Phase sensitive observation of coherent emitted signals from excitons in quantum well structure

D. S. Chemla, J.‐Y. Bigot, Mary‐Ann Mycek +2 more

We investigate the temporal evolution of the instantaneous frequency of ultrashort-pulse four-wave mixing in GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of excitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission or quantum beats. In the latter case we determine that the beat \ensuremath{\pi}-phase shift is very fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism takes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximations in the theory: the statical treatment of screening, and the Lorentzian description of dephasing. In both cases, this indicates the need for theoretical refinements requiring a microscopic description of Coulomb scattering and dephasing processes in a non-Markovian theory.

Duncan G. Steel, Steven T. Cundiff, H. Wang

Kazuhiro Komori, Gary R. Hayes, B. Deveaud +3 more

We have investigated the ultrafast coherent dynamics of exciton-polaritons and excitons in semiconductor quantum nanostructures, including InGaAs/AlGaAs multi quantum well Bragg structures and AlGaAs/GaAs crescent shaped quantum wire samples, by using a phase-locked spectroscopic technique. In the multi quantum well Bragg sample, a two-stage decay due to the superradiant exciton-polariton mode is clearly observed at the resonant condition. In the crescent shaped quantum wire sample, the highly sensitive coherent control measurement enables us to observe the dephasing characteristics and a very fast dephasing time T-2 of 200 to 300 Is with power dependence was observed.

Martín Koch, J. Feldmann, G. von Plessen +8 more

T. Stroucken, A. Knorr, P. Thomas +1 more

The optical response of radiatively coupled semiconductor multiple-quantum-well structures is investigated theoretically. It is shown that the transverse optical field leads to a coupling of exciton states within each well which causes a radiative decay and a mixing of excitonic resonances. Simultaneously, the field-induced long-ranged coupling between different wells leads to collective effects that are very sensitive to the detailed geometry of the structure. For a quantum-well spacing equal to an integer multiple of half the optical wavelength inside the medium, it is predicted that the collective effects cause a stimulated decay of electronic excitations that should be observable in either transmission or reflection geometry. On the other hand, in a quarter wavelength structure, the light-induced coupling causes an interwell energy transport and a splitting of the excitonic resonances, that should be observable as quantum beats in the time-resolved transmitted or reflected signal.

J. Kühl, Matthias Hübner, T. Stroucken +5 more