EXCITONIC OPTICAL NONLINEARITIES AND DYNAMICS IN II-VI HETEROSTRUCTURES AND LASER-DIODES

The extremely strong Coulomb correlation in wide-gap II-VI semiconduct ors results in considerable excitonic contributions to optical nonline arities still at excitation densities sufficient for stimulated emissi on and lasing. In pure ZnSe and ZnCdSe QWs, even more complex exciton systems like biexcitons may dominate the emission spectra at moderatel y high densities. In ternary wells of compositions typical for laser d iodes, the exciton-biexciton kinetics is still described without the a ssumption of strong localization. In highest-quality ZnSe wells with e xcitonic line widths as small as 0.75 meV and large biexciton binding energies, the nontrivial polarization and intensity dependence of exci ton and biexciton contributions to coherent four-wave mixing is for th e first time systematically investigated and explained by a microscopi c density-matrix model taking into account higher-order correlations. Whereas biexcitons have also been proved to give rise to stimulated em ission 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 s trongly Coulomb-correlated electron-hole plasma in operational, inject ion laser diodes. Its dynamical behavior after ps-pulse excitation rev eals novel features of kinetic and spectral hole burning and a couplin g of the longitudinal ground and higher lateral modes in resonator str uctures. Eventually, an overall understanding of the lasing properties of II-VI diode structures is nowadays possible. (C) 1998 Elsevier Sci ence B.V. All rights reserved.