Physical review. B, Condensed matter

Four-wave-mixing studies of quantum-well excitons in a magnetic field

O. Carmel, Israel Bar-Joseph

Published Mar 15, 1993• 27 citations

Abstract

We report the results of four-wave-mixing experiments in GaAs quantum wells in a magnetic field. We find that the time evolution of the excitonic signal is strongly modified when a magnetic field is applied. Quantum beats, rotation of the echo polarization, recovery of the echo for cross-polarized exciting beams, and an increase in the dephasing times are observed.

DOI: 10.1103/physrevb.47.7606

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Physical review. B, Condensed matter

Four-wave-mixing studies of quantum-well excitons in a magnetic field

O. Carmel, Israel Bar-Joseph

Published Mar 15, 1993• 27 citations

Publisher

Abstract

DOI: 10.1103/physrevb.47.7606

Related papers

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Influence of exciton-exciton interactions on the coherent optical response in GaAs quantum wells

K. Bott, O. Heller, D. Bennhardt +7 more

Physical review. B, Condensed matterDec 15, 1993192 citations

We have investigated, by fully-time-resolved degenerate four-wave mixing (FWM), excitonic coherence phenomena in single

\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}

quantum wells which are known to exhibit localization. A magnetic field

(0--12\phantom{\rule{0.3em}{0ex}}\mathrm{T})

was applied in Faraday geometry as a means of altering the nature of the excitonic transitions in a controlled manner. When the system is excited by a sequence of pulses tuned in resonance with the heavy-hole excitons, the predominant behavior observed at positive interpulse delays is beating in the FWM emission due to polarization interference between uncoupled excitons localized by monolayer islands. For negative interpulse delay the variation of beat phase with delay changes sign as the magnetic field is increased. In order to understand this feature, we modeled a coupled three-level system and included local field effects as a simple phenomenological approach to describing many-body terms. Comparison with experiments shows that with a larger magnetic field, the contribution to the signal from the coupled system reduces while that from two noninteracting two-level systems increases. We attribute this to the increased confinement of the excitons with field, leading to reduced wavefunction overlap, and hence reduced coupling.

Recent transient four-wave-mixing experiments in the two-pulse photon-echo configuration performed on GaAs quantum wells have yielded the surprising result that in some samples the

{\mathit{T}}_{2}

time obtained from the polarization decay can be shorter for cross-polarized input fields than for parallel input fields. A concomitant change from photon-echo to free-induction-decay behavior has also been observed. A phenomenological model is presented which explains these observations by means of a disorder-induced coupling of the ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ exciton transitions which is inhomogeneous in coupling strength. Such coupling also leads to a remarkable change in the light-hole--heavy-hole exciton quantum beat phase as a function of pulse delay for a critical relative orientation of the polarization of the incident pulses. Experimental observations of this effect are presented to support the validity of the model.

We investigate the properties of coherent exciton states in GaAs/

{\mathrm{Al}}_{\mathit{x}}

${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells using the four-wave-mixing technique with subpicosecond time resolution. In the first part of this work, we show that many-body interactions in a solid manifest themselves in a line shape of the four-wave-mixing signal that is substantially different from the predictions of the generally used simple two-level model. In particular, the coherent interaction of the quasiparticles leads to a signal at negative delay times, in agreement with recent theoretical predictions. In the second part, we report in detail about experiments where energetically close exciton transitions are excited coherently with a short laser pulse. The quantum or polarization interference leads to the observation of quantum beats in the decay of the diffracted signal. These results demonstrate that quantum-beat spectroscopy can be applied to study intrinsic excitations in solids.