Study On Measurement Techniques Based On Terahertz Time Domain Spectroscopy

Terahertz time domain spectroscopy(THz-TDS) is used in the terahertz spectroscopy of material,material identification and quantitative analysis of material.Exfoliation of THz spectra is also studied.The terahertz spectra of some common pesticides are obtained by THz-TDS. Quantum chemical calculation is employed to analyze the THz spectra of pesticides theoretically. Pattern recognition is applied to process the THz spectra of material in order to classify the material.Correlation analysis,linear regression and partial least square(PLS) method are adopted to quantify material based on their terahertz data.Gauss function and Lorentz function are used to exfoliate the THz spectra of mixture.The major contributions in dissertation are as follows:(1) THz-TDS is used to measure the terahertz time domain waveform of seven common pesticides.The frequency spectroscopy,absorption and refractive spectra are obtained from the time waveform.The results show that the seven pesticides have obvious different characteristics in their absorption spectra.This study provides experimental results for the construction of fingerprint spectra database of pesticides and for deep understanding of the conformation and configuration of the pesticides.(2) In order to better understand the experimental spectra of the pesticides,quantum chemical calculation software Gaussian03 is applied to simulate the vibrational modes of dimethoate molecule and methomyl molecule.The vibrational modes of these two molecules in terahertz region are calculated by Becke-3-Lee-Yang-Parr(B3LYP) algorithm of density functional theory.The theoretical data are compared to the experimental absorption spectra.The results show that the theoretical calculations are in good agreement with the experimental data. Simultaneously,the results verify the experimental absorption characteristics of the investigated pesticides.According to the results from density functional calculations,the origin of the observed vibrational frequencies are interpreted as the intra-molecular and inter-molecular vibrations with the aid of GaussView 3.07.After further analysis,the intra-molecular vibrations are assigned as the collective vinrational modes and the torsion modes of the skeleton.(3) A method to identify textile fiber is proposed based on terahertz time domain spectroscopy.Principle component analysis is used to select their feature.Then the Mahalanobis distance discriminant method is employed to classify them.The experimental results show that the fiber can be well classified according to their terahertz response.(4) A method to classify tea is put forward based on terahertz time domain spectroscopy. Using the absorption spectra as feature,tea are classified by support vector machine.The results show that tea can be accurately classified without using complicated feature extraction.This indicates that the THz spectra of tea can well reflect their characteristics.Simultaneously,it indicates that support vector machine is appropriate to process the THz data of materials.(5) Correlation analysis,linear regression and partial least square(PLS) method are used to analyze the THz spectra of tartaric acid of different concentration with the aim of predicting the concentration of the investigated tartaric acid.Two methods are proposed.First,correlation analysis is applied to select the absorption coefficient at the most effective frequency.Then linear regression is employed to predict the concentration.Second,the THz spectra at a certain frequency region of tartaric acid are processed by partial least square(PLS) method. Leave-one-out is used to conform how many principle components are used in the PLS analysis. These two methods can both predict the concentration accurately,and are possible to be widely used in the practical applications.(6) A method to mesure the basis weight of paper sheet is studied based on terahertz time domain spectroscopy.Two principles are proposed.One is based on the amplitude attenuation and the other is based on delay time.Experiments are carried out to verify which one is better to characterize the basis weight of paper sheet.The experimental results show that delay-time-based method is better than amplitude-attenuation-based method in terms of linearity and accuracy.The research still shows that terahertz technique can be applied in the quantitative analysis of many dry nopolar materials and it is a promising technique for practical applications in the quantitative analysis of these materials.(7) Exfoliation and simulation of THz spectra are investigated.Gauss function and Lorentz function are used to exfoliate the THz spectra of the mixture of tartaric acid and L-ascorbic acid.The research shows that Lorentz function is more suitable to exfoliate the THz spectra.This result indicates that the Lorentz model is more appropriate to simulate THz spectra of compounds.