TY - JOUR A1 - Kim, Dai-Sik A1 - Shah, Jagdeep A1 - Cunningham, J. E. A1 - Damen, T. C. A1 - Schäfer, Wilfried A1 - Hartmann, Michael A1 - Schmitt-Rink, Stefan T1 - Giant excitonic resonance in time-resolved four-wave mixing in quantum wells JF - Physical Review Letters N2 - We report a strong (300:1) excitonic resonance in time-resolved (100 fs) four-wave mixing in GaAs quantum wells as a function of the incident photon energy and show that it results from the large difference in the dephasing rates of excitons and free carriers. We conclude that excitons dominate even when they make up only a small fraction of the excited states. These results, in conjuction with new results in exciton-phonon scattering, provide new insights into previous studies performed with spectrally broad ultrashort pulses. KW - Physics Y1 - 1992 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 68 SP - 1006 EP - Keine Angabe - No details ER - TY - JOUR A1 - Hartmann, Michael A1 - Schäfer, Wilfried T1 - Real Time Approach to Relaxation and Dephasing Processes in Semiconductors JF - Physica Status Solidi (B) N2 - Starting from the kinetic equations for the nonequilibrium Green's functions, a practical method is proposed which allows to solve the set of equations for the two-time propagators numerically. In contrast to the usual semiconductor Bloch equations with phenomenological damping relaxation and dephasing processes are treated microscopically for the case of coupling to a bath of LO-phonons. Beside a standard approximation for the treatment of LO-phonon scattering, no further approximations are performed. For the low-density limit we compare exact results with approximate treatments, as the Marcovian and non-Marcovian approximations resulting from a density matrix approach. First results concerning nonlinear optical properties are presented. KW - Physics KW - Real time approach KW - Semiconductors Y1 - 1992 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5980 VL - 173 IS - 1 SP - 165 EP - 176 ER -