Spin and polarization dynamics in magnetic and non-magentic semiconductor quantum dots

Abstract

We report on time-resolved optical investigations on the polarization and the spin dynamics in non-magnetic and magnetic self-assembled II VI semiconductor quantum dots. In case of CdSe/ZnSe quantum dots, no transient loss of polarization is found within the time scale of exciton recombination, if one excites the excitons strictly resonant in the quantum dot ground state with a laser pulse linearly polarized along the [110] or [1-10] crystal axes. This indicates a high temporal stability of the exciton state, which is a coherent superposition of spin-up and spin-down exciton states. Even after replacing some Cd atoms in the crystal matrix by magnetic ions Mn²⁺, the polarization is being conserved as long as the average Mn²⁺ concentration is about (formula available in paper)or less, despite the pronounced exchange interaction between the manganese ion spins and the carrier spins. In case of magnetic semiconductor quantum dots with a large concentration of (formula available in paper)ions, the spin spin interaction between charge carriers and manganese ions results in the formation of a quasi-zero dimensional ferromagnetically aligned spin complex, the exciton magnetic polaron. For (formula available in paper)Se quantum dots this transient spin alignment is directly evidence by a transient shift of the emission energy. We deduce a typical time constant of 125 ps at T = 2 K for the dynamical response of the magnetic ion spins.