S. Hunsche

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

We use femtosecond time-resolved transmission spectroscopy to study the density and time dependence of transient absorption changes of CdSe nanocrystals. Our data show pronounced absorption saturation up to complete bleaching of the lowest optical transition. At high carrier density the nonlinear spectra show several peaks that can be related to the two lowest quantized electron states. Thetime dependence of the carrier-induced absorption changes indicates an ultrafast relaxation process within the strongly broadened absorption lines.

Time-resolved study of intervalence band thermalization in a GaAs quantum well

The thermalization of optically excited cold holes in a GaAs quantum well is investigated by femtosecond two‐color pump–probe measurements. Clear evidence is found for scattering from heavy‐holes into the lowest light‐hole band due to LO‐phonon absorption. We obtain firm data on scattering times which depend strongly on lattice temperature. They vary from 230 fs at room temperature to 900 fs at T=105 K. The experimental data are well reproduced by numerical calculations.

Details of coherent phonon motion in tellurium

We study impulsively excited coherent phonons in tellurium at various lattice temperatures and excitation densities. From a comparison with temperature-dependent reflectivity and X-ray scattering data, we infer the direction and estimate the amplitude of the coherent ionic motion. In addition, we show quantitatively that electronic weakening of the crystal forms the dominant contribution to a density-dependent red-shift of the phonon frequency.

Nonlinear optics in semiconductors and multiple quantum wells

The confinement of electrons and holes in quantum wells allows the fabrication of structures exhibiting enhanced nonlinear optical effects associated with excitonic transitions. By applying femtosecond spectroscopy, it has become possible to study the coherent and incoherent properties of these systems with high temporal resolution, thus gaining a detailed insight into the processes involved in the interactions of photons, electrons and the crystal lattice on a subpicosecond time scale. We present investigations of the nonlinear optical behaviour governed by the relaxation and thermalization dynamics in multiple quantum wells and in low-temperature grown GaAs. Experiments with high temporal and energetic resolution provide a detailed insight into the inter-valence-band thermalization in quantum wells and the influence of shallow bound defect levels in low-temperature grown GaAs.