We show ultrafast spin injection from a diluted magnetic semiconductor (DMS) into self-assembled quantum dots
(QDs), where excitons or carriers are highly spin-polarized in the DMS under magnetic fields and are subsequently
injected into the QDs resulting from the energy relaxation due to the potential difference. Two types of the sample
structure have been studied for exploiting efficient spin injection by using time-resolved circularly polarized
photoluminescence: one is the exciton-spin injection structure of CdSe QDs stacked with a Zn0.80Mn0.20Se layer and the
other one is the electron-spin injection structure of QDs coupled with a Zn0.68Cd0.22Mn0.10Se quantum well. In the former
structure, exciton-spin injection takes place from the DMS layer into the QDs with a time constant of 10 ps after the
pulse excitation for the DMS, followed by spin transfer among the QDs and spin relaxation in the QDs. In the latter case,
we realize efficient electron-spin injection via quantum tunneling with a time constant of 20 ps, where the spin injection
is resonantly assisted by LO-phonon scattering. These results imply importance of the spin-injection dynamics for the
future applications of the QDs coupled with the DMS to ultrafast spintronic and spin-functional optical devices.
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