We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.

We report on the observation of a terahertz radiation-induced photon drag effect in epitaxially grown nand p-type (Bi 1−x Sb x ) 2 Te 3 three-dimensional topological insulators with different antimony concentrations x varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a dc electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent can be generated in surface states. It arises due to the dynamical momentum alignment by time-and space-dependent radiation electric field and implies the radiation-induced asymmetric scattering in the electron momentum space. We show that the photon drag current may also be generated in the bulk. Both surface states and bulk photon drag currents behave identically upon variation of such macroscopic parameters as radiation polarization and photocurrent direction with respect to the radiation propagation. This fact complicates the assignment of the trigonal photon drag effect to a specific electronic system.

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We demonstrate that the terahertz/infrared radiation induced photogalvanic effect, which is sensitive to the surface symmetry and scattering details, can be applied to study the high frequency conductivity of the surface states in (Bi1−xSbx)2Te3 based three dimensional (3D) topological insulators (TI). In particular, measuring the polarization dependence of the photogalvanic current and scanning with a micrometre sized beam spot across the sample, provides access to (i) topographical inhomogeneity's in the electronic properties of the surface states and (ii) the local domain orientation. An important advantage of the proposed method is that it can be applied to study TIs at room temperature and even in materials with a high electron density of bulk carriers.Electronic, optical and opto-electronic properties of topological insulators (TI) have attracted continuously growing attention yielding challenging fundamental concepts and being of potential interest for novel applications in the fields of spintronics and opto-electronics 1-5 . Hence, the fabrication of high quality topological insulators and their characterization yielding feedback to technologists is of particular importance. Until now a large variety of materials was proposed and confirmed to host topological protected surface states in three-dimensional (3D) TI and edge channels in two-dimensional (2D) TI. Particular examples are (Bi 1−x Sb x ) 2 Te 3 based 3D TI. Their fabrication in view of good insulating properties of the bulk at room temperature, homogeneity of a large area growth materials is still a challenging task. The former problem is caused by the high density of residual impurities serving parallel channels to the surface transport 6-8 . A promising way to overcome this problem serves the recent progress in growth of 3D TI applying molecular-beam-epitaxy (MBE) technique, see e.g., [9][10][11][12]. Owing to the progress in material growth, low temperature electron transport and magneto-transport studies becomes possible providing information on average electronic properties of Dirac fermions and carrier scattering mechanisms in 3D TIs 13-21 as well as to observe the quantum anomalous Hall effect reported for Cr or V doped (Bi 1−x Sb x ) 2 Te 3 based 3D TI 22-25 . An important issue for improvement of the material properties is their characterization allowing insights in the material properties and providing a feedback for technologists. For that a palette of methods has been developed and widely used. An insight into the band structure of the surface states of 3D TIs, especially proof for the single Dirac cone, is obtained by varios modifications of the angle resolved photoemission spectroscopy (ARPES) 26-32 , including spin-resolved and time-resolved ARPES, as well * permanent address: Ioffe Physical-Technical Institute, St. Petersburg, Russia † permanent address: Saint-Petersburg State Polytechnic University, St. Petersburg, Russia as by time-resolved two-photon photoelectron (2PPE) spectroscopy [33][34][35] , with which an enlightening pi...

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