B. Haase

Intensity dependence of signals obtained in four-wave-mixing geometry: influence of higher-order contributions

The nonlinear optical response of a single ZnSe quantum well is investigated in four-wave-mixing geometry. Contributions above third order in the driving fields are quantified by intensity-dependent measurements. Comparison with a microscopic density-matrix approach yield good agreement between experiment and theory. It is found that substantial contributions above third order are generated even at lowest excitation levels necessary in current experiments.

Line shape of four-wave-mixing signals : dependence on sample geometry and excitation conditions

We present an analysis of the dependence of the line shape of four-wave-mixing (FWM) signals on the sample geometry and the excitation conditions based on ZnSe single quantum wells with varying width and barrier composition. We analyze the geometry dependence of the biexciton binding energy and observe a strong influence of reabsorption and of the signal polarization on the spectral structures. The results exhibit excellent agreement with calculations based on a microscopic density-matrix theory considering the biexcitonic continuum for a 2D model.

Correlated Coulomb quantum kinetics in optically excited semiconductors

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.

Excitonic optical nonlinearities and dynamics in II–VI heterostructures and laser diodes

Abstract The extremely strong Coulomb correlation in wide-gap II–VI semiconductors results in considerable excitonic contributions to optical nonlinearities still at excitation densities sufficient for stimulated emission and lasing. In pure ZnSe and ZnCdSe QWs, even more complex exciton systems like biexcitons may dominate the emission spectra at moderately high densities. In ternary wells of compositions typical for laser diodes, the exciton-biexciton kinetics is still described without the assumption of strong localization. In highest-quality ZnSe wells with excitonic line widths as small as 0.75 meV and large biexciton binding energies, the nontrivial polarization and intensity dependence of exciton and biexciton contributions to coherent four-wave mixing is for the first time systematically investigated and explained by a microscopic density-matrix model taking into accounthigher-order correlations. Whereas biexcitons have also been proved to give rise to stimulated emission at temperatures ⩽ about 150 K in undoped QW structures, they play no role for stimulated emission in favor of a strongly Coulomb-correlated electron-hole plasma in operational, injection laser diodes. Its dynamical behavior after ps-pulse excitation reveals novel features of kinetic and spectral hole burning and a coupling of the longitudinal ground and higher lateral modes in resonator structures. Eventually, an overall understanding of the lasing properties of II–VI diode structures is nowadays possible.