  We study experimentally and theoretically polarization-dependent luminescence
from an ensemble of quantum-dot-like nanostructures with a very large in-plane
shape anisotropy (quantum dashes). We show that the measured degree of linear
polarization of the emitted light increases with the excitation power and
changes with temperature in a non-trivial way, depending on the excitation
conditions. Using an approximate model based on the k.p theory, we are able to
relate this degree of polarization to the amount of light hole admixture in the
exciton states which, in turn, depends on the symmetry of the envelope wave
function. Agreement between the measured properties and theory is reached under
assumption that the ground exciton state in a quantum dash is trapped in a
confinement fluctuation within the structure and thus localized in a much
smaller volume of much lower asymmetry than the entire nanostructure.
