Laser processes and optical nonlinearities in ZnSe heterostructures.

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Optical nonlinearities and the characteristics of the stimulated emission in ZnSe/${\mathrm{ZnS}}_{\mathit{y}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ double heterostructures up to room temperature are studied under quasistationary optical excitation. We identify the underlying gain mechanisms as 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, which simultaneously provide the nonlinear absorption changes at the spectral position of the excitonic resonance. We perform theoretical line-shape analysis of the experimental gain as well as luminescence spectra. Further information is drawn from the temperature-dependent redshifts of the stimulated emission and of the excitonic absorption maximum. At low lattice temperature we find that the onset of lasing at pump intensities around 50 kW/${\mathrm{cm}}^{2}$ is due to stimulated recombination involving inelastic exciton-exciton collision. Electron-hole plasma recombination is responsible for the gain at higher excitation levels (500 kW/${\mathrm{cm}}^{2}$). The changes of the optical density are characterized by induced absorption due to band-gap renormalization and a loss of excitonic oscillator strength in the center of resonance. But the exciton is still well preserved at the onset of lasing. Above 100 K the stimulated emission 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 dominates the nonlinear absorption changes, especially for pump intensities beyond laser threshold. This proves that the gain originates from the plasma phase.