  We report GaAs/AlGaAs nanowires in the one-dimensional (1D) quantum limit.
The ultrathin wurtzite GaAs cores between 20-40\,nm induce large confinement
energies of several tens of meV, allowing us to experimentally resolve up to
four well separated subband excitations in microphotoluminescence spectroscopy.
Our detailed experimental and theoretical polarization-resolved study reveals a
strong diameter-dependent anisotropy of these transitions: We demonstrate that
the polarization of the detected photoluminescence is governed by the symmetry
of the wurtzite 1D quantum wire subbands on the one hand, but also by the
dielectric mismatch of the wires with the surrounding material on the other
hand. The latter effect leads to a strong attenuation of perpendicularly
polarized light in thin dielectric wires, making the thickness of the AlGaAs
shell an important factor in the observed polarization behavior. Including the
dielectric mismatch to our k.p-based simulated polarization-resolved spectra of
purely wurtzite GaAs quantum wires, we find an excellent agreement between
experiment and theory.
