Measurement And Analysis Of Organic Molecular Solution By Terahertz Time Domain Spectroscopy

Terahertz (THz) spectrum is corresponding to vibration and rotation of organic and biological molecules, which is suitable to identify and study those molecular dynamics. Especially, THz Time-domain spectroscopy(THz-TDS) can enable us to extract the material parameters or dielectric spectrum that show material micro-structure and dynamics by measuring amplitude and phase from coherent terahertz pulses. Furthermore, THz wave is very sensitive to water, through which we can directly detect the water dynamics in hydration layer between water and solute molecules. In this paper, material parameter extraction technology for THz-TDS is investigated. Different regressive calibration methods for THz spectrum quantitative analysis are compared. A THz metamaterial sensor is designed for the concentration measurement of the solution. Mechanism of dielectric response for glucose and sucrose in THz band is discussed. The details of the above research work are listed as follows:1) An innovative optical material parameter extraction algorithm based on waveform rebuilding technology for Terahertz time-domain spectroscopy (THz-TDS) is proposed. Unlike previously reported extraction algorithms in frequency domain, which have to be compromised between amplitude and phase, or the real part and imaginary part of the transfer function, we calculate material parameters by minimizing the difference between the measured sample waveform and the rebuilt one in time domain. Our cost function is more meaningful than those extraction methods in frequency domain, and thus the results are more accurate. The new method can also be used effectively in multilayer transmission mode or in reflection mode.2) THz Spectrum of edible oil is quantitatively analyzed by Support Vector Regression(SVR). The terahertz absorption spectrum of edible oil in 30 samples is measured by THz-TDS, and they are randomly divided into training set and testing set. Regression models for oil's oleic acid concentration, acid number and peroxide value are built by SVR algorithm and reference values from standard methods with training set, then the values of acid concentration, acid number and peroxide value in testing set are predicted by those regressive models. The results show that the models by SVR have significant advantage compared with traditional Partial Least Square(PLS) method.3) Hydrated water parameters in glucose and sucrose solutions such as the number and the radius of the hydrated water molecule are estimated by Onsager's molecular polarization theory for the case of a time-varying field in THz band. By THz dielectric spectrum of glucose and sucrose solutions it is confirmed that the relaxation responses of hydrated water is slowed down to GHz or MHz with its resonant response unchangeable, which enable us to calculate the number of hydrated water in solutions. At the same time, the dielectric spectrum of hydrated water is calculated. This is very important for monitoring the hydration dynamically.4) The feasibility of the solution's concentration sensor by THz metamaterial is evaluated by Finite Element Method, and a sensor with a double golden finger structure on a flexible PCB is designed and tested by THz-TDS. It is shown that there are sharp resonant peaks in reflection coefficient which are shifted with solution concentration changing. The sensitivity is relative with cell structure and the number of layers in THz metamaterial, which is worth further research.