For further understanding of the mechanism regarding terahertz (THz) wave generation in laser-induced plasma, the pump wavelength dependence of THz emission is examined when two-color laser fields are mixed in pre-formed plasma created by another 800nm laser pulse. In our experiment, the effect of pre-formed plasma is investigated using an orthogonal pumping geometry. With a pre-formed plasma, the power of THz wave generated by the pump pulse reduces significantly, and the THz modulation increases with the growth of pump wavelength. Possible reason for the result is discussed in terms of tunneling ionization in the THz generation mechanism.
In our experiments, terahertz radiation via two-color generated laser plasma gas targets is studied using nitrogen and the noble gases (helium, neon, argon, krypton, and xenon) as the generation media. Carried out at the infrared beam of the advanced laser light source, we studied the effects of different pump wavelengths (between 1200 nm and 1600 nm) on THz generation. Terahertz pulse energy is measured as functions of input pulse energy, gas species, gas pressure. The experimental results show that the terahertz pulse energy approach a maximum value of 0.0578 μJ per pulse in xenon gas when the input 1600 nm pulse energy is 0.4 mJ per pulse.