Research On Material Terahertz Detection Method Based On Finite Difference Time Domain Method

Terahertz time-domain spectroscopy?THz-TDS?is a spectral detection method for extracting the internal information of a material through broadband terahertz pulses carrying medium information.In this thesis,the finite-difference time-domain?FDTD?method is used to study the transmission characteristics of terahertz waves,and a theoretical model of terahertz defect detection is established.The specific waveforms of different defects are analyzed and compared with the actual detection results.The method makes up for the standard library of the defect type characterization.Firstly,in this thesis,the mathematical model of optical parameters is constructed,and the optical parameters of the material are calculated including the refractive index,absorption coefficient and extinction coefficient.Based on the error theory of materials,the system error sources in the process of parameter extraction are analyzed.Secondly,the finite-difference time-domain method is applied to the terahertz non-destructive testing technology.By studying the basic theory of the finite-difference time-domain method,a simulation model of the material is constructed.Take the single dielectric material,debonding,and inclusion defects as examples.Samples were simulated based on the FDTD method,and simulation waveforms for single media and defect detection were extracted.Finally,the real-time flight-time results were collected and analyzed by using time-domain spectroscopy to detect the single dielectric material and the bottom hole defect material.By comparing the simulated thickness calculations with the calculated thickness of the THz-TDS technique,it can be concluded that the thickness errors of the single dielectric material and the defects are within the order of 10^-2,but the error of the simulated thickness is smaller and the thickness reproduction is more accurate.The experimental results prove the feasibility of the finite difference method based on non-destructive testing.At the same time,based on the percentage of the amplitude of the THz-TDS detection to the simulation amplitude,the surface scattering effect of the ceramic matrix composite is better than that of the HDPE material.