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Physical Review Letters

Influence of Two-Pair Continuum Correlations Following Resonant Excitation of Excitons

V. M. Axt, Björn Haase, U. Neukirch

Published May 14, 2001• 32 citations
Publisher

Abstract

The memory structure induced by coherent transitions to the exciton-exciton scattering continuum is shown to have significant influence on spectrally resolved four-wave-mixing signals even under selective excitation of 1s excitons. Comparisons between experiments and calculations that account nonperturbatively for these quantum kinetic Coulomb correlations demonstrate large compensations between mean-field contributions and transitions to the two-pair continuum. Experiments with different polarizations of the laser pulses show that two-pair continuum correlations are responsible for delay-time dependent shifts of the excitonic emission as well as for substantial deformations of the line shape.

DOI
10.1103/physrevlett.86.4620

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The memory structure induced by coherent transitions to the exciton-exciton scattering continuum is shown to have significant influence on spectrally resolved four-wave-mixing signals even under selective excitation of 1s excitons. Comparisons between experiments and calculations that account nonperturbatively for these quantum kinetic Coulomb correlations demonstrate large compensations between mean-field contributions and transitions to the two-pair continuum. Experiments with different polarizations of the laser pulses show that two-pair continuum correlations are responsible for delay-time dependent shifts of the excitonic emission as well as for substantial deformations of the line shape.
The combined influence of biexcitons and the two-exciton scattering continuum is analyzed. The analysis is based on a density matrix description of a two-band semiconductor accounting for mean-field contributions, bound biexcitons and the correlated parts of the two-exciton scattering continuum on a common footing. Consistent with the microscopic formulation, the influence of biexcitons as well as that of the two-exciton continuum is represented by a correlation kernel which is obtained from an integral equation. For a one-dimensional model four-wave mixing signals are calculated solving numerically the resulting coupled nonlinear equations for excitation conditions in the vicinity of the 1s resonance. A strong compensation between the exciton–exciton continuum and the mean-field contribution is found for all excitation conditions studied. Surprisingly, biexcitonic signatures turn out to be enhanced when continua are included in the description.
Correlations due to biexcitons as well as to two-pair scattering states are direct manifestations of Coulomb quantum kinetics. Effects resulting from the memory structure of the corresponding contributions to the optical interband polarization are analyzed taking a ZnSe single-quantum well as model system. Our calculations are based on a microscopic density matrix description using the dynamics controlled truncation scheme for closing the hierarchy of higher-order density matrices. A detailed comparison between experiment and theory allows for the identification of numerous effects directly reflecting different aspects of pair-correlations. In particular, we demonstrate various influences of the two-pair scattering continuum on four-wave-mixing signals. Furthermore, we show that signals with a dynamically changing polarization sensitively monitor a competition between coherent parts of the dynamics and signal components induced by incoherent exciton densities. Surprisingly, the phase of these signals is found to be almost unaffected by the presence of incoherent densities. This result can be traced back to a compensation of the influences on the phases resulting from different incoherent parts of the dynamics. Six-particle correlations representing transitions from incoherent densities to two-pair correlations turn out to be indispensable for a proper description of this effect.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The optical phase of four-wave-mixing (FWM) signals from a ZnSe single quantum well is analyzed experimentally and theoretically. The focus is on excitation conditions where the polarization state of the emitted light is dynamically changing rather than being fixed by selection rules, a case known to be sensitive to high-order Coulomb correlations. The phase is measured using an iterative procedure taking as input FWM signals with real-time resolution as well as frequency regime data. The results are compared with a microscopical density-matrix description using the dynamics controlled truncation (DCT) scheme. Mean-field Coulomb interactions, Pauli blocking and transitions to biexcitons and the two-pair scattering continuum are accounted for. Calculations in the coherent limit are able to reproduce the measured phases. The correlated two-pair scattering continuum is shown to have a significant impact on the phase.
We show that the exciton–exciton scattering continuum has a significant influence on the transient non-linear optical response of semiconductor microcavities. Comparison between transient shifts observed in pump and probe experiments and calculations demonstrates almost complete cancellation between mean-field and two-exciton continuum contributions. Moreover, we show that suppression of non-linear absorption of the lower polariton observed in many experiments is determined by the specific spectral shape of these correlations. In particular, we find that the pump induced dephasing of polariton resonances is proportional to the spectrum of four-particle correlations calculated at the energy sum of the pumped and probed polaritons.
The optical phase of four-wave-mixing (FWM) signals from a ZnSe single quantum well is analyzed experimentally and theoretically. The focus is on excitation conditions where the polarization state of the emitted light is dynamically changing rather than being fixed by selection rules, a case known to be sensitive to high-order Coulomb correlations. The phase is measured using an iterative procedure taking as input FWM signals with real-time resolution as well as frequency regime data. The results are compared with a microscopical density-matrix description using the dynamics controlled truncation (DCT) scheme. Mean-field Coulomb interactions, Pauli blocking and transitions to biexcitons and the two-pair scattering continuum are accounted for. Calculations in the coherent limit are able to reproduce the measured phases. The correlated two-pair scattering continuum is shown to have a significant impact on the phase.
Two-pair correlations are shown to induce dependences of the nonlinear optical response of semiconductors that can be used for a coherent manipulation of the resulting emission. Influences of biexcitonic transitions as well as transitions to two-pair scattering states are discussed within a microscopic density matrix approach. It is shown that the coherent control of biexciton beats is selective with respect to the propagation direction of the emitted signal, in contrast to the control of beats where all beat partners are generated by single photon processes. Furthermore, it is demonstrated that memory structures induced by exciton?exciton scattering states enable coherent manipulations of the excitonic line shape that allow us to estimate lower bounds for the corresponding memory time from experimentally accessible data.