  A wide range of disordered materials contain electronic states that are
spatially well localized. In this work, we investigated the electrical response
of such systems in non-equilibrium conditions to external electromagnetic
field. We obtained the expression for optical conductivity valid for any
non-equilibrium state of electronic subsystem. In the case of incoherent
non-equilibrium state, this expression contains only the positions of localized
electronic states, Fermi's golden rule transition probabilities between the
states and the populations of electronic states. The same form of expression is
valid both in the case of weak electron-phonon interaction and weak
electron-impurity interaction that act as perturbations of electronic
Hamiltonian. The derivation was performed by expanding the general expression
for AC conductivity in powers of small electron-phonon interaction or
electron-impurity interaction parameter. Applications of the expression to two
model systems, a simple one- dimensional Gaussian disorder model and the model
of a realistic three-dimensional organic polymer material, were presented, as
well.
