Investigation Of Some Kinds Of Material By THz Time-domain Spectroscopy

Since THz spectra (emission, transmission and reflection) of material contain abundant physical and chemical information, the spectra response in THz range is significant to the study on structure of material. In the work, we used terahertz time-domain spectroscopy (THz-TDS) technology to study the chemical process that Ag₂O thermal decomposes to Ag; chlorophyll protein 43 and 47 (CP43, CP47) denaturation induced by the treatment of guanidine hydrochloride, light irradiation and heating; the coupling of surface plasmon wave to free-space radiation beyond the interaction zone via random metallic particles and polarization dependent enhanced transmission of THz radiation through one-dimensional metallic cylindrical gratings. Additionally, the efficient generation of subpicosecond terahertz radiation by nonlinear difference frequency effect in a LiNbO₃ crystal was studied.1. We studied the generation of subpicosecond terahertz (THz) electromagnetic radiation in a nonlinear electric-optical crystal LiNbO₃. Changing the pump power, the intensity of THz wave changes, it should be a quadratic linear relationship. But the deviation turns up for the absorption of LiNbO₃ crystal causing by the decaying of TO phonon-polaron disresonance into two acoustics phonons. By cooling the crystal, we decrease the absorption and increase the efficiency. 2. The chemical process that Ag₂O thermal decomposes to Ag has been observed experimentally using a THz-TDS. SEM is used to evaluate the particles'size and distribution and the colloidal Ag particles have strong reciprocity and turn to be uniform, as the temperature rising with the power of fs-pulse laser beam. We find the relative percentage total power transmission in different decomposing state shows a nonlinear dependence on the heating power of the laser. Optical absorption spectra of the different specimens are studied in the frequency range 0.3–1.4 THz. The data are analyzed by the relevant theory of the effective medium approach combined with the Drude-Lorentz model. The analysis suggests that optical properties stem from Drude for metallic phase variation and Lorentz term for insulator Ag₂O in the complex system.3. Thermal decomposition of Ag₂O thin layer generates optical near-field, both propagating and evanescent field have been achieved using terahertz time-domain spectroscopy (THz-TDS) with or without a GaAs substrate. We also used scanning electron microscope (SEM) to evaluate the particle size and distribution.4. The application of terahertz time-domain spectroscopy to discriminate protein conformation change of the chlorophyll protein CP43 and CP47 induced by the treatment of guanidine hydrochloride, light irradiation and heating. The results indicate that THz transmission spectroscopy could be used for monitoring protein denaturation and associated conformation change processes in a feasible and effective way.5. We experimentally discovered that the efficient coupling of surface plasmon wave to free-space radiation beyond the interaction zone via random metallic particles. Coupling was achieved for focusing the terahertz (THz) wave to the sub-wavelength particles and the THz spot size is less than the cross section of the sample. The enhanced transmission amplitude induced by delocalized plasmon decreased as augmenting the noninteraction zone of the particles, even total absorption is observed. Absorption in the particles beyond the irradiation zone indicates that the surface plasmon wave can spread along the surface of particles, thus quenching optical emission .This phenomenon in random metallic particles opens a way to realize SP-based photonic switch and more possible applications are under-investigated.6. An experiment is performed of one-dimensional metallic cylindrical gratings for P-polarized and S-polarized incident THz radiation. We fabricate two kinds of gratings by brass rods and the size of the rods is chosen as: 150μm and 200μm respectively, which are both smaller than the radiation wavelengthλ=500μm. Sharp resonances and enhanced transmission are observed. Polarization dependence of transmission spectra show that the combined effects: surface plasmon resonances and cavity resonances generate the transmission enhanced. These results are in good agreement with the theoretical calculation by FDTD.As mentioned above, we did experiments in a creative way at Ag₂O thermal decomposing to Ag and chlorophyll protein denaturation by using THz time-domain spectroscopy. Referenced to the results obtained by other researching technology, our data not only gave the property of material in THz wave band, but also indicated that THz transmission spectroscopy can be used for monitoring changing process of material in a feasible and effective way. Moreover, as to the popular sub-wavelength system, some new phenomena were found by using THz spectroscopy to study the metallic particles and gratings as well as some theoretical work. In addition, THz time-domain spectroscopy will provide a new research way in the field of groundwork, industrial application, iatrology, military affairs and biology.