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physica status solidi (b)

Coherent Excitation of Magnetoexciton Wavepackets

Steven T. Cundiff, M. Koch, Jagdeep Shah, Wayne H. Knox, W. Stolz

Published Mar 1, 1998• 0 citations
Publisher

Abstract

The coherent response of magnetoexcitons is measured using two-pulse transient-four-wave-mixing. Spectral resolution shows that the magnetoexcitons are interacting and that they act as a single quantum mechanical system. Measurements at zero magnetic field yield insight by demonstrating that interactions between the exciton and continuum states can dramatically modify the coherent response. This is most evident in an experiment where coherent emission is observed from the 1s exciton resonance, although it is not excited by one of the pulses.

DOI
10.1002/(sici)1521-3951(199803)206:1<77::aid-pssb77>3.0.co;2-o

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The coherent response of magnetoexcitons is measured using two-pulse transient-four-wave-mixing. Spectral resolution shows that the magnetoexcitons are interacting and that they act as a single quantum mechanical system. Measurements at zero magnetic field yield insight by demonstrating that interactions between the exciton and continuum states can dramatically modify the coherent response. This is most evident in an experiment where coherent emission is observed from the 1s exciton resonance, although it is not excited by one of the pulses.
The presence of interactions between carriers in differing states, nondegenerate interactions, is experimentally demonstrated to result in strong, new contributions to the optical coherent response of semiconductors. These interactions are manifest in a new, state-selective, two-pulse four-wave-mixing technique as emission from a state that is excited by only one pulse. The emission arises due to interactions with another state that is excited by both pulses. These interactions are observed between the 1s1s1s exciton and continuum states and also between distinct magnetoexciton states. The resulting contributions must be included to properly understand the coherent response.
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The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.
Summary form only given. Despite recent effort, there are aspects of the transient four-wave-mixing (TFWM) response of semiconductors that remain puzzling. By performing partially nondegenerate TFWM (PND-TFWM) we show that for broadband excitation, the signal emitted at the 1 s exciton is dominated by contributions that correspond to excitation of higher states by the first pulse. The interference of these contributions results in the rapid decay of the excitonic signal. Our results demonstrate that, because TFWM is inherently a many-particle phenomenon, the many-body interactions always couple the excitonic transitions.
We present an experimental and theoretical investigation of ultrafast transient four-wave mixing of GaAs/ Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells for coherent excitation of exciton and continuum states. The signal appears at the exciton resonance and is shown to consist of two contributions: an intense spectrally narrow part only present for short time delays and a weak exponentially decaying part reflecting the intrinsic coherence properties of the exciton states. The intense part of the signal is due to the excitation of the continuum states mediated by interaction-induced nonlinearities. \textcopyright{} 1996 The American Physical Society.
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.