Coupling-dependent spin polarization of quantum dots in double layer geometry

We have studied the effect of coupling on spin polarization in two asymmetric double-layer quantum dot (DLQD) systems consisting of adjacent CdSe and CdZnSe layers. The thickness of the ZnSe barrier separating the CdSe QDs from the CdZnSe QDs in the DLQD structures was 20 monolayer (ML) and 60 ML respectively. Experiments as a function of excitation power reveal that the inter-dot coupling is relatively strong in DLQD with the 20 ML barrier, while it is rather weak when the barrier is thick (60). When a magnetic field is applied, the PL emitted by CdSe and CdZnSe QDs is partially circularly polarized due to the Zeeman splitting of the spin states in each layer material. While the degree of circular polarization emitted by the two layers was approximately the same when the barrier separating the layers was thick (60 ML), a large difference in the degree of polarization between the PL emitted by the CdSe and by the CdZnSe QD layers was observed in the sample with the 20 ML barrier. This barrier-dependent difference observed in the two DLQD structures is discussed in terms of coupling-dependent spin interaction between carriers localized in the adjacent QD layers.