Efficient terahertz radiation generation from air plasma with mid- and near-infrared three-color femtosecond pulses.

We report on the generation of the terahertz (THz) radiation from air plasma driven by a three-color femtosecond field with the fundamental pulse in the near- (1700 nm and 800 nm) and mid-infrared (3900 nm) ranges. With an inline optical setup for three-color pulse generation and relative phase control, we observed that the THz pulse energy can be enhanced up to 6 times compared with the conventional two-color excitation scheme, dependent on the incident fundamental pulse energy. The polarization of the enhanced THz pulse was found to be elliptical, with the second and third harmonics components of the three-color pump also being elliptically polarized. With the 3.9 µm driving pulses, the maximum THz pulse energy reaches 39.4 µJ with a conversion efficiency of about 1%. Based on the plasma current model, we analytically and numerically reproduced the THz energy enhancement, as well as its dependence on incident pulse energy. The THz enhancement can be attributed to the two additional ionization wave mixing channels introduced with the addition of the third harmonic.