Terahertz wave amplification in gases with the excitation of femtosecond laser pulses

We report the observation of terahertz wave amplification in gases excited by femtosecond laser pulses. When a seed terahertz wave is sent into the ∼5mm long plasma created by focusing a 800nm beam and its second harmonic into nitrogen gas, a peak amplification factor of about 65% is measured with total optical excitation intensity of 8×1014W∕cm2. The amplification effect occurs within a time scale of less than 400fs of the onset of ionization processes due to the optical excitation pulse duration. Four-wave-mixing parametric processes during the plasma formation are proposed to be responsible for the amplification effect.