  Optical injection and detection of charge currents can complement
conventional transport and photoemission measurements without the necessity of
invasive contact that may disturb the system being examined. This is a
particular concern for the surface states of a topological insulator. In this
work one- and two-color sources of photocurrents are examined in epitaxial,
thin films of Bi2Se3. We demonstrate that optical excitation and terahertz
detection simultaneously captures one- and two- color photocurrent
contributions, as previously not required in other material systems. A method
is devised to isolate the two components, and in doing so each can be related
to surface or bulk excitations through symmetry. This strategy allows surface
states to be examined in a model system, where they have independently been
verified with angle-resolved photoemission spectroscopy.
