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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Spectral speckle analysis: a new method to measure coherence and dephasing in semiconductor nanostructures

Gerrit Kocherscheidt, W. Langbein, R. Zimmermann

Published Jun 5, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.513755

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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.
A linear optical method to measure coherence and dephasing of excitations in solids is presented. The spectrally resolved degree of coherence of resonantly scattered light is deduced from the intensity fluctuations over its emission directions (speckles). The spectral intensity correlation gives a direct measure of the dephasing rate within the inhomogeneously broadened ensemble. For localized excitons in semiconductor quantum wells, the combination of static disorder, phonon scattering, and radiative decay leads to a spectral dependence of the emission coherence and the dephasing rate, which is well described by model calculations.
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.
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.
The technique of the speckle analysis of resonant secondary emission is reviewed. It is a novel, linear optical technique to measure the microscopic intensity and polarization dynamics in inhomogeneously broadened ensembles, that represents an alternative for established non-linear optical techniques such as photon-echo or spectral hole burning. Both time-resolved and spectrally resolved variants of the speckle analysis are discussed. Experimental results obtained on semiconductor quantum wells using both techniques are presented.
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 optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.