Measurement And Extraction Algorithm Of Complex Dielectric Constant For Materials In The Terahertz Frequency Band

The dielectric properties of materials in the terahertz(THz)band are different from those in the microwave and optical bands.Accurate acquisition of the dielectric constant(optical parameters)of the material is a prerequisite for theoretical calculation and applied research.This thesis focuses on the free electron laser terahertz source device(FEL-THz)under construction at Huazhong University of Science and Technology.The transmittance of vacuum windows(TPX,quartz crystal and HRSi)in THz transmission process was measured accurately,and the complex dielectric constant of the vacuum window material in the terahertz frequency band was extracted by the advanced iterative algorithm.Firstly,the thesis introduces and compares two mature terahertz spectrum measurement techniques: terahertz time-domain spectroscopy(TDS)and Fourier transform infrared spectroscopy(FTIR).The calculation model of complex dielectric constant and commonly used methods for extracting spectral measurement data were reviewed.Secondly,based on two THz time-domain spectroscopy platforms(self-established TDS system and commercial TDS system),the time-domain waveforms and spectral curves of the THz pulse transmitted through the dielectric material were measured,and the sample thickness was optimized.In combination with the derived theoretical formula of field transmittance,the MATLAB processing program was written using the Nelder–Mead simplex algorithm.The complex dielectric constant,refractive index and extinction coefficient of the window material were calculated with the frequency range 0.6 ? 5 THz.The results extracted by the two sets of TDS measurement systems were compared.The relative error is less than 4% when compared with the results at some discrete frequency points reported in the literature.The thesis also conducted algorithm optimization research.Through interpolation zero-filling method,the spectrum resolution was improved and more stable phase was obtained.The actual thickness of the three materials was calculated by the second-order variation method,and the effect of echo pulse in TDS measurement on the extracted pareameters was evaluated quantitatively.Finally,with the known sample thickness and complex permittivity,the power transmission model of the single-layer material under plane wave normal incidence is derived.Combined with the experimental data measured by the laboratory Fourier transform infrared spectrometer(1 ? 8 THz),the complex dielectric constant of the material was obtained using the same core algorithm.The comparision with the TDS method further verifies the correctness of the experimental method and the accuracy of the extraction algorithm.The complex dielectric constant of the vacuum window material obtained through two independent measurement techniques,combined with the structure of the terahertz transmission line and the parameters of the optical elements therein,could be utilized to accurately calculate the absolute power of the THz signal radiated from the downstream mirror of the optical resonator in the FEL facility.In addition,the MATLAB program completed in the thesis will also be used in the laboratory's future THz applied research(spectroscopy and imaging).