Photogalvanic probing of helical edge channels in two-dimensional HgTe topological insulators

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 approximate to 3 mu m 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.