We study the dynamical behavior of excitons in a bimodal distribution of CdSe/ ZnSSe quantum dots by intensity dependent, temperature dependent, and time resolved photoluminescence. The effect of exciton localization is investigated, both experimentally and theoretically by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe a dominant exciton emission from high energy dots ͑QDs1͒ and weaker emission from low energy dots ͑QDs2͒ at 10 K and at low excitation levels. At high excitation densities a CdSe QD-precursor state becomes visible at the high energy side of the QDs1 emission. Temperature dependent photoluminescence studies reveal a thermally activated exciton transfer from QDs1 to QDs2 resulting in an enhanced QDs2 emission above 60 K. Time resolved photoluminescence measurements allow us to estimate the characteristic radiative and nonradiative decay rates as well as the trapping rate from the QD-precursor layer. The experimentally observed photoluminescence is reasonably reproduced using a coupled rate equation model.

We investigate the spectral and thermal dependence of an exciton resonant all-optical and phase coherent photorefractive effect in a ZnSe single quantum well by four-wave mixing using 90 fs pulses. The observed phase coherence is attributed to an exciton-induced formation of an electron grating within the quantum well. The diffracted signal exhibits an exponential decay that is equal to the exciton dephasing rate and it shows a pronounced suppression at the temporal overlap of incident pulses. With decreasing photon energy the diffracted signal traces reveal faster decay and the exciton transition is shifted towards lower energies indicating an increasing electron concentration within the quantum well. In temperature dependent measurements we observe a filling of the signal dip at pulse overlap and a transition from the photorefractive effect to a ͑3͒ four-wave-mixing process below 35 K. Additional experiments using a cw pump laser are consistent with the proposed mechanism of the observed photorefractive effect. The phase coherent photorefractive signal is reproduced by a phenomenological model that is based on the optical Bloch equations for a five-level system.

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