Illuminating Hidden Symmetries in Topological Insulator Thin Films

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 $D_{3\mathrm{d}}$, generated by threefold rotational symmetry about the z-axis $C_{3}$, inversion symmetry $i$, and mirror reflection the x-axis $\sigma_{\hat{\chi}}$. However, direct and indirect measurements of helical photocurrents in Bi3Se3 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.