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Quantum Electronics and Laser Science Conference

Coherent emission from excitons in quantum well structures

Ch. Nacke, Frank Kieseling, H. Stolz

Published May 7, 2000• 0 citations
OpenAlex

Abstract

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.

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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.
We have performed spectral interferometry with Rayleigh scattered light coming from a single speckle and emitted after resonant creation of excitons in GaAs quantum wells. The experiment confirms the coherence of the secondary emission at early times, regardless of sample parameters, but underlines the stochastic nature of a single speckle. The scattered electric field is calculated, correctly accounting for the center-of-mass quantization in the disordered potential. We find that the amplitudes of the electric fields emitted by different excitons are uncorrelated, so that the well-known Poisson distribution for speckle intensities applies.
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 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.
A new method to measure the coherence of inhomogeneously broadened optical excitations in semiconductor nanostructures is presented. The secondary emission of excitons in semiconductor quantum wells is investigated. The spectrally-resolved coherence degree of resonantly-excited light emission is deduced from the intensity fluctuations over the emission directions (speckles). The spectral correlations of the speckles give direct access to the homogeneous line width as function of spectral position within the inhomogeneously broadened ensemble. The combination of static disorder and phonon scattering leads to a partially coherent emission. The temperature dependence of the homogeneous line width is well explained by phonon scattering.
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.
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.