The Physical Mechanism Of Novel Broadband Terahertz Radiation Source

Terahertz science and technology is an attractive research topic nowadays. Theterahertz science has wide range application on communication, sensing, imagingand fundamental science research areas. The broadband terahertz radiation source isnormally achieved by pumping nonlinear optical media with the femtosecond laserpulse, which has attracted considerable interest worldwidely. The dissertationfocuses on the physical mechanism of several broadband terahertz radiation sources.The nonlinear property of medium can be expressed by a nonlinearsusceptibility tensor. In different polarization orientation, the medium experiencesdifferent physical processes. Hence, the reconstruction of complete terahertz vectoris important to understand the physical mechanism. The two-dimensionalpolarization terahertz electro-optics sampling technique is developed in the thesis.The detection crystal is sensitive to the terahertz polarization, which is feasible tomeasure the orthogonal polarized terahertz electric field individually. Based on thenew setup, the accurate and complete terahertz instantaneous electric field can bereconstructed, which is used to vector analyse the nonlinear process. Thetwo-dimensional polarization terahertz electro-optics sampling setup is the keyexperimental tool in this dissertation.Terahertz radiation based on the rectification effect in nonlinear crystal, thathasthe advantage of simple experimental setup and ultra-broadwidth terahertz pulse, iswidely-used terahertz radiation source. However, the nonlinear optics property hasnot been fully investigated, which limits the development of this field. In the thesis,the second order nonlinearity tensor in terahertz region is investigated based on the2D polarization sampling technique. In order to demonstrate the new method, thenonlinearity tensor of BBO (β-Barium Borate) crystal is systematically studied. Thesusceptibility coefficient d22, which has dominamt role in the visible and IR region,can be neglected in terahertz region. It helps to optimize the energy conversionefficiency and phase matching condition in the future research. The terahertz pulse radiated from the air plasma can achieve400kV/cm electricfield and35THz bandwidth. This new type terahertz source has potential applicationon the advanced spectroscopy technique, and fundamental physical research. Thegeneration and detection both in the air plasma, which can be applied in terahertzremote sensing, is also an attractive topic in this field. The terahertz radiation fromfemtosecond laser filamentation shows a great potential in terahertz remote sensing.This type of terahertz pulse source is investigated experimentally in the dissertation.With the2D polarization terahertz time domain spectroscopy, the polarization fromthe filmentation is investigated in the thesis. The ellipitical polarization terahertzpulse from femtosecond filamentaton is first time observed. After the analysis, thefour wave mixing effect is revealed to be the reason for the terahertz pulse radiationfrom filamentation.The terahertz radiation induced by the two-color laser field scheme isconsidered as a reliable high-power, broadband terahertz source. However, theunderlying physical mechanism is not in agreement yet. The mechanism of thismethod is systematically studied. The instantaneous terahertz electric field isreconstructed by the two-dimensional electrooptics sampling method. Our workclarifies and extends the understanding of the four wave mixing model in the2-colorscheme.The linear polarization is first time observed in the experiment, and the crossterm χxyxyis proved to be the most dominant term in the third-order susceptibilitytensor,which is meaningful to improve energy conversion efficiency of the new-typeterahertz radiation source.