The effect of strong terahertz radiation on the current-voltage characteristics in one-dimensional superlattices is studied using the method of two-time nonequilibrium Green's functions. It is shown that the broadening of discrete spectrum lines due to elastic scattering plays a fundamental role in this problem. A new phonon-free type of photon-induced charge transfer in the hopping mode is predicted. Under certain conditions, the constant components of current and electric field may have opposite directions in this regime. The phonon-free current cannot be described in principle in the formalism of the one-time density matrix and reflects the two-time nature of correlation functions.
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