast nonlinear optical response.

We study the ultrashort dynamics and the dephasing processes of one-dimensional Lorentzian and Fano magnetoexcitons in GaAs, using temporally and spectrally resolved four-wave mixing. The dephasing of the Lorentzian magnetoexcitons is determined by quasi-particle scattering, dominated by exciton – exciton scattering at low temperatures. In contrast, the dephasing time of the Fano magnetoexcitons is intrinsically limited by the Fano coupling to the continuum. Quantum interference is observed if Fano magnetoexcitons contribute to the four-wave mixing signal. This effect is absent in the case of purely Lorentzian dynamics. We show that Lorentzian and Fano magnetoexciton resonances are coupled by Coulomb correlation and discuss the consequences of this coupling.

We study the dynamics of optical excitations in solid films of ladder-type poly(para-phenylene) (LPPP). A low-energy emission band is observed in the photoluminescence spectrum of solid films of this π-conjugated polymer, which is absent in LPPP solutions. We show that the low-energy luminescence can be attributed to aggregates between subunits of the polymer chains. Energy transfer into aggregate states strongly affects the dynamics of excitations in solid LPPP. We further discuss the influence of non-radiative defects on the dynamics.

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.

We have set up a laser system which simultaneously provides synchronized picosecond pulses in the visible and at a wavelength of 1.06 µm with a repetition rate of 76 MHz. The set-up consists of a dye laser synchronously pumped by the second harmonic of a cw mode-locked Nd:YAG laser and a fiber-grating compressor for the fundamental wavelength of the Nd;YAG laser. Crosscorrelation measurments reveal the time jitter between the two pulse trains to be less than 10 ps. As a first application we have performed non-degenerate transient grating experiments in semiconductors. The non-degenerate technique allows to use excitation energies well above the bandgap energy and to separate non-linear refractive index effects from photoinduced absorption or transmission changes.

The complex transfer function of a microwave probe is determined by a time-domain sampling method between 10 and 300 4;GHz. This technique allows the characterisation of non-coplanar high-frequency components at frequencies beyond the spectral range of standard electronics.

A technique that allows for the transfer of 3.3 (4.9) picosecond voltage pulses to electronic devices with coaxial (coplanar) transmission line geometry is presented. Using this method, electronic circuits can be characterised at frequencies well above 100 4;GHz.

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