  We present energy-momentum mapping of surface Dirac photocurrent in the
topological insulator Sb$_2$Te$_3$ by means of time- and angle-resolved
two-photon photoemission spectroscopy combined with polarization-variable
mid-infrared pulse laser. It is demonstrated that the direct optical transition
from the occupied to the unoccupied part of the surface Dirac-cone permits the
linear and circular photogalvanic effect which thereby enables us to coherently
control the surface electric-current by laser polarization. Moreover, the
surface current mapping directly visualizes ultrafast current dynamics in the
Dirac cone in the time domain. We unravel the ultrafast intraband relaxation
dynamics of the inelastic scattering and momentum scattering separately. Our
observations pave the pathway for coherent optical control over surface Dirac
electrons in topological insulators.
