Polarization Dependence Of THz Air Biased Coherent Detection And Application Of Polarized THz Waves

Among the various techniques generating THz waves, tilted-pulse-front method involving LiNbO3crystal is developing rapidly, for its high conversion efficiency, high THz-wave intensity and stable performance. This thesis studies the method to optimize the tilted-pulse-front system. Through mathematical calculation, we can obtain the proper grating incident angle, the distance from the lens to the grating and LiNbO3crystal, and the cut angle of the crystal, which can lead to the optimal pump pulse tilt angle in the crystal when building the system. However, in the practical application, due to the misalignments of the system, the pump pulse tilt angle is sometimes suboptimal. Thus, we use finite element simulation to analyze the relation between the pump pulse tilt angle and the THz beam property. The simulation results show that when the pump pulse tilt angle is optimal, the wavefront of THz waves is plane wave like near the vicinity of the crystal surface and the bandwidth is the broadest.Recent research has shown that the polarization state of THz waves provides information on the chiral properties of antiferromagnets and metamaterials. The THz-wave polarization state is also used in measuring the magneto-optical response. The traditional THz-wave polarization measurement involves solid detectors such as electro-optical crystal and photoconductive antenna. The time-resolved polarization can be reconstructed from the THz waveforms measured along two orthogonal directions by rotating a wire-grid polarizer or detector. However, this technique has limited detection bandwidth and intensity ceiling, making it incompatible with ultra-broadband and high power THz sources with versatile polarization control and arbitrary electrical-field control. THz air-biased coherent detection (ABCD) is a promising detection technique for these novel THz sources, due to its superior detection bandwidth and high intensity ceiling. This thesis introduces the principle and method to build and optimize the THz ABCD system. Then we investigate the polarization dependence of THz ABCD, i.e., the relation between the polarization direction of the THz-field induce second-harmonic, the fundamental wave and the DC bias. The experimental results show that due to the plasma birefringence, the polarization state of THz waves cannot be determined by the polarization state of the second-harmonic with the traditional polarization measurement method. Consequently, the orientations of the wire-grid polarizer, DC bias, and FW must all be collinear and rotate together in order to produce accurate orthogonal THz waveforms. The research results also show that the intensity, ellipticity, and polarization angle of the THz-field induced second-harmonic are affected by the air plasma birefringence and can be predicted by cross-phase modulation theory. The DC bias induced second-harmonic also participates in the cross-phase modulation, which will influence the detected polarization state of second-harmonic.In this thesis, we also conducted a comprehensive study on nuclear graphite properties with THz polarization imaging. By imaging nuclear graphite samples in reflection mode at nine different incident polarization angles using THz time-domain-spectroscopy, we find that different domain distributions and levels of porosity will introduce polarization dependence in the THz reflectivity. A sample with a higher density is less porous and has a smaller average domain distribution. As a consequence, it is less polarization-dependent and the maximum polarization frequency is higher. The results also show that samples oxidized at higher temperatures tend to be more polarization dependent. Because polarization-dependent imaging of graphite is related to the pore size and domain distribution, it would provide us with a method to determine the structural features of the graphite as well as new criteria to qualify new graphite grades.At last, this thesis discusses the feasibility of predicting human disease through measuring human sweat gland function with THz waves. In the development of some diseases such as type Ⅱ diabetes, small nerve fiber impairment usually happens in the early stage, leading to a malfunction of the sweat gland, which is measurable. Optical coherence tomography of human skin shows that the structure of human skin is an antenna in THz regime. Thus, we can use THz-wave reflectivity to measure the function of sweat gland. The experimental results shows that the THz-wave reflectivity of human skin will increase during the period of calming down after sport. This indicates that due to the high sweat gland activity, human skin is highly absorptive to the THz waves. Theoretically, Due to the irregularity of sweat, we could quantify the sweat gland function through polarization imaging of human skin.