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The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.

The ultrafast vibronic response of organic dye molecules in solution is studied in pump-probe experiments with 30 fs excitation pulses resonant to S 0 -S n transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S 0 -S 1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S 0 -S n and the S 0 -S 1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S 0 -S 1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S 1 states. The data demonstrate that the relaxation of S n states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the S n -S 1 relaxation in other molecules.

High-resolution amplitude and phase linear spectroscopy of high-quality bulk GaAs are reported. The detailed structure of the observed full complex transmission is consistently explained by polariton effects on the basis of microscopic calculations. The coupled equations for the excitonic polarization and the light field in the slab configuration are evaluated using appropriate boundary conditions for the electromagnetic field and the excitonic wave function without reference to additional boundary conditions for the macroscopic polarization.

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