  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.
