  We experimentally investigate the back-scattering properties of an array of
atoms that is evanescently coupled to an optical nanofiber in the strongly
non-paraxial regime. We observe that the power and the polarization of the
back-scattered light depend on the nanofiber-guided excitation field in a way
that significantly deviates from the predictions of a simple model based on
two-level atoms and a scalar waveguide. Even though it has been widely used in
previous experimental and theoretical studies of waveguide-coupled quantum
emitters, this simple model is thus in general not adequate even for a
qualitative description of such systems. We develop an ab initio model which
includes the multi-level structure of the atoms and the full vectorial
properties of the guided field and find very good agreement with our data.
