Abstract

Topological insulators (TIs) such as Bi₂Se₃ host topological surface states [1], where spin-momentum locking engenders a helicity-dependent photoresponse [2,3] and crystal symmetries are encoded in nonlinear optical (NLO) responses under strong optical stimulation [4]. A perfect Bi₂Se₃ crystal obeys the point group D_3d, generated by threefold rotational symmetry about the z-axis C₃, inversion symmetry i, and mirror reflection the x-axis ${\sigma }_{\widehat{x}}$. However, direct and indirect measurements of helical photocurrents in Bi₂Se₃ thin films have until now only displayed weak to no threefold symmetry due to the presence of twinned domains that impart an effective sixfold symmetry. We report on new low-twinning thin film growth that realizes for the first time the emergence of clear threefold symmetries in the azimuthal dependence of topological photocurrents, which are attributed to a measurable photon drag effect (PDE) by NLO analysis of intrinsic crystal symmetries.

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