Measurement And Simulation Study On Terahertz Spectra Of Organics

Terahertz(THz) radiation has the advantages of high resolution, strong penetrability and low photon energy, etc. This technology can be applied widely in security, medical treatment, communications and other fields. Uncooled terahertz detectors have attracted considerable attention due to their low cost, without refrigeration device, and high stability. In general, the absorbing materials exploited in traditional uncooled infrared detectors are inorganic materials that are usually limited by their poor selectivity for incident light, low control, and severe preparation conditions. This makes it inevitable to explore suitable organic materials as the absorbing materials for terahertz detectors. But the vibrational absorbent modes of organic material at the wavelengths of terahertz are very complicated, and up to now, we have only known little about the spectra characteristics of organic material. These strongly limit the development of superior organic THz sensitive materials.In this thesis, we first investigated some simple organic small molecules, and then gradually focused on complex organic materials suitable for applications in terahertz detectors. Combined experimental test with theoretical calculation, the spectral features of organic materials in the terahertz range were systematically investigated. Our main results include:(1) Through studying the THz spectra for six kinds of saturated monohydric alcohols, namely methanol, ethanol, propanol, isopropanol, butanol and isobutanol, we found that the monohydric alcohols with higher hydroxy concentration possess lower average terahertz transmission. In addition, the average terahertz transmissions of linear chain monohydric alcohols are higher than those of the branched chain ones. After compared the simulation calculation of the methanol molecules, we found that the absorption of the methanol in terahertz is attributed to the collective vibrations of different kinds of polymer, and that the polymer for methanol is mainly trimmer.(2) Through investigating the THz spectra for saturated straight chain organic molecules of triacontane, triacontanol and triacontanoic acid, we deduced that different organic functional groups exhibit different terahertz spectral characteristics. The characteristic absorption peaks of triacontanoic acid at 74 cm-1 have been assigned. Notably, red-shift hydrogen bonding(O?H···O) and blue-shift hydrogen bonding(C?H···O) phenomena were disclosed to be not only existed in infrared spectra, also existed in THz spectra. In addition, DFT was employed to simulate the terahertz spectra of saturated straight-chain alkane, alkanol and acid, respectively. And the simulation results of dimer structures meet well with the measured spectra than those simulated from monomer molecule structures.(3) By studying the THz spectra of organic nonlinear optical material of DAST, we experimentally demonstrated that DAST indeed is an excellent THz sensitive material, exhibiting great potential for THz detectors. Combining the simulation of DAST single molecular structure based on DFT, the characteristic absorption peaks of DAST at 66, 98, 135, 158, 214 and 247 cm-1 have been assigned.