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内容記述 |
In this thesis the development of an integrated optics for the first principle analysis of terahertz (THz) spectrum of selected biomolecules is presented. Two designs of THz waveguides were characterized and two biomolecules were studied. In each of the works, THz spectra were obtained both experimentally and by using theoretical calculation models in order to complement each other's veracity.<br /> Planar photonic crystal waveguide (PPCW), a sheet-like polymer-based material with high transparency from the deep ultraviolet to the far infrared regime, was investigated in the THz region. Frequency-selective characteristic of the PPCW was revealed and found to have center frequency about 0.45 THz. At a length of 30 mm, single mode propagation was achieved. The calculation using finite difference time domain (FDTD) method was also implemented and used to compare to the experimental results. Relative agreement between the experimental data and theoretical calculation was attained. The coupling loss was estimated to be 1.2 dB while the broadband propagation loss value of 3.6 dB/cm. <br /> The transmission of THz radiation using microstructured polymer optical fiber was also analyzed. The time domain reference signal showed a single pulse while that of the fibers manifested two pulses, with the second pulse delayed by about 20 ps with respect to the first pulse. The first pulse is attributed to the THz wave that propagated through the hollow core while the second signal is surmised as the component of the THz wave that propagated in the microstructured. The snapshots of the FDTD simulation show that such delay is present as the THz wave is propagating through the fiber. When the frequency spectra of the fibers were plotted, a shift in the central frequency was observed. The experimental transmission band is 0.8 － 1.4 THz for fiber 1 and 1.0 － 1.6 THz for fiber 2; its width is approximately 50% of the centre frequency. FDTD calculation also showed this shift in the frequency bands but its center frequency is different to that of the experiment, which was surmised to be due to the elliptical microstructured cladding of the fibers while circular cladding were used in the calculation. These fibres were also reported to guide in the visible and infrared by the photonic bandgap mechanism, and a theoretical investigation of the photonic bandgaps of such fibres were reported for visible and telecommunications wavelengths. Propagation losses were estimated to be as low as 0.9 dB/cm using the same method in obtaining the losses of PPCW.<br /> Far infrared spectra of naphthalene and 1,4-dihydroxynaphthalene (1,4-naphthol), which exhibits estrogenic like activity and potentially mimic natural hormones, were measured in the region from 0.5 to 6.3 THz at 4 K and room temperature using a GaP THz wave generator. Quantum chemical calculations were also performed to obtain normal mode coordinates and frequencies for an isolated molecule and unit cell case using density functional theory (DFT). A shift in the absorption peaks in the 4 K spectra of naphthalene and 1,4-naphthol with respect its room temperature spectra were observed. This is related to the temperature dependence of sample density and average hydrogen-bond strength, as well as anharmonic distribution of vibrational states. Correlation field splitting was also observed in the low temperature spectra that originated from the dipole-dipole coupling between the molecules that split the intramolecular vibrational modes in the crystal. Another origin of the splitting is deduced to be due to changing of vibrational motions of between two pairs of 1,4-naphthol brought about by their orientation in the unit cell. Density functional theory (DFT) calculations for both the isolated molecule and unit cell models predicted the shift in the absorption peaks of the experimental spectra brought about by the presence of the hydroxyl group. Normal modes were also assigned which are both the intra- and intermolecular interaction.<br /> Retinal isomers, 9-<i>cis</i>, 13-<i>cis</i>, and all-<i>trans</i>, which are chromophores found in vertebrates, are investigated from 0.5-6.5 THz in the low temperature environment. Shift in absorption peaks were observed. Such changes were attributed to changes in the intramolecular interaction of each retinal isomer near the ground state condition. DFT calculations showed that the modes are dominated by intramolecular interaction and supported the assertion that shifts in the absorption peaks are due to the changes in the intramolecular interaction of each retinal isomer. <br /> This work showed first principles analysis of selected biomolecules and the development of integrated optics with low losses in the THz. However, there are still challenges that are needed to be address before fully functional THz optic devices are realized for biomolecular applications. First, though the presented THz waveguides here are so far has the lowest reported loss in their class, its design and flexibility is yet to be optimized. Second, since vibrations in this region correspond to collective motions of large portions of the molecules with considerably large moving masses and weak potential forces, its spectra is rather complicated and its vibrational mode is difficult to assign. Single molecule or unit cell model are not sufficient to accurately predict these parameters and periodic boundary condition is therefore needed. |
要旨・審査要旨 (314.5 kB)
