High‐Frequency Nonlinear Transport and Photogalvanic Effects in 2D Topological Insulators

Excitation of a topological insulator by a high‐frequency electric field of a laser radiation leads to a dc electric current in the helical edge channel whose direction and magnitude are sensitive to the radiation polarization and depend on the physical properties of the edge. An overview of theoretical and experimental studies of such edge photoelectric effects in 2D topological insulators based on semiconductor quantum wells is presented. First, a phenomenological description is given of edge photocurrents, which may originate from the photogalvanic effects or the photon drag effects, for edges of all possible symmetry. Then, microscopic mechanisms of photocurrent generation for different types of optical transitions involving helical edge states are discussed. They include direct and indirect optical transitions within the edge channel and edge‐to‐bulk optical transitions. Excitation of a topological insulator by polarized laser radiation leads to a dc spin‐polarized electric current in the helical edge channels. The article presents an overview of theoretical and experimental studies of such photocurrents originating from edge photogalvanic effects and photon drag and discusses underlying microscopic mechanisms for different types of optical transitions involving topological edge states.