  DarkSide-50 has demonstrated the high potential of dual-phase liquid argon
time projection chambers in exploring interactions of WIMPs in the GeV/c$^2$
mass range. The technique, based on the detection of the ionization signal
amplified via electroluminescence in the gas phase, allows to explore recoil
energies down to the sub-keV range. We report here on the DarkSide-50
measurement of the ionization yield of electronic recoils down to
$\sim$180~eV$_{er}$, exploiting $^{37}$Ar and $^{39}$Ar decays, and
extrapolated to a few ionization electrons with the Thomas-Imel box model.
Moreover, we present a model-dependent determination of the ionization response
to nuclear recoils down to $\sim$500~eV$_{nr}$, the lowest ever achieved in
liquid argon, using \textit{in situ} neutron calibration sources and external
datasets from neutron beam experiments.
