THz Time-domain Spectroscopy Of Opaque Materials

With the development of 6G technology,we have entered the terahertz(THz)com-munication era.This requires a clear understanding of the THz spectral responses of ma-terials.In the past 20 years,the linear spectral properties of many materials in THz band have been measured by THz time-domain spectroscopy.However,the "position error" in THz time-domain reflection spectroscopy limits the study of "optically thick" materials(such as high dielectric ceramics)in THz band.On the other hand,the lack of strong THz sources also limits the study of THz nonlinear spectra.Therefore,we focus on the THz linear spectral measurement of "optical thick" materials and the nonlinear spectral response of the materials under the strong THz waves.We have preliminarily solved the measurement problem of the "optical thick" material by THz time-domain spectroscopy,and studies the responses of several materials under 500 kV/cm strong THz field.The major results of this thesis are outlined as following:(1)The "position error" in THz time-domain reflection spectroscopy have been stud-ied.We analyzed the key factors on the results' accuracy and precision in THz time-domain reflection spectroscopy in detail,and a phase correction method based on ana-lytic continuation is proposed,which reduced "position error" to the order of 0.1?m.The response of PMN-PT with different poled degrees to THz waves was studied,which reflected the ferroelectric domain size in the materials.We studies the properties of Sn1-x(Zn1/3+Nb2/3)xO2 in THz frequency,which is a material with high dielectric and low loss in microwaves,and found two Debye relaxations in THz frequency band.(2)We designed a THz time-domain ellipsometry with simultaneous polarization measurement.The system significantly suppressed the common mode noises of the laser,and the reliable frequency range was extended to the low frequency and high frequency bands with lower dynamic range of the THz waves generated from the photoconductive an-tenna.At the same time,the "position erroyr" of THz time-domain reflection spectroscopy was canceled,and effectively improved the experimental efficiency.(3)A strong THz system based on the tilted pulse front(TPF)technique in lithium niobate was constructed,and the responses of several materials were studied under the strong THz field.We compared the THz pulse energies,beam shapes and focal spots of the strong THz system in detail,and measured the THz peak field strength as high as 500 kV/cm.We also analyzed the factors that affected the THz radiation intensity,and proposed a scheme to further enhance the radiation intensity.Then the responses of graphene,PZT ferroelectric thin-films,z-cut and x-cut lithium niobate crystals under strong THz electric field were studied,and the experimental phenomena were preliminarily explained.