Investigation of fingerprints for small polar molecules by using a tunable monochromatic THz source

Over the past 20 years, considerable efforts have been dedicated to the generation and the application of electromagnetic waves in the Terahertz (THz) regime. Among all of the proposed applications, the THz spectroscopy is probably the most mature and promising one. According to the previous reports, the THz spectroscopy has been extensively applied into many analysis fields, including the investigation of vibrational modes for the crystalline solids, the characterization of electron transport in the condense matters and the identification of explosive materials at a standoff distance. More interestingly, since most gas phase chemicals exhibit unique transition peaks in the THz spectra, one could in principle achieve highly accurate molecular fingerprinting and chemical sensing as well. However, all of the practical THz spectroscopy applications were still greatly hampered by the lack of suitable sources and detectors.;In this thesis, a unique approach to measure the THz spectrum is developed based on a novel tunable narrowband source. Unlike the previous THz systems, high power THz pulses were generated by the difference frequency generation processes between two collinearly propagated near infrared laser beams. To tune the output THz signal frequency, one can simply adjust one of the incident beam frequencies. Therefore, based on a convenient wavelength tuning scheme, the transmission spectra can be measured for a series of polar gases with either similar or distinct molecular structures. According to the measured spectra, it is found that the obtained transition frequencies, absorption intensities and molecular constants are all in good agreement with the theoretical results tabulated in the molecular spectroscopic databases, such as the HITRAN database. By further analyzing the transition frequencies, it is also discovered that one can confidently identify each polar molecule and differentiate between various isotopic variants based on their characteristic molecular parameters (rotational constants). Compared to the commercially available FT-IR system, we have found that this technique has reached the same measurement accuracy.;To quantitatively analyze the gas mixtures, we have measured the THz transmission spectra for two mixture samples (12CO/1 3CO, HBr/HCl) as well. Based on the Beer - Lambert Law, the partial pressure of the mixture constituent has been determined by comparing the transition peak absorption intensities. Considering the pressure broadening effect, further analysis on the detection limit of our tunable THz source is also provided.;Compared to the previous techniques, this novel approach has demonstrated several advantages, including high accuracy, simplicity, robustness, larger dynamic range and longer lifetime. Therefore, it is believed that our tunable THz source has a great potential in the THz molecular spectroscopy and chemical sensing applications.