Influence of Coulomb correlations on gain and stimulated emission in (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) quantum-well lasers

The influence of Coulomb correlations on gain and stimulated emission in (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) quantum-well lasers is studied under stationary conditions. Systematic temperature-dependent measurements under application of different spectroscopic techniques were performed. Optical gain is measured by means of the variable stripe-length method, whereas excitonic bleaching under lasing conditions is analyzed through two-beam photoluminescence excitation (PLE) spectroscopy. Furthermore, complementary low-density single-beam PLE spectra are recorded in order to study the temperature dependence of the heavy-hole exciton peaks. The experimental data as a whole are shown to be inconsistent with any of the usually quoted excitonic models for lasing in II-VI heterostructures. The experiments are more adequately explained by a strongly correlated electron-hole plasma described by Bethe-Salpeter-like equations for the optical response and recombination rates in the excited medium. The nonequilibrium Green’s-function approach used consistently includes, at a microscopic level, band structure, quantum-confinement, and many-body effects.