Terahertz Radiation Generation, Detection, And Terahertz Time Domain Spectroscopy Of Molecules And Solids

Terahertz (THz, 1THz=1012Hz) radiation, which occupies a large portion of the electromagnetic wave, is in the range between the far-infrared and microwave region. In the last decade, rapid progress in free electron laser and ultrafast laser technology provides a steady and reliable optical source for the THz pulse generation. This greatly promotes the research in THz technique, which has attracted much attention from many research groups both in fundamental and applied fields.Chapter one introduces the development process and research actualities of THz radiation. THz radiation characters are also described. In Chapter two, photoconduction and optical rectification for producing THz radiation are theoretically explained and two detection methods, photoconductive sampling and free-space-electro-optic sampling, are also illuminated. We especially emphasize the description of THz experimental set up and make a detailed statement for the operation of our experimental set up with some analyses for controllable factors that affect the generation and detection of THz electromagnetic field. In Chapter three, we study the optical and dielectric response of GaAs, single crystal diamond, polycrystalline CVD diamond and fused quartz by THz time-domain spectroscopy over the frequency range from 0.2 to 1.6 THz at room temperature. The refractive index; dielectric constants and transmission properties of these solid samples are obtained. Synchronously, classical Lorentz model is used to certificate and simulate our measured results for GaAs refractive index and absoption coefficient in THz region. The THz spectra of some large biomolecules such as polycrystalline glucose, polycrystalline maltose monohydrate and polycrystalline lactose monohydrate have also been measured using THz time-domain spectroscopy. The low frequency vibrational modes of these molecules are assignend based on a quantum chemical calculation. We interpret tentatively the origin of the observed resonances absorption peaks. In Chapter four, vibrational spectra of 2,4-dinitrotoluene molecule, in THz region has been calculated using DFT method based on different basis sets. The study is focused on solvent effect by using polarizable continuum model (PCM) for this molecule in different solutions, for example, toluene, ethanol and water. The results suggest that the diffuse and polarization functions are necessary in the calculations and the solvent molecule has a significant influence on the geometric structure of 2,4-dinitrotoluene molecule. In Chapter five, we study experimentally the interaction between high-intense, ultrashot laser pulses and air which produces plasma for generating the emission of coherent, short-pulse THz radiation. This THz radiation can be generated by focusing either an ultrashort fundamental-wave laser pulse in 800 nm or an ultrashort laser pulse composed of both the fundamental-wave and its second-harmonic-wave into air. We measured the spatial pattern of the THz radial emission due to the ponderomotive forces. The dependence of the THz output on some experimentally controllable parameters such as the applied external electric field, the relative phase of the fundamental-wave and its second-harmonic-wave, is also studied. Finally, in Chapter six we give a summary of this thesis work and discuss the challenges and opportunities of THz technology in future studies.