Optical gain characteristics and excitonic nonlinearities in II–VI laser diodes

Gain characteristics and lasing of (Zn, Cd)Se/Zn(S, Se)/(Zn, Mg)(S, Se) separate confinement heterostructures are investigated by optical-gain spectroscopy and by high-excitation spectroscopy under quasi-stationary conditions. The optical gain is measured by means of the stripe-length method whereas excitonic bleaching under lasing conditions is analyzed through two-beam photoluminescence excitation (PLE) spectroscopy. At low lattice temperature, we find a rather low threshold density of 15–20 kW/cm2 for both laser structures and the excitonic enhancement is still preserved at the onset of lasing. The red shift of the gain maximum with respect to the low-density PLE exciton peak increases with temperature. The results indicate a considerable influence of Coulomb correlations even at the high densities necessary for stimulated emission and are more adequately explained by a strongly correlated electron-hole plasma model that goes beyond the simple “excitonic lasing” concept.