Source: Dissertation Abstracts International, Volume: 76-07(E), Section: B.

Adviser: Charles A. Schmuttenmaer.

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Terahertz (THz) spectroscopy is a powerful technique for studying a variety of phenomena in the far-infrared region of the electromagnetic spectrum. One area that has yet to be studied by THz spectroscopy is the optical activity of chiral molecules. Molecular optical activity manifests itself as optical rotation (OR), which is the difference in refractive index of left and right circularly polarized light, and circular dichroism (CD), which is the differential absorption of left and right circularly polarized light. Vibrational CD (VCD) has become a standard tool in the infrared; however, it has only become routine in last few decades, mainly due to the fact that optical activity roughly scales with frequency squared. The ratio of VCD to linear absorption is ~10 -4 and in the THz region this ratio will be on the order of 10 -4 to 10-6 with an accompanying transmitted THz pulse OR on the order of 0.01° to 0.001°.

THz spectroscopy offers unique advantages since it is a sub-picosecond time-domain measurement with phase sensitivity. A technique that fully characterizes the polarization state of every frequency component in a broadband THz pulse was developed which is referred to as THz time-domain spectroscopy polarimetry (THz- TDP). In a single scan, both the vertical and horizontal components of the THz electric field are measured, and the Fourier transform of both components yields their respective amplitudes and phases: Ax(o), deltax(o), Ay(o), and deltay(o). These four quantities provide a complete description of the transmitted THz pulse in terms of its magnitude, polarization rotation, and ellipticity angles, or equivalently in terms of the more commonly employed Stokes parameters. Traditional VCD measurements determine the difference in absorption of left and right circular polarized light, but not the optical rotatory dispersion (ORD). Given that this technique measures the full polarization state of every spectral component, it can also determine ORD as well as VCD.

Metamaterials were used to demonstrate the capabilities of THz-TDP, where polarization rotation and ellipticity from Archimedean spiral arrays was on the order of 10° to 30°. The magnetoelectric responses were monitored for several series of spirals wherein different parameters such as turn spacing, number of turns, array period, and their orientation were sequentially changed. Their complex transmission matrix and eigenpolarizations were also characterized. Highly accurate 3D electromagnetic simulations were performed using CST Microwave Suite and provided another avenue for understanding the complicated interactions between the THz beam and Archimedean spiral arrays.

Two chiral molecular systems were also investigated: (5)-ibuprofen and polycarbodiimide helical polymers. While neither sample produced verifiable VCD signals, the limitations of the current technique were determined. New methods to improve the sensitivity of THz-TDP are discussed, with the leading candidate being a multi-contact THz emitter that will dramatically increase the modulation frequency and phase stability of the experiment.