Formation and phase relaxation of negatively charged excitons in ZnSe single quantum wells

We investigate the formation, interaction, and dephasing of negatively charged excitons (trions) in ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}}_{0.9}{\mathrm{Mg}}_{0.1}\mathrm{Se}$ single quantum wells by polarization, intensity, and temperature dependent spectrally resolved transient four-wave mixing. The trion transition shows a binding energy of 2.8 meV and is discriminated from the biexciton by its polarization dependence. The coherent dynamics of the four-wave mixing response is in fair agreement with calculations based on extended optical Bloch equations. Temperature dependent measurements exhibit a significant enhancement of the exciton dephasing below 40 K due to the presence of trions and incoherent electrons which are optically excited from the GaAs substrate and captured by the quantum well.