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 ∼5 mm long plasma created by focusing a 800 nm beam and its second harmonic into nitrogen gas, a peak amplification factor of about 65% is measured with total optical excitation intensity of 8x1014 W/cm2. The amplification effect occurs within a time scale of less than 400 fs 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