Optical orientation and alignment of excitons in self-assembled CdSe ZnSe quantum dots: The role of excited states

We studied the optical orientation of excitons in an ensemble of self-organized CdSe ZnSe quantum dots (QDs) via photoluminescence (PL) under quasiresonant excitation. The observed PL spectra clearly show the high efficiency of phonon-assisted emission processes. The results demonstrate unambiguously that the QD symmetry is lower than D2d, which results in the splitting of optically active exciton states into two linearly polarized dipoles. The corresponding QD axes have different directions, defined by shape and strain anisotropies. It is shown that the spin-relaxation time of excitons in the ground state noticeably exceeds the exciton lifetime. We have further observed that the degree of optical orientation is small at zero magnetic field, but dramatically increases as the field is increased. The experimental results are interpreted in terms of a cascade model, in which the exciton is initially created in the excited state (which is believed to be a coupled state with phonons) and emits from the ground state after energy relaxation. This model provides an explanation for the experimentally observed conversion of the linear polarization from one set of axes to another. It was shown that the excited states have a significant influence on the polarization of the ground-state emission, and thus it is possible to measure their short characteristic times. We were able to estimate the lifetime τ2 of the exciton in the excited state to be ∼1ps and the value of the anisotropic exchange-splitting Ωx,y to be ∼0.5meV.