Ultrafast spin injection into self-assembled quantum dots

Abstract

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 Zn 0.80 Mn 0.20 Se layer and the other one is the electron-spin injection structure of QDs coupled with a Zn 0.68 Cd 0.22 Mn 0.10 Se 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.