Dynamics of exciton localization in CdS/HgS quantum-dot quantum wells

Localization of carrier wave functions to the quantum-well portion of the C dS/HgS quantum-dot quantum well (QDQW) is investigated. Nanosecond hole-bur ning (HB) spectra measure the photoinduced exciton coupling to a 250-cm(-1) HgS phonon mode indicative of localization. Femtosecond pump-probe spectro scopy of these QDQW, however, show the photoinduced exciton couples to cohe rent 300-cm(-1) CdS longitudinal optical-phonon modes, which is indicative of delocalization throughout the QDQW. Femtosecond HE and three pulse pump- dump experiments reveal these results are dependent on the time scale of th e experiment. These experiments indicate that the initially photoexcited el ectron and hole wave functions are weakly confined to the HSS monolayer. On ly after long times (similar to 400 fs) will the exciton localize to the Hg S well. These results indicate that the primary optical interaction excites electrons from a delocalized QDQW ground state and nor from a localized Hg S well state. [S0163-1829(99)10307-2].