  Sub-picosecond ac spin current pulses are generated optically in GaAs bulk
and quantum wells at room temperature and 90K through quantum interference
between one-photon and two-photon absorptions driven by two phase-locked
ultrafast laser pulses that are both circularly polarized. The dynamics of the
current pulses are detected optically by monitoring in real time and real space
nanoscale motion of electrons with high-resolution pump-probe techniques.
