  In a recent preprint (arXiv:1401.1128v1) Cilibrizzi and co-workers report
experiments and simulations showing the scattering of polaritons against a
localised obstacle in a semiconductor microcavity. The authors observe in the
linear excitation regime the formation of density and phase patterns
reminiscent of those expected in the non-linear regime from the nucleation of
dark solitons. Based on this observation, they conclude that previous
theoretical and experimental reports on dark solitons in a polariton system
should be revised. Here we comment why the results from Cilibrizzi et al. take
place in a very different regime than previous investigations on dark soliton
nucleation and do not reproduce all the signatures of its rich nonlinear
phenomenology. First of all, Cilibrizzi et al. consider a particular type of
radial excitation that strongly determines the observed patterns, while in
previous reports the excitation has a plane-wave profile. Most importantly, the
nonlinear relation between phase jump, soliton width and fluid velocity, and
the existence of a critical velocity with the time-dependent formation of
vortex-antivortex pairs are absent in the linear regime. In previous reports
about dark soliton and half-dark soliton nucleation in a polariton fluid, the
distinctive dark soliton physics is supported both by theory (analytical and
numerical) and experiments (both continuous wave and pulsed excitation).
