  We apply a pump- and probe-scheme to coplanar stripline circuits to
investigate the photocurrent response of GaAs photoswitches in time and space.
We find a displacement current pulse, as has been reported earlier. A
time-delayed second pulse is interpreted by a transport current. A
time-of-flight analysis allows us to determine the velocity of the
photogenerated charge carriers in the transport current. It exceeds the Fermi
and the single-particle quantum velocities. This suggests that the excitation
of a collective electron-hole plasma and not single charge-carriers dominates
the ultrafast transport current in GaAs photoswitches.
