Unipolar terahertz pulse formation in a nonequilibrium plasma channel formed by an ultrashort uv laser pulse

We perform an alternative approach to produce highly unipolar terahertz pulses. The idea is based on the nonuniform amplification of seed ultrashort carrier-envelope phase (CEP) pulses in nonequilibrium fast relaxing plasma of air or nitrogen. If the gain coefficient drops significantly within the duration of a one-cycle CEP pulse, it undergoes significant distortion where the leading edge of the pulse is amplified and the trailing tail of opposite polarity is absorbed. The obtained results involve a self-consistent solution of the second-order wave equation and kinetic Boltzmann equation for the electron velocity distribution function relaxation.