Terahertz Time Domain Spectroscopy of Anisotropic Systems: techniques, analysis and applications

Abstract

<p>The method of terahertz time domain spectroscopy (THz-TDS) allows determination of the broadband electromagnetic response of materials over a typical spectral region of 0.3 to 3 THz. Past work in this field has generally been dominated by the investigation of isotropic materials and the significant potential of using THz-TDS to study anisotropic materials has generally be under utilized. This work encompasses several stages on the path to develop THz-TDS for anisotropic material studies. First, the design, construction and optimization of a high sensitivity THz-TDS polarimetry setup is shown. The extinction ratio between electromagnetic field components in this setup is on the order of 105:1, over two orders of magnitude better than the typical 103:1 found in previous studies. Birefringent and dichroic polymer and crystalline materials are then studied to show the utility of the polarimetric system when compared to typical THz-TDS systems. To further the goal of developing broadband THz optics, a method for designing and constructing achromatic waveplates is discussed. Using this method, a proof of concept waveplate is fabricated and shown to induce an ellipticity of at least 0.75 over the THz bandwidth of 0.75 to 2.25 THz. Discussion then shifts to the field of gas spectroscopy using THz-TDS, and the THz absorption spectra of water, methanol, ethanol, isopropanol and acetone are measured. The measured water spectrum shows evidence for the formation of a tertramer of water molecules, which has a collective vibrational mode around 2.05 THz. The thesis concludes with a discussion of possibilities for measurements of circular dichroism using THz-TDS.</p>