  We propose a method to generate high field terahertz (THz) radiation with
peak strength of GV/cm level in the THz frequency gap range 1-10 THz using a
relativistic laser interaction with a gaseous plasma target. Due to the effect
of local pump depletion, an initially Gaussian laser pulse undergoes leading
edge erosion and eventually evolves to a state with leading edge being step
function. Interacting with such a pulse, electrons gain transverse residual
momentum and excite net transverse currents modulated by the relativistic
plasma frequency. These currents give rise to the low frequency THz emission.
We demonstrate this process with one and two dimensional particle-in-cell
simulations.
