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We observe enhanced three-fold symmetric helicity-dependent topologicalphotocurrents using time-domain THz spectroscopy in epitaxially-grown Bi2Se3with reduced crystallographic twinning. It is established how twinned crystaldomains introduce competing responses that obscure inherent nonlinear opticalresponses of the intrinsic crystal structure. Minimizing this defect revealsstrong nonlinear optical response currents whose magnitude and direction dependon the alignment of the excitation to the crystal axes and follow thethree-fold rotational symmetry of the crystal structure. Notably, the azimuthaldependence of the photoresponse persists for helical excitations -- anunprecedented result we attribute to the photon drag effect, where the photonmomentum acts as an applied in-plane field that is stationary in the laboratoryframe. Additionally, the sign of the resultant THz signal inverts when thehelicity of incident light is switched from right to left circularly polarized,indicating a reversal of the photocurrent. Our results demonstrate that evenextended domain defects can obscure intrinsic physical processes, making thestudy of single domain thin films crucial to the observation of phenomena thatcouple topological order and crystal symmetries.

Emergence of Three-fold Symmetric Helical Photocurrents in Epitaxial Low Twinned Bi$_2$Se$_3$