Computational and experimental THz spectroscopic characterization of multi-state molecules for enhanced sensing in the THz region

While the infrared spectroscopy provides information about chemical bonds such as bond lengths and angles, the terahertz (THz) regime (0.1-10 THz) between the infrared and millimeter regions of the electromagnetic spectrum can provide information about intramolecular phonon modes. Since these modes are unique to a particular molecule, very low frequency vibrational modes (<30 cm -1) of molecules within the THz regime may allow the identification of unknown substances by a spectral 'terahertz fingerprint'.; The goal of the dissertation is to investigate theoretically and experimentally the feasibility of a new Multi-State Spectral Sensing (MS3) concept where bio-signature information can be collected from multiple metastable conformations for enhanced sensing in the THz region. Several geometric isomeric molecular systems have been investigated including 2-butenes, retinals, and stilbenes, in order to (1) study THz spectra of different molecular conformations; (2) illustrate a basic procedure through which optical excitation can be used to modify molecular conformations.; Hartree-Fock (HF) method and density functional theory (DFT) with proper basis sets have been chosen to model various molecular systems. The comparison of THz spectra of different conformations shows that multiplication of THz spectral information can be achieved by modification of molecular conformations. A relatively repeatable and reliable technique to measure the THz spectra of the optical sensitive materials has been developed. Experimental results from the measurement demonstrate that it is possible to detect conformation change in retinal molecules using THz spectroscopy.; To realize the spectral multiplication, an appropriate interface to the molecular level needs to be defined so that a new conformation with the resultant extra phonon mode information can be obtained. In the dissertation, a basic procedure has been illustrated through which optical excitation can be used to modify the molecular conformational states. We investigated the optical absorption characteristics of different conformations, modeled the excited states of different conformations and predicted the excitation conditions.; The unique THz and optical absorption characteristics of multi-state molecular systems may also lead to other molecular inspired devices to process data with carrier frequencies in the THz region. The schematic structure of a molecule-based optically tunable filter in THz region has been demonstrated as an example of potential devices. For ultimate application of molecule-based devices, extensive efforts have been made to develop feasible methodologies of integration of functional molecules into semiconductor substrates and DNA lattices. We have conducted the preliminary investigation of the THz spectral and optical absorption characteristics of the integrated systems to understand how integration environments affect THz and optical properties of functional molecules.