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physica status solidi (b)

Four‐wave‐mixing theory beyond the semiconductor Bloch equations

V. M. Axt, Á. Stáhl, E. J. Mayer, P. Haring Bolívar, S. Nüsse, Klaus Ploog, Klaus Köhler

Published Mar 1, 1995• 30 citations
Publisher

Abstract

Abstract Four‐wave‐mixing (FWM) experiments using a dynamical density matrix model of the semiconductor band edge are discussed. Higher‐order correlation functions are retained which are neglected in the commonly used RPA treatment leading to the semiconductor Bloch equations. In order to terminate the hierarchy of the equations of motion for the higher‐order density matrices systematically a truncation scheme controlled by orders in the driving field is applied. For any prescribed order n in the exciting field a closed set of equations is obtained from which the dielectric response up to order n can be calculated exactly. In addition it turns out that in a coherently driven system part of the remaining density matrices become redundant and can be eliminated. Four‐wave‐mixing experiments are dominated by third‐order contributions. Applying the above‐described results one ends up with only two functions in this case. These are the excitonic and the biexcitonic transition densities. As an application of our method the example of a GaAs single quantum well is studied. Two pulses with finite lengths are assumed such that both heavy‐and light‐hole excitons are excited. The influence of the biexciton contribution on the polarization properties of the FWM signal is analyzed and compared with experimental results.

DOI
10.1002/pssb.2221880142

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Quantum Electronics and Laser Science Conference•Jan 20, 2003•0 citations
Abstract Four‐wave‐mixing (FWM) experiments using a dynamical density matrix model of the semiconductor band edge are discussed. Higher‐order correlation functions are retained which are neglected in the commonly used RPA treatment leading to the semiconductor Bloch equations. In order to terminate the hierarchy of the equations of motion for the higher‐order density matrices systematically a truncation scheme controlled by orders in the driving field is applied. For any prescribed order n in the exciting field a closed set of equations is obtained from which the dielectric response up to order n can be calculated exactly. In addition it turns out that in a coherently driven system part of the remaining density matrices become redundant and can be eliminated. Four‐wave‐mixing experiments are dominated by third‐order contributions. Applying the above‐described results one ends up with only two functions in this case. These are the excitonic and the biexcitonic transition densities. As an application of our method the example of a GaAs single quantum well is studied. Two pulses with finite lengths are assumed such that both heavy‐and light‐hole excitons are excited. The influence of the biexciton contribution on the polarization properties of the FWM signal is analyzed and compared with experimental results.
The paper presents a description of two-pair correlations in an electron-hole system in laser-excited direct-gap semiconductors in terms of the exciton-boson formalism. The dependence of the states of quasi-particles on their angular momentum orientation is properly taken into account following group theory rules. It is shown for the example of heavy hole excitons in GaAs quantum wells that Coulomb two-exciton correlations include effective repulsion between excitons with nonzero total momenta and effective attraction between excitons with zero total momenta. They generate exciton and biexciton nonlinearities, respectively, in the exciton-biexciton system. The Heisenberg equations of motion derived are applied to four-wave mixing in the coherent mode. Two components of Coulomb two-exciton correlations were found to generate coherent four-wave mixing either separately, one at the cocircular and the other at the perpendicular-linear polarization of applied pulses, or jointly at parallel-linear polarization. General equations are obtained for the amplitude of four-wave mixing signals with time resolution in these polarization configurations, which, in the limit of ultranarrow pulses, reduce to analytic functions of time and excitation and sample parameters. These equations are used to analyze signal characteristics in every polarization configuration. The results give a correct picture of biexciton effects and the polarization dependence of coherent four-wave mixing in GaAs quantum wells.
In order to optimize opto-electronic applications of semiconductor nanostructures, a deep understanding of optical nonlinearities is essential.One fundamental method currently used within this field of research is ultrafast optical spectroscopy.This work presents a theoretical study of optical nonlinearities in semiconductor quantum wells in the coherent time regime.The theory is based on the equations of motion for the coherent exciton and biexciton transition amplitudes.These equations are derived using two different ways: Heisenberg equations of motion in combination with the dynamics-controlled truncation theory and alternatively the non-equilibrium Green´s function technique. For the coherent regime both approaches are equivalent and can be used to formulate a coupled set of equations for the coherent excitation dynamics that is correct up to the third order in the optical field.In contrast to the dynamics-controlled truncation theory the non-equilibrium Green´s function technique allows the inclusion of incoherent scattering processes into the description.The theory is evaluated to describe pump and probe as well as four-wave-mixing experiments with ZnSe quantum wells.Numerical calculations for the coherent control of the macroscopic polarization andthe dependence of transient four-wave-mixing on the light-polarization and intensity are both in good agreement with experiments.In order to correctly describe elevated excitation conditions for the latter investigation, the theory is self-consistently extended such that the included excitonic and biexcitonic nonlinearities contribute up to arbitrary order in the optical field.
A systematic analysis of excitonic and biexcitonic contributions to four-wave mixing in dependence on the polarization state and on the intensity of the exciting beams is presented for ZnSe/Zn1—xMgxSySe1—iiy quantum wells. The relative signals of excitons and biexcitons for different polarization configurations are discussed in terms of a microscopic density matrix model revealing excellent correspondence to the complex experimental data. In configurations which do not allow for a third-order contribution of the biexciton of zero angular momentum higher-order correlations are shown to become important already at densities as low as 1010 excitons cm—2/pulse.
A theory of degenerate four-wave mixing (DFWM) in the low density excitation limit at the semiconductor band edge is developed on the basis of the bosonic representation of the electron-hole Hamiltonian. By accounting for the exciton-exciton interactions of the lowest order we arrive at the practical few-level density matrix description of the third-order excitonic nonlinearity. The developed theory accounts phenomenologically for the phase space filling, and excitation induced dephasing effects and well reproduces the results of the recent DFWM experiments in high- and low-fineness microcavity. We show that polarization-sensitive four-wave mixing measurements enable to discriminate the contributions to the DFWM signal from the exciton-exciton interactions of different symmetry.
We present a unified microscopic approach to four-wave mixing (FWM) in semiconductors on an ultrashort time scale. The theory is valid for resonant excitation in the vicinity of the excitonic resonance and at low densities. The most important many-particle effects, i.e., static and dynamical exciton-exciton interaction as well as biexcitonic effects are incorporated. The internal fields resulting from these interaction processes give rise to pronounced many-particle effects in FWM signals. Our results explain the dependence of FWM signals on the polarization geometry, especially if biexcitons contribute. Time-resolved (TR) FWM experiments show that the diffraction of the interaction induced fields dominate the FWM signals completely. This dominance of the interaction induced field at low temperatures is true regardless of density, detuning, or polarization geometry. While spectrally resolved FWM (-FWM) shows biexcitonic or bound excitonic contributions under various experimental conditions, TR-FWM is always completely delayed, peaking roughly at the dephasing time after both beams passed through. \textcopyright{} 1996 The American Physical Society.
Summary form only given. Here, we use the polarization selection rules for four-wave mixing (FWM) and a test that was originally designed to distinguish between quantum beats and polarization interference to demonstrate the presence of strong exciton-exciton correlations in GaAs-AlGaAs multiple quantum wells. We show that when the four-wave-mixing signal is produced by two pulses having the same circular polarizations that quantum-beat-like behavior is observed. The latter observation is contrary to the predictions based on either the density matrix equations for two independent three-level systems (without many body effects) or the conventional semiconductor Bloch equations (SBE), both of which predict that polarization interference, but not quantum beats, should be observed. Consequently, these observations provide clear evidence for exciton-exciton correlations and evidence that those correlations go beyond those contained in the SBE. The procedure described here provides a simple effective test for such processes. Finally, we show that the quantum-beating-like behavior can be reproduced by including a density-dependent dephasing term (so-called excitation-induced dephasing, or EID) in the density matrix equations.