Exciton spin dynamics in spherical CdS quantum dots

Abstract

Exciton spin dynamics in quasispherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the six-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal-field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wave functions in the lowest exciton state are of $S$ type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron-spin-relaxation rate on the radius of quantum dots agrees well with that computed theoretically.