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physica status solidi (a)

Temporal Coherent Control in Semiconductor Quantum Structures

P. Renucci, M. Paillard, X. Marie, T. Amand, J. Barrau, Cristiano Ciuti

Published Mar 1, 2000• 4 citations
Publisher

Abstract

Temporal coherent control experiment have been performed on semiconductor quantum well structures, using a sequence of polarized excitation pulses. The oscillatory behaviour of the populations, spin and alignment of the exciton states are observed through the time resolved secondary emission when the time separation between the two excitation pulses is varied. We show that, in specular detection direction, for bare quantum wells at 10 K and low exciton densities, the oscillations amplitude decay is determined by the exciton inhomogeneous broadening. In non-specular detection directions however, when the SE is dominated by resonant Rayleigh scattering, the decay is longer and becomes temperature dependent, which strongly supports that it is now determined by the exciton dephasing time.

DOI
10.1002/1521-396x(200003)178:1<373::aid-pssa373>3.0.co;2-m

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physica status solidi (a)•Mar 1, 2000•7 citations
Temporal coherent control experiment have been performed on semiconductor quantum well structures, using a sequence of polarized excitation pulses. The oscillatory behaviour of the populations, spin and alignment of the exciton states are observed through the time resolved secondary emission when the time separation between the two excitation pulses is varied. We show that, in specular detection direction, for bare quantum wells at 10 K and low exciton densities, the oscillations amplitude decay is determined by the exciton inhomogeneous broadening. In non-specular detection directions however, when the SE is dominated by resonant Rayleigh scattering, the decay is longer and becomes temperature dependent, which strongly supports that it is now determined by the exciton dephasing time.
Summary form only given. The coherence decay of optically excited electronic systems provides one of the most powerful tools to investigate interaction processes of excited states. In semiconductors, the optical coherence phenomena are usually investigated by nonlinear techniques such as four wave mixing (FWM), photon echo, or reflectivity experiments. Exciton-phonon, exciton-exciton, or exciton-free carriers scattering processes have been explored in recent years with these techniques. As demonstrated recently, a direct measurement of the optical dephasing time can be obtained through the temporal coherent control of electronic excitations. In these experiments, the emission following the excitation by an ultrashort optical pulse sequence, is detected in a non-specular direction (secondary emission). This provides an alternative approach to FWM techniques, which allows us to analyse the scattering processes either with phonons or due to mutual collisions. Using the coherent control technique monitored through the secondary emission (SE), we report on the coherent dynamics of excitons in bare quantum wells (QW) and compare it to that of exciton-polaritons in microcavity (MC) embedded quantum wells.
Phase-locked pulse pairs are used for resonant excitation of excitons confined within GaAs/AlGaAs quantum wells. The time-resolved secondary emission displays strong dependence on the relative phases of the two pulses for inter-pulse delays of the order of 1ps or less. Constructive interference is combined with an increase in the rise time of the emission following the second pulse. For longer inter-pulse delays the different phases of inhomogeneous excitons complicate the effect of the second pulse, with the apparent effect changing with time after the second pulse. To model this, the optical Bloch equations for a line of three-level systems are integrated over time and the resulting polarisations summed over a range of angles. The importance of the inhomogeneity in descriptions of coherent control of secondary emission is shown.
We have investigated the manipulation of the phase and amplitude of the state of a three-level exciton system by excitation with pairs of phase-locked optical pulses. We demonstrate coherent control over the phase of quantum beating between the light- and heavy-hole excitons which are the components of the system. The state of the system is monitored by time-resolved detection of the resonant secondary emission coupled out of the system in non-phase-matched directions by virtue of the static disorder. We show that the amplitude of the coherent manipulation must diminish as a function of time delay between the components of the pulse pair in a way which takes into account both the dephasing of the excitons and their unphasing which arises from inhomogeneous broadening.
A new method to measure the time-dependent coherence of both homogeneously and inhomogeneously broadened optical excitations in solids is presented. The coherence degree of resonantly excited light emission is deduced from the intensity fluctuations over the emission directions (speckles). This method determines the decays of intensity and coherence separately, thus distinguishing lifetime from pure dephasing. The secondary emission of excitons in semiconductor quantum wells is investigated. Here the combination of static disorder and inelastic scattering leads to a partially coherent emission. The temperature dependence is well explained by phonon scattering.
Coherent Rayleigh scattering and incoherent luminescence are studied by femtosecond upconversion under resonant excitation as a function of exciton density in GaAs quantum wells. Comparing samples with different well widths allows to establish the relation between inhomogeneous broadening and the shape of resonant Rayleigh scattering. We find an excellent agreement between experiment and a classical model. In samples with moderate inhomogeneous broadening we provide evidence for the transition from coherent to incoherent emission. The comparison with excitonic dephasing times T2 obtained from four-wave-mixing experiments demonstrates that the incoherent exciton population rises within T2/2, at any exciton density when changed by more than an orders of magnitude. The experimental results confirm very clearly the concept predicting that the coherent polarization made up by localized excitons decays into an incoherent exciton population under conservation of the number of excited quanta.
The excitonic coherence in semiconductor quantum wells is measured by time-resolved speckle analysis and four-wave mixing. It is found that in the case of single GaAs quantum wells, the results of both methods are in agreement, and can be understood in the framework of an inhomogeneously broadened distribution of localized excitons which are subject to phonon-scattering and radiative recombination. The speckle analysis gives the additional distinction between recombination and pure dephasing processes. In multiple quantum wells, the radiative coupling between the quantum wells leads to a pronounced change of the observed dynamics. In the secondary emission, a strong coherent component with an oscillating time-structure is observed for small scattering angles. This is attributed to the beating of the radiative eigenstates of the stacked structure.