| Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool toinvestigation spectroscopic properties of matters at THz frequencies. The invention ofTHz-TDS not only delivers a wide band radiation source in the THz range, but alsofills up the “THz Gap” in spectroscopy. With the development of THz-TDS, studies ofsubstance characteristics have spread into various of fields, such as biology, chemistry,medicine, and security. So the realistic applications of THz-TDS technology havebecome one of the main development trends. By focusing on this task in my PhDwork, several issues on standardized definition of the main parameters to measure thesignal quality of THz-TDS have been pursued for the first time. Based on thestandardized parameters defined here, factors appeared in the THz signal generation,detection, transmission and data acquisition have been studied and their influencerules to the signal qualities have been explored systematically according to theexperiments. By the optimized parameters acquired in former experiments, the THzsignal acquiring and processing parts have been improved to increase the hardwareresource utilization ratio and the effective information extraction ratio. At last, themelamine powder has been characterized using the improved THz-TDS system. Anew absorption peak never been reported has been identified.The main achievements of this PhD work are listed in the following:1. Issues on the definition of three main parameters in THz-TDS have beendiscussed. A method for evaluating the spectral resolution precision has beenintroduced. Two kinds of definition principles, which are using the noise level as thereference and using spectral maximum as the reference, have been proposed. A newconcept of standard signal-to-noise ratio has been defined to evaluate the performanceof different systems quantitatively.2. The quality factors occurred during the generation and detection parts of THz-TDS have been analyzed according to the current surge model, some of whichhave been studied experimentally. The optimized parameters have been acquired fromthe experiments.3. The quality factors occurred in the transmission part have been analyzedaccording to the principles of weak signal detection. The optimized parameters havebeen acquired from the experiments on the chopper and lock-in amplifier.4. The advantages and disadvantages of program flow P1and P2have beencompared. According to the result, an optimum THz signal acquiring program flowand data format have been determined by combining the advantages together. Itsvalidity has been proved by experiment.5. Using the optimum parameters, a new CD552lock-in amplifying circuit hasbeen designed to replace the former THz transmission part which uses the commerciallock-in amplifier SR830. The hardware resource is used more effectively.6. The data acquiring and processing program in the host computer is alsoimproved. The multi-page structure design considering the demands of users atdifferent professional levels makes the interface more user-centered. The addedprogram for spectral characteristics calculation makes the software with highpracticability.7. The melamine powder sample has been tested on the improved THz-TDSsystem. Three peak absorptions, which are at2.00,2.25and2.60THz, respectively, inthe THz range have been observed. The first two pesks have been reported by otherresearchers during their THz experiments while the3rd is not. The3rd spectrum isalso proved to be the essential characteristic of melamine with the help of the softwarefor vibrational and rotational spectra simulation of chemicals used in quantumchemistry. Also, the simulation result indicates that some spectral characters ofmelamine can not be distinguished under the0.05THz spectrum resolution precision,which set a still higher demand of THz-TDS system. |
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