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

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 account higher-order correlations. Whereas biexcitons have also been proved to give rise to stimulated emission at temperatures less than or equal to 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. (C) 1998 Elsevier Science B.V. All rights reserved.