  Fluorescent semiconductor nanoplatelets (epitaxial quantum wells) can be
synthesized with excellent monodispersity and self-assembled in highly-ordered
structures. Modifications of their electronic and luminescence properties when
stacked, due to strong mechanical, electronic or optical interactions between
them, have been the topic of intense recent discussions. In this paper, we use
Fourier imaging to measure the different dipole components of various
nanoplatelet assemblies. By comparing different measurement conditions and
corroborating them with polarimetric analysis, we confirm an excellent
precision on the dipole components. For single nanoplatelets, only in-plane
dipoles (parallel to the platelet plane) are evidenced. For clusters of 2-10
platelets and chains of 30-300 platelets, on the other hand, a clear
out-of-plane dipole component is demonstrated. Its contribution becomes more
significant as the number of platelets is increased. We review possible
explanations and suggest that the added out-of-plane dipole can be induced by
strain-induced nanoplatelet deformations.
