  We investigate picosecond spin-currents across Au/iron-garnet interfaces in
response to ultrafast laser heating of the electrons in the Au film. In the
picoseconds after optical heating, interfacial spin currents occur due to an
interfacial temperature difference between electrons in the metal and magnons
in the insulator. We report measurements of this interfacial longitudinal spin
Seebeck effect between Au and rare-earth iron-garnet insulators, i.e. RE$_3$
Fe$_5$O$_{12}$, where RE is Y, Eu, Tb, Tm. We use time domain thermoreflectance
(TDTR) measurements to characterize the thermal response of the bilayer to
ultrafast optical heating. We use time-resolved magneto-optic Kerr effect
(TR-MOKE) measurements of the Au layer to measure the time-evolution of spin
accumulation in the Au film. We observe a spin Seebeck effect between Au/TmIG
that is three times larger than for an Au/YIG bilayer. The interfacial thermal
conductance between electrons in the Au and magnons in the TmIG layer is ~ 3
$\frac{MW}{m^2 K}$.
