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.
Terahertz(THz) wave modulator technology, due to its important value of imaging and detecting research. In the paper, we discuss the Polyethylene lens based the terahertz wave front modulation, which is benefit the terahertz wave image technology. Simulation results show that the optical system can extent the depth of imaging field of test objects based on continuous terahertz source. The way to get the image has significant meaning for detection and large image quality.