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Physical Review Letters

Resonant Secondary Emission from Two-Dimensional Excitons: Femtosecond Time Evolution of the Coherence Properties

Michael Woerner, Jagdeep Shah

Published Nov 9, 1998• 71 citations
Publisher

Abstract

Using a novel technique, we simultaneously study the femtosecond dynamics of the amplitude and the coherence of resonant secondary emission from GaAs quantum wells. Two phase-locked pulses resonantly excite excitons in the semiconductor and the subsequently emitted radiation is detected by femtosecond up-conversion. In experiments at various delays and phases between the excitation pulses we identify three different contributions to the secondary emission with different coherence properties in time.

DOI
10.1103/physrevlett.81.4208

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Using a novel technique, we simultaneously study the femtosecond dynamics of the amplitude and the coherence of resonant secondary emission from GaAs quantum wells. Two phase-locked pulses resonantly excite excitons in the semiconductor and the subsequently emitted radiation is detected by femtosecond up-conversion. In experiments at various delays and phases between the excitation pulses we identify three different contributions to the secondary emission with different coherence properties in time.
Summary form only given. The femtosecond dynamics and the coherence properties of resonant secondary emission (RSE) from quantum wells is a subject of considerable current interest. We use a novel approach in which the sample is resonantly excited by a pair of phase-locked pulses, and the spontaneously emitted radiation is detected by femtosecond upconversion. This novel experimental technique allows us, for the first time, to investigate the temporal evolution and the coherence properties of RSE simultaneously.
We report on the observation of coherent secondary emission from resonantly excited bound and unbound two-exciton states in quantum wells. The emission is intrinsic, in contrast to the well-known Rayleigh scattering from excitons that requires disorder. We measure the spectral amplitude and the phase (and, hence, the femtosecond dynamics) of this coherent emission using ultrafast spectral interferometry, and obtain direct information on the contributions to the nonlinear susceptibility from exciton-exciton correlations beyond the mean-field approximation.
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.
Summary form only given. The question of the coherence properties of the radiation from two-dimensional electron-hole-pair states in systems of reduced dimensionality has recently gained new interest both in theory and experiment. In these experiments the light emission from a short-pulse excited semiconductor QW in a direction different from the excitation or reflection directions (secondary emission, SE) was studied either by spectral interferometry or speckle analysis. As origin of the SE either disorder or Coulomb interaction is under discussion. We report in our contribution a careful study of both the specular and non-specular resonant-emission from GaAs quantum well exciton states applying interferometric controlled spectral interferometry with an accuracy of about 50 attosetonds.
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.
Secondary emission from GaAs quantum wells following resonant pulsed excitation is studied in the time domain. Selective excitation of heavy- and light-hole excitons is accomplished in a two-pulse excitation experiment which preserves sub-ps time resolution. After the initial rise the emission decays exponentially at a fast rate and then with another much slower exponential decay at longer times. The variations in the initial decay times with excitation density and sample temperature are similar to the variation in dephasing rates measured in four-wave mixing experiments. The decay in the visibility of heavy- and light-hole exciton beating is consistent with the decay rates measured for both species separately.