  We use an atomistic model to consider the effect of shape symmetry breaking
on the optical properties of self-assembled InAs/GaAs quantum dots. In
particular, we investigate the energy level structure and optical activity of
the lowest energy excitons in these nanostructures. We compare between quantum
dots with two-fold rotational and two reflections (C2v) symmetry and quantum
dots in which this symmetry was reduced to one reflection only (Cs) by
introducing a facet between the quantum dots and the host material. We show
that the symmetry reduction mostly affects the optical activity of the dark
exciton. While in symmetric quantum dots, one of the dark exciton eigenstates
has a small dipole moment polarized along the symmetry axis (growth direction)
of the quantum dot, in non-symmetric ones, the two dark excitons' dipole
moments are predominantly cross-linearly polarized perpendicular to the growth
direction and reveal pronounced polarization anisotropy. Our model calculations
agree quantitatively with recently obtained experimental data.
