  We observe enhanced three-fold symmetric helicity-dependent topological
photocurrents using time-domain THz spectroscopy in epitaxially-grown Bi2Se3
with reduced crystallographic twinning. It is established how twinned crystal
domains introduce competing responses that obscure inherent nonlinear optical
responses of the intrinsic crystal structure. Minimizing this defect reveals
strong nonlinear optical response currents whose magnitude and direction depend
on the alignment of the excitation to the crystal axes and follow the
three-fold rotational symmetry of the crystal structure. Notably, the azimuthal
dependence of the photoresponse persists for helical excitations -- an
unprecedented result we attribute to the photon drag effect, where the photon
momentum acts as an applied in-plane field that is stationary in the laboratory
frame. Additionally, the sign of the resultant THz signal inverts when the
helicity of incident light is switched from right to left circularly polarized,
indicating a reversal of the photocurrent. Our results demonstrate that even
extended domain defects can obscure intrinsic physical processes, making the
study of single domain thin films crucial to the observation of phenomena that
couple topological order and crystal symmetries.
