Correlated Coulomb quantum kinetics in optically excited semiconductors

摘要

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.