Intense few cycle terahertz pulses exhibit complex non-linear behavior under interaction with heavily n-doped Si. Fast increase in the transmission of a 700 fs pulse (central frequency 1.5 THz) through the Si sample (low field transmission of 0:02 %) saturates at 8 % for the external field of 5 MV/cm and then drops twofold at 20 MV/cm. An electro-optical sampling measurements revealed formation of a single cycle terahertz pulse at this field due to formation of a thin ionized layer by the first intense oscillation of the terahertz field.
