Polarization dynamics in quantum dots: The role of dark excitons
We study an impact of the fine structure of the heavy--hole ground state exciton confined in semiconductor quantum dots on the photoluminescence polarization dynamics solving the relevant system of the rate equations. The presence of the dark excitons is usually ignored and the polarization decay is assumed to be caused by direct transitions within the radiative doublet. We demonstrate that in strongly confined quantum dots the dark excitons, which are energetically well below the bright excitons, have actually a decisive effect on the polarization dynamics due to their persistent nature. The linear polarization shows nonexponential decay controlled by a conversion of the dark into a bright exciton. To get quantitative answers for specific quantum dot structures, all the necessary information can be obtained already from experiments on the luminescence dynamics following nonresonant excitation in these dots.