  Spin injection across interfaces driven by ultrashort optical pulses on
femtosecond timescales constitutes a new way to design spintronics
applications. Targeted utilization of this phenomenon requires knowledge of the
efficiency of non-equilibrium spin injection. From a quantitative comparison of
ab-initio time-dependent density functional theory and interface-sensitive,
time-resolved non-linear optical experiment, we determine the spin injection
efficiencies (SIE) across ferromagnetic/metal interfaces and discuss their
microscopic origin, i.e. the influence of spin-orbit coupling and the interface
electronic structure. Moreover, we find that the SIE can be optimized through
laser pulse and materials parameters, namely the fluence, pulse duration, and
substrate material.
