Terahertz (THz) wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. The most popular and efficient laser-plasma scheme used for THz generation is the two-color scheme, in which a laser pulse at the fundamental frequency is supplemented by its second harmonic that is obtained with the use of a nonlinear crystal. The type-I β-barium borate (BBO) crystal plays a very important role in second harmonic generation. In this research, we investigate the THz generation efficiency with changing the thicknesses of the BBO crystals. Moreover, the wavelength of the excitation laser is tunable from 1200 nm to 1600 nm. The THz generation efficiency is characterized by rotating the BBO crystal with the same tilt angle, changing laser wavelength with invariant pump power. And we also record the two orthogonal components of THz electric field by rotating the ZnTe crystal. We think that the thickness of BBO crystal affects the phase difference between the two components along the ordinary axis and extraordinary axis, resulting in the change of the polarization state of the fundamental wave. Meanwhile, the frequency doubling efficiency of BBO has an impact on the power ratio of the two-color laser. This provides a practical way to control the polarization of THz pulses for potential applications.
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.