The talk overviews experimental and theoretical studies of terahertz (THz) radiation induced second order opto-electronic phenomena in three dimensional topological insulators (TI). Two kinds of direct photocurrents are discussed: the photogalvanic [1-5] and the photon drag effect .
It will be shown, that photogalvanic spectroscopy, like previously used for study of DF in graphene , is caused in (Bi,Sb)Te based 3D TIs by asymmetric scattering of the Dirac fermions, driven in the applied alternating electric field. It will be shown, that photogalvanic spectroscopy opens up new opportunities for probing Dirac fermions in (Bi,Sb)Te based 3D TIs even in materials with substantial conductance in the bulk. It allows insight into the scattering details and mechanism of high frequency conductivity of the surface states [2,6], can be applied to study carrier dynamics, to map the domain orientation and to verify the homogeneity of the electronic properties of the surface states . An advantage of photogalvanic spectroscopy is that it probes, due to symmetry arguments, only the surface states - even at room temperature, where classical magneto-transport techniques can be hindered by a high bulk carrier concentration. The competing photon drag effect - resulting from the additional transfer of the light momentum to charged carriers - is caused by the dynamical momentum alignment by time and space dependent radiation electric field and includes different scattering probabilities for different half periods of the electromagnetic wave . The nonlinear transport phenomena are discussed in terms of a model, a phenomenological and a microscopic theory. The latest state of the art in this field, possible applications and an outlook will be presented.
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