  Optically induced spin currents have proven to be useful in spintronics
applications, allowing for sub-ps all-optical control of magnetization.
However, the mechanism responsible for their generation is still heavily
debated. Here we use the excitation of spin-current induced THz spin-waves in
noncollinear bilayer structures to directly study optical spin-currents in the
time domain. We measure a significant laser-fluence dependence of the spin-wave
phase, which can quantitatively be explained assuming the spin current is
proportional to the time derivative of the magnetization. Measurements of the
absolute spin-wave phase, supported by theoretical calculations and
micromagnetic simulations, suggest that a simple ballistic transport picture is
sufficient to properly explain spin transport in our experiments and that the
damping-like optical STT dominates THz spin-wave generation. Our findings
suggest laser-induced demagnetization and spin-current generation share the
same microscopic origin.
