Nonlinear optical absorption and photocurrents in topological insulators

Theory of optical absorption of linearly and circularly polarized light in surface and edge states in topological insulators is developed for a nonlinear light intensity regime. The absorbance for surface states and the absorption width for the edge states both decrease as 1/???I at high light intensity I. Elastic scattering of the photoexcited electrons and holes is taken into account. The absorption bleaching at high intensity is shown to be strongly suppressed by the elastic scattering. The linear-circular dichroism of one- and two-photon absorption in surface states, emerging in the nonlinear intensity regime, decreases due to the elastic scattering of photocarriers. Absorption in the edge states of two-dimensional topological insulators also depends on the elastic scattering rate being suppressed when it is stronger than the energy relaxation rate. Linear dichroism???the dependence of the absorption length on the polarization plane orientation???is studied at arbitrary light intensities. We show that a degree of linear dichroism is governed by the ratio of the elastic and inelastic relaxation times. The photocurrents generated in the edge states by both linearly and circularly polarized light change their intensity dependencies from the linear one at low I to the ??? ???I at high intensities. The variation of the photocurrents with both polarization and intensity is strongly dependent on the elastic scattering rate. The considered effects can be observed in the terahertz (THz) frequency range at laser intensities used in modern experiments.