  One of the key challenges for spintronic and novel quantum technologies is to
achieve active control of the spin angular momentum of electrons in nanoscale
materials on ultrafast, femtosecond timescales. While conventional
ferromagnetic materials and materials supporting spin texture suffer both from
conceptional limitations in miniaturization and in efficiency of optical and
electronic manipulation, non-magnetic centrosymmetric layered materials with
hidden spin polarization may offer an alternative pathway to manipulate the
spin degree of freedom by external stimuli. Here we demonstrate a novel
approach to generate transient spin polarization on a femtosecond timescale in
the otherwise spin-unpolarized band structure of the centrosymmetric 2H-stacked
group VI transition metal dichalcogenide WSe$_{2}$. Using ultrafast optical
excitation of a fullerene layer grown on top of WSe$_{2}$, we trigger an
ultrafast interlayer electron transfer from the fullerene layer into the
WSe$_{2}$ crystal. The resulting transient charging of the C$_{60}$/WSe$_{2}$
interface leads to a substantial interfacial electric field that by means of
spin-layer-valley locking ultimately creates ultrafast spin polarization
without the need of an external magnetic field. Our findings hence open a novel
pathway for optically engineering spin functionalities such as the
sub-picosecond generation and manipulation of ultrafast spin currents in 2D
heterostructures.
