Magneto-optical studies of (Zn,Mn)Se/ZnTe quantum dots

We have recorded the circular polarization $P$ of photoluminescence from (Zn,Mn)Se/ZnTe quantum dots (QDs) as function of magnetic field $B$. The polarization at a fixed temperature increases monotonically with $B$ and saturates for B \textgreater 3 tesla at $P_{sat}$. The value of $P_{sat}$ depends strongly on the laser photon energy. When we excite above (below) the ZnMnSe gap with photons of energy of 3.81 eV (2.54 eV), we measure $P_{sat}=$55$\% (P_{sat}=$20$\% )$. We interpret these results as due to the difference in the Zeeman band splitting between the magnetic (Zn,Mn)Se matrix and the non-magnetic ZnTe QDs. For 3.81 eV excitation, electron-hole pairs are generated mainly in the (Zn,Mn)Se matrix. The majority of the holes relax to the $+$3/2 state before capture by the ZnTe QDs. With 2.54 eV excitation, all electron-hole pairs are excited in the QDs where the Zeeman splitting is negligible. Thus, $P_{sat}$ is determined in this case by the relatively small Zeeman splitting of ZnMnSe conduction band. We relate these findings to our previous results for magnetic type-II QDs, where $P_{sat}$ does not depend on the exciting photon energy.