We report on the coherent dynamics of excitonic wave packets observed in transient four-wave mixing experiments on strained InGaAs/GaAs quantum-well structures. The excitonic noneigenstates are created by using laser pulses shorter than the electron round trip time within the exciton orbital. A characteristic temporal modulation of the diffracted signal is observed, which resembles the coherent dynamics of an electron wave packet in a hydrogen atom expected for the case of a simultaneous excitation of all bound and the lowest continuum states.

Absorption measurements on single crystals of C60 fullerite have been performed as a function of temperature, hydrostatic pressure, and magnetic field in the optical visible range. Various structures observed in the fundamental absorption edge are assigned to excitonic transitions based on comparison with calculations, including many-electron and electron-hole interaction effects. The strength of the electron-phonon interaction is discussed in view of these results.The electronic structure of the fullerites has attracted much interest from both the theoretical as well as the experimental point of view. ' Theoretically, different approaches have been employed to study this material, including the local-density approximation (LDA) and the ab initio quasiparticle method. These methods provide the same band ordering for C60 crystals in the Em3 structure, which consists of highest occupied molecular orbitals (HOMO's) of H, symmetry and lowest unoccupied molecular orbitals (LUMO's) of T, "symmetry, but the respective band gap changes from 1.04 eV (in the LDA calculation) to 2.15 eV (in the GW quasiparticle picture ). Experimentally, direct and inverse photoemission experiments gave values for the gap closer to the GW approach prediction than to the LDA one. This has to be compared to results of microwave conductivity experiments providing excitation energies of about 1.85 eV whereas luminescence experiments' reported an emission band at around 1.7 eV. The discrepancy with the optical gap could be due to excitonic effects, but this remains to be demonstrated. In order to investigate the fundamental properties of the absorption edge of this semiconductor, we have performed systematic transmission measurements on fullerite single crystals as a function of temperature, hydrostatic pressure, and magnetic field. Calculations including electron-hole interactions have also been made, and comparison between experimental and theoretical data leads to the conclusion that the absorption edge is essentially made of states of a Frenkel excitonic nature, strongly coupled to vibrational modes. ' Performing reliable absorption measurements on C6O single crystals is difficult, due to their mechanical and chemical fragility. Any type of polishing results in damaging irreversibly the sample. Because fullerites are easily polluted by water vapor or any solvent, samples have been heated up to 130 C for at least 12 h before starting the experiments.The single crystal platelets, characterized by x-ray analysis, were grown by a vapor-transport technique, in an openended quartz tube subjected to a temperature gradient. C60 powder of very high purity was sublimated at one end at 600 C, and driven by a purified (02-free) argon flow to the growth area at a temperature ranging between 500 and 550 C. Such platelets with thicknesses ranging from a few p, m to about 400 p, m were selected and investigated with accurate spectrometry equipment.The temperature was monitored with a helium-Aow cryostat, also allowing lowtemperature investigations under mag...

The electron-bound hole luminescence of a GaAs-based heterojunction has been investigated as a function of magnetic fields at low temperatures. The study of the intensities originating from different Landau levels allows one to obtain precious information about the relaxation rates within these levels and provides values far above those predicted by standard theories. A new mechanism involving impurity-assisted phonon emission is proposed which gives good agreement with the experimental results. [S0031-9007(97)05136-3]