  Theory of optical absorption of linearly and circularly polarized light in
surface and edge states in topological insulators is developed for a nonlinear
in light intensity regime. The absorbance for surface states and the absorption
width for the edge states both decease as $1/\sqrt{I}$ 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 in 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 that the energy
relaxation rate. The linear dichroism - a dependence of the absorption length
on the polarization plane orientation - is studied at arbitrary light
intensities. We show that a degree of the 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
$\propto \sqrt{I}$ at high intensities. The variation of the photocurrents with
both polarization and intensity are strongly dependent on the elastic
scattering rate. The considered effects can be observed in THz frequency range
at laser intensities used in modern experiments.
