No abstract

Time-resolved four-wave-mixing experiments are usually interpreted in terms of noninteracting two-level systems in order to obtain information on the polarization dephasing time T2. Recent experiments involving excitonic resonances in semiconductor quantum wells (including results presented in this paper) show striking qualitative deviations from this simple picture. In particular, an exponential tail is observed at low excitation for negative time delays. At high excitation, the four-wave-mixing signal is found to evolve into two distinct temporal maxima. We demonstrate that the microscopic origin of this time dependence can be understood in terms of coherent exciton-exciton interactions. We show in fact that this behavior is more general and should be seen in numerous dense media where strong nonlinear interactions of polarizations occur. In addition to presenting rigorous numerical results, we analyze two simple situations in which such interactions exist: dielectric media with local-field effects and the anharmonic oscillator. We derive analytical expressions for their time-dependent four-wave-mixing response and discuss the physical origin of these new nonlinear-optical effects.

We present the first observation of the spatial dynamics of a wavepacket in a solid. Using an ultrashort laser pulse, we create an excitonic wavepacket in one well of an asymmetric double quantum well structure. The oscillation of this wavepacket from one well to another and back is traced by time- resolved pump-probe spectroscopy as well as time- resolved degenerate fourwave-mixing. We present results for two GaAs/AlGaAs double quantum wells with oscillation periods of about 1.3ps and 800fs, respectively. The experimental observations are compared with a theory that shows that the two experimental techniques give complementary information about the relaxation dynamics of the coupled system. The analysis of the experiment explains the strong damping of the oscillations by the fast thermalization between the delocalized states. We present the first study of the dynamics of an extended electronic wave packet in a solid. The wave packet is created in a GaAs/AlGaAs double-quantum-well stru cture by ultrashort pulse excitation. We observe the oscillatory motion of the wave packet between the two wells by using time-resolved degenerate four-wave-mixing and pump-and-probe spectroscopy

We present investigations of fs time resolved coherent wave mixing under high magnetic field. Our experiments reveal a new regime at high magnetic field and low excitation density dominated by the Coulomb interaction. This regime is inconsistent with the semiconductor Bloch equations. A model which includes exciton-exciton correlation successfully describes many features of this regime.[S0031-9007(97)