A. Euteneuer

Electronically tunable external-cavity laser diode

We present a new concept for an electronically tunable diode laser. It is based on an external-cavity configuration with simultaneous feedback and intracavity spatial separation of the lasers spectral components. The electronical tunability is achieved by insertion of a liquid-crystal array as an electronically controlled aperture into the region of spatial separation of the spectral components. Wavelength tunability without mechanical movement over a range of 10 nm and two-color operation are demonstrated with a 670-nm laser diode.

Quantum beat spectroscopy on excitons in GaN

We present four wave mixing experiments on excitons in GaN. The four wave mixing transients exhibit strong modulations yielding a splitting of the A- and the B-exciton of 7.98 meV. In addition, we obtain a biexciton binding energy for A-excitons of 5.7 meV.

Confinement Dependence of Biexcitonic Binding Energies in Semiconductor Quantum Wells

We study the dependence of the biexciton binding energy on confinement in semiconductor quantum wells. A set of symmetrically strained (GaIn)As/Ga(PAs) quantum wells with different well depths and equal well widths is investigated by quantum beat spectroscopy based on transient degenerate four-wave mixing. The ratio of biexcitonic to excitonic binding energy increases with stronger confinement. This new experimental result is discussed in the framework of recent theoretical predictions and experimental results.

Analysis of disorder-induced dephasing

A number of experiments yielded a polarization-dependent decay of the time-integrated four-wave-mixing (TI-FWM). Often, the TI-FWM decays more rapidly for linear perpendicular than for linear parallel polarized incident pulses. It was suspected that this difference in the decay times is related to the inhomogeneous linewidth of the exciton, i.e. to disorder. On the other hand, it was demonstrated that the understanding of polarization-dependent FWM and pump-probe experiments following excitonic excitation requires the treatment of Coulomb correlations beyond the Hartree-Fock level. For not too strong disorder, these correlations still remain important and influence the nonlinear optical response. A meaningful analysis of disorder effects thus has to include correlations. We analyze the origin of the polarization-dependent disorder-induced dephasing in TI-FWM by performing microscopic model calculations which include both energetic disorder and Coulomb correlations. Correlations are treated exactly up to third-order in the field and energetic disorder is taken from random distribution functions over which the optical response is averaged.

Coherent Excitation Spectroscopy of Disordered Quantum-Well Structures

We study semiconductor quantum-well samples with different disorder strengths by coherent excitation spectroscopy with particular emphasis on disorder-mediated couplings of exciton subensembles. Our experiments show that there is no coherent coupling of excitons in different quantum islands and in a strongly inhomogeneously broadened sample. Latest results with polarization-resolved coherent excitation spectroscopy indicate a disorder-induced dephasing of biexcitons in the inhomogeneously broadened sample.

Investigation of Disordered Semiconductor Quantum Wells by Coherent Excitation Spectroscopy

First, we study coupling of excitons in different quantum-well islands by coherent excitation spectroscopy and find that excitons in different islands are not coherently coupled. Second, we investigate exciton and biexciton binding energies in inhomogeneously broadened (GaIn)As/Ga(PAs) multi quantum wells. A strong increase of the 1s–2s-splitting with increasing exciton localization is found, whereas the biexciton binding energy shows a very weak decrease with increasing localization.

Distribution of exciton binding-energies in disordered semiconductor quantum-wells

Summary form only given. Partly non-degenerate four-wave mixing is able to detect coherent coupling between excitonic resonances like e.g. light-hole and heavy-hole excitons or between an exciton and its continuum states. We apply an extension of this method, which we call selective coherent spectroscopy (SCS), to symmetrically strained (GaIn)As/Ga(PAs) multiple quantum-wells. The samples are excited simultaneously by a picosecond pulse with wavevector k/sub 1/ and a 100 fs pulse with wavevector k/sub 2/. The central wavelength of the picosecond pulse is scanned over the entire spectrum of the 100 fs pulse and the four-wave mixing signal in the direction 2k/sub 2/-k/sub 1/ is detected at a fixed energy. This scan provides an SCS spectrum.