We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from a right-handed to a left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a ≈3 μm distance at CNP approaches 2e 2 /h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with experimental data. DOI: 10.1103/PhysRevB.95.201103 The quantum spin Hall (QSH) effect occurs in 2D TIs and rests on the existence of conducting helical edge states while the bulk is insulating [1][2][3][4]. In contrast to the quantum Hall effect, the formation of these edge states requires no magnetic field: they stem from the band inversion caused by strong spin-orbit interaction and are topologically protected by time reversal symmetry. Given that the spin-up and spin-down electrons propagate along an edge in opposite directions, i.e., the spin projection is locked to the k vector, the edge channels are helical in nature. The first experimental evidence for the QSH effect was obtained in HgTe quantum wells (QWs) [5] by observing a resistance plateau around h/2e 2 in the longitudinal resistance of a mesoscopic Hall bar. Here h is Planck's constant and e is the electron charge. This observation was further confirmed by nonlocal experiments in the ballistic [6] and diffusive [7] transport regime. Conducting edge channels were later probed by scanning SQUID microscopy [8], scanning gate microscopy [9], microwave impedance microscopy [10], and by analyzing supercurrents [11]. The spin polarization of the edge states was investigated so far by electrical means only: by detecting the spin to charge conversion in devices utilizing the inverse spin Hall effect [12] or with ferromagnetic contacts [13].Here we use circularly polarized terahertz radiation to excite selectively spin-up and spin-down electrons circling clockwise and counterclockwise around a sample. We show that the excitation causes an imbalance in the electron distribution between positive and negative wave vectors. This is probed as the associated photogalvanic [14,15] current, which reverses its direction upon switching the helicity.The experiments have been carried out on Hg 0.3 Cd 0.7 Te/HgTe/Hg 0.3 Cd 0.7 Te single QW structures with a well width of 8 nm having inverted band ordering. We used this width to maximize the energy gap to about 25 meV [4,5]. Structures were grown by molecular beam epitaxy o...

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