  Exciton spin and related optical polarization in self-assembled
InAs/In$_{0.53}$Ga$_{0.23}$Al$_{0.24}$As/InP(001) quantum dashes emitting at
1.55 {\mu}m are investigated by means of polarization- and time-resolved
photoluminescence, as well as photoluminescence excitation spectroscopy, at
cryogenic temperature. We investigate the influence of highly non-resonant and
quasi-resonant optical spin pumping conditions on spin polarization and spin
memory of the quantum dash ground state. We show that a spin pumping scheme,
utilizing the longitudinal-optical-phonon-mediated coherent scattering process,
can lead to the polarization degree above 50%. We discuss the role of intrinsic
asymmetries in the quantum dash that influence values of the degree of
polarization and its time evolution.
