  Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy
with in-situ reflection high-energy electron diffraction. The films are shown
to be high-quality by X-ray reflectivity and diffraction and atomic-force
microscopy. Quantum interference control of photocurrents is observed by
excitation with harmonically related pulses and detected by terahertz
radiation. The injection current obeys the expected excitation irradiance
dependence, showing linear dependence on the fundamental pulse irradiance and
square-root irradiance dependence of the frequency-doubled optical pulses. The
injection current also follows a sinusoidal relative-phase dependence between
the two excitation pulses. These results confirm the third-order nonlinear
optical origins of the coherently controlled injection current. Experiments are
compared to a tight-binding band structure to illustrate the possible optical
transitions that occur in creating the injection current.
