Role of Biexcitons in the Stimulated Emission of Wide-Gap II-VI Quantum Wells

On high-quality ZnCdSe/ZnSe quantum wells the occurrence of a new emission feature energetically below the exciton transition is demonstrated from which stimulated emission and gain emerge at elevated excitation levels (10 kW/ ${\mathrm{cm}}^2$). Application of a magnetic field allows us to distinguish this feature from bound excitons and evidence its biexciton nature. Subpicosecond excite-and-probe measurements reveal the scenario of exciton-exciton interaction in the presence of alloy disorder (localization). A model accounting for bound and antibound two-exciton states provides good agreement with the experimental data.