We demonstrate features of carrier transport, which are based on optical nonlinearity in highly photoexcited GaAs crystals due to spatial redistribution of electron gas heated by THz-frequency ac-field. Ultrafast processes of carrier heating, inter-valley transfer, and field shielding are taken into account in modelling a nonlinear transport in spatially modulated structure. Numerical calculations suggest a possibility to create an efficient space charge wave, that follows an external 1 THz frequency field and exceeds the diffusion field up to 250 times.