LASER PROCESSES AND OPTICAL NONLINEARITIES IN ZNSE HETEROSTRUCTURES

Optical nonlinearities and the characteristics of the stimulated emiss ion in ZnSe/ZnSySe1-y double heterostructures up to room temperature a re studied under quasistationary optical excitation. We identify the u nderlying gain mechanisms its a function of excitation conditions and sample temperature. For this purpose we use a combination of different spectroscopic techniques. Optical gain spectra are determined by the variable stripe-length method and by pump-and-probe measurements, whic h simultaneously provide the nonlinear absorption changes at the spect ral position of the excitonic resonance. We perform theoretical line-s hape analysis of the experimental gain as well as luminescence spectra . Further information is drawn from the temperature dependent redshift s of the stimulated emission and of the excitonic absorption maximum. At low lattice temperature we find that the onset of lasing at pump in tensities around 50 kW/cm(2) is due to stimulated recombination involv ing inelastic exciton-exciton collision. Electron-hole plasma recombin ation is responsible for the gain at higher excitation levels (500 kW/ cm(2)). The changes of the optical density are characterized by induce d absorption due to band-gap renormalization and a loss of excitonic o scillator strength in the center of resonance. But the exciton is stil l well preserved at the onset of lasing. Above 100 K the stimulated em ission experiences a pronounced redshift as a function of temperature, which we attribute to inelastic scattering processes, including free carriers. At room temperature bleaching of the excitonic enhancement d ominates the nonlinear absorption changes, especially for pump intensi ties beyond laser threshold. This proves that the gain originates from the plasma phase.