Optical measurement of exciton transport in quantum wells and self-assembled quantum dots

In diluted magnetic semiconductors Zeeman splitting causes a giant Faraday rotation in an external magnetic field. This effect can be used to probe the dynamical magnetization of the magnetic ions after intense photoexcition of spin-polarized excitons in a time-resolved experiment. A polarization modulation method is able to make a sensitive measurement of Kerr rotation, instead of the Faraday rotation, using a photoelastic modulator. A strained Zn0.98 Mn0.02Se epilayers directly grown on a GaAs substrate is used in the experiment. The investigation shows a strong thickness dependence in Kerr spectra due to multiple reflections between the sample surface and the interface of sample and substrate. The data also match Zeeman splitting in the polarization photoluminescence measurement.; From time-resolved and magnetization saturation experiments the type-II exciton magnetic polarons are very long-lived. Thus, the spin transport may be observed in such a material. Using spatially- and temporally-resolved photoluminescence we can study these transport properties in Zn0.86Mn0.14Se/ZnSe multiple quantum wells. At low excitation density the exciton magnetic polarons are localized as expected from theory. However, when the incident power is greater than a threshold, the driven transport suddenly becomes significant. A rapid expansion away from the excitation center occurs at high densities. The expansion velocity is even over twice the speed of sound. Moreover, the expansion velocity is strongly temperature and magnetic field dependent. The velocity quickly increases to reach the maximum at 6 K, and then decreases with temperature. On the other hand, the velocity decreases by a factor of 2 as magnetic field increases to nearly 3 tesla.; From micro- and nano-photoluminescence and time-decay experiment, there exist two distinct states in CdSe/ZnSe self-assembled quantum dots. The broad emission corresponds to the short-lived state, and the sharp emission corresponds to the long-lived state. We can modify the setup of spatial scan to study the transport properties of excitons in such structures. We vary energies, the incident powers, and temperatures. The measurement shows that there is almost no expansion at all even up to 9 ns, so that the excitons are strongly localized for both states.