Preliminary Study Of The Use Of Terahertz Detection Of Contaminants In Water

With economic development, the problem of water pollution has become more prominent. Water pollution is not only constantly challenging the public’s heart endurance, but also a great threat to people’s lives and property safety. This prompted the society to become more and more desired for water pollution monitoring and governance requirements. Nowadays, most water samples have to be taken back to laboratory for testing. These method can prove exact results. But it will take some time to take back and check the samples, and this will give the opportunity to pour out sewage. What’s more, this method take more people, money, and time. So people are looking forward for a new kind of method, which can detect the sewage automatically. The terahertz radiation has several distinctive properties over other electromagnetic ranges. Because of these characteristics, terahertz technology can be a effective method applied in sewage detecting. But there are still some problems, which need to be resolved. For example, how to overcome the problem that water can absorb terahertz wave intensely? Additionally, the algorithm for extracting optical parameters of the THz-TDS in the reflection modes is complicated. Whether there is a new method with simple algorithm can be found in terahertz gap? Whether a new method for real-time automatic monitoring of water pollution with terahertz wave can be achieved? Whether a terahertz band sensor can be designed and make it more suitable for liquid measurement? Based on the above issues, we do the following work:Firstly, the introduction of the background which includes two parts:one is a brief introduction of water pollution, including water pollution situation at home and abroad, the major pollutants in the water as well as the existing water quality testing methods; the one is a brief introduction of terahertz technology, including the methods for THz wave generation and detection, the characteristics of terahertz time-domain spectroscopy system and the development of terahertz technology at home and abroad.Then, the terahertz time domain spectroscopy system is employed to detect8kinds of herbicides,13kinds of fertilizer, and2kinds of plasticizers. What’s more, the refractive index and the absorption coefficient in terahertz band are also obtained. Two herbicides (Tribenuron-methyl and Sodium2,4-dichlorophenoxyacetate) are found to have characteristic absorption peak in the terahertz band. Three kinds of Glyphosate-sodium hydrosol with different concentration are found to have different absorption coefficient and refractive index in terahertz band. The plasticizer content in plasticizing agent with sesame oil mixture can also be detected using terahertz time-domain spectroscopy data.13kinds of fertilizer samples are not found to present characteristic absorption peak in the terahertz band. These optical data of contaminants from water in terahertz band have laid a good foundation for future research on water pollution. For the convenience of the query, a database of optical properties of water pollutants in terahertz band was establish based on the measured data.What’s more, in order to overcome the strong absorption of terahertz wave by water, a angled reflection-type terahertz time-domain spectroscopy system was structured. To obtain the best signal-to-noise ratio and a higher-resolution, the effects of reflection-type system configuration parameters, such as the incidence angle, the refractive index of the sample cell, the thickness of the sample cell and the sample cell material to terahertz single were researched. The incidence angle are found to affect the energy distribution of the terahertz wave before and after the both surfaces of the reflection peak, while the thickness and refractive index of the sample cell will affect the position of the rear surface reflection peak. Since the time window width between the front surface reflection peak and system echo is determined, the best spectral resolution can be achieved by setting the reflection peaks of the rear surface just at the middle of the time window. According to the theoretical calculations and experimental results, the plastic Petri dishes are the best choice to be used as the sample pool. Distilled water was tested by reflection-type system, and the refractive index and absorption coefficient of the distilled water in terahertz band were calculated, and were compared with the results in the literature, to validate the experimental methods and algorithms are feasible. Thereafter, three different concentrations aqueous solution of sodium2,4-dichlorophenoxyacetate were detected and the data were processed. There is no absorption peak found as the solid state sodium2,4-dichlorophenoxyacetate. We speculate that this is because in the process of dissolution, the interaction of the water molecules and the sodium2,4-dichlorophenoxyacetate destroy the lattice structure and the oscillation mode of the solid sample.Although the use of reflection-type terahertz time-domain spectroscopy system can obtain the information of the aqueous solution in terahertz band, but its data processing are more complex, and not conducive to the realization of the real-time monitoring. Therefore, the grating structure based on Rayleigh abnormal effects in terahertz band are chose to be designed as refractive index sensor. The effect of the duty cycle, cycle, environmental refractive index to vertical reflection grating Rayleigh abnormal response are studied by theoretical modeling and experimental studies to select the most suitable parameters for refractive index sensor. Three liquid samples (alcohol, water, and carbon disulfide) are monitored with the refractive index sensor. The experimental results and theoretical simulation results agree very well, which verifies the reasonableness of the experimental sets. At the same time, the experimental peak calculated on the basis of refractive index data agrees well with the data in literature reports. This proves that the experimental method is effective and reasonable. Thus provide a new way to achieve real-time automatic monitoring of water quality with the terahertz technology.Finally, the paper concluded the works presented in this thesis and pointed out the future works in this subject.