Emergence of threefold symmetric helical photocurrents in epitaxial low twinned Bi₂Se₃

We present evidence of a strong circular photon drag effect (PDE) in topological insulators (TIs) through the observation of threefold rotationally symmetric helicity-dependent topological photocurrents using THz spectroscopy in epitaxially-grown Bi₂Se₃ with reduced crystallographic twinning. We establish how twinned domains introduce competing nonlinear optical (NLO) responses inherent to the crystal structure that obscure geometry-sensitive optical processes through the introduction of a spurious mirror symmetry. Minimizing the twinning defect reveals strong NLO response currents whose magnitude and direction depend on the alignment of the excitation to the crystal axes and follow the threefold rotational symmetry of the crystal. Notably, photocurrents arising from helical light reverse direction for left/right circular polarizations and maintain a strong azimuthal dependence—a result uniquely attributable to the circular PDE, where the photon momentum acts as an applied in-plane field stationary in the laboratory frame. Our results demonstrate new levels of control over the magnitude and direction of photocurrents in TIs and that the study of single-domain films is crucial to reveal hidden phenomena that couple topological order and crystal symmetries.

本研究通过采用分子束外延生长的Bi₂Se₃拓扑绝缘体，并利用太赫兹光谱观测到了三重旋转对称的螺旋角依赖性拓扑光电流，从而提供了拓扑绝缘体中强圆光子拖曳效应（PDE）的证据。我们揭示了孪晶域如何通过引入虚假的镜面对称性，引入了与晶体结构内在的竞争性非线性光学（NLO）响应，从而掩盖了与几何敏感的光学过程。通过最小化孪晶缺陷，我们发现了强烈的NLO响应电流，其大小和方向取决于激发与晶体轴的相对取向，并遵循晶体的三重旋转对称性。值得注意的是，来自螺旋光的电流在左右圆偏振下反转方向，并保持强烈的方位角依赖性——这一结果可归因于圆光子拖曳效应的独特性，其中光子动量充当实验室参考系中静止的平面场。我们的研究结果展示了在拓扑绝缘体中对光电流大小和方向的全新控制水平，并强调了研究单晶薄膜对于揭示拓扑序与晶体对称性耦合的隐蔽现象的重要性。