Doctor of Philosophy
Institute for Superconducting and Electronic Materials Centre for Medical Radiation Physics
Pogson, Elise Maree, Terahertz Applications in Medicine, the Environment and Optics, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials Centre for Medical Radiation Physics, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3819
Current and future applications for Terahertz (1012 Hz) have been obtained from within the existing literature. Terahertz spectroscopy techniques and analysis tools for; Terahertz Time Domain Spectroscopy (THz-TDS), Continuous Wave (CW) Spectroscopy, and Terahertz Imaging, have been developed to explore optical, environmental and medical applications. Two Nematic Liquid crystals have been characterised for future optical applications. Fiber Drawn metamaterials have also been explored using THz-TDS and CW spectroscopy for their optical applications. Spectroscopy differentiated various Biochars and sought their possible constituents to aid in enhancing soil fertility. The use of Terahertz spectroscopy in radiation dosimetry using two detection methods: Film and radiosensitive gel, were found to not be feasible. Various x-ray and Terahertz imaging systems have been compared.
Nematic Liquid crystals, E7 and K15, optical properties have been investigated by THz- TDS. The birefringence of K15 was found to be 0.103 ± 0.004, whilst E7 was 0.144 ± 0.004 between (0.15-1.00) THz.
Spectroscopy has been used to characterise Fiber Drawn Metamaterials. Fiber Drawn metamaterials were shown to have resonances between (0.1-0.4) THz at their theoretically established frequency under Transverse Magnetic polarisation. This resonance may be tailored by selecting the appropriate orientation of the slotted resonators in the spooling production process. The resonance frequency may also be increased by increasing the incident angle for longitudinally invariant metamaterials. This shift in frequency to higher frequencies due to an increase in incident angle may be overcome by patterning. Both sputtered and direct fiber drawn metamaterials were shown to have negative magnetic permeability in the Terahertz region, with negative effective permeability near the resonance frequency. Stacking and drawing of two dimensional direct drawn metamaterials resulted in 3 dimensional layered structures with magnetic resonances. The resonances increase with increasing layers and care must be taken to keep Bragg peaks from interfering with this resonance. These three dimensional structures may be extended into development of sub-wavelength waveguides.
Biochar has been analysed by THz-TDS which can discriminate between Biochar types to aid in field detection, from their absorption and refractive index differences. BMC5 Biochar has the same index of refraction (N=1.29) as its constituents of dried chicken manure and Saligna Biochar. Terahertz frequency “fingerprints” may be used to determine Biochars constituents from pelletised samples if a quantitative analysis is performed. This is a complex structure and any insight into its mechanics is highly sought after. THz-TDS cannot distinguish between chemically treated Biochar, or thermal and biological prepared Biochars compared to unprepared Biochars.
THz-TDS has been explored in radiation dosimetry, to better understand dosimeters (Film and radiosensitive gel) and in an attempt to determine if THz techniques could be used for dosimetry readout. Terahertz radiation could not establish the dose of EBT2 Gafchromic Film or PAGAT gel. The only possible application from this is a dual THz/X-ray system as the Terahertz radiation cannot read out the Film but does have a negligible effect.
Terahertz imaging has been explored on both broadband and CW systems, and directly compared to X-rays. The literature has established that Terahertz radiation has many advantages over other frequencies, some examples include having less scatter than higher frequencies, being non-ionising and non-destructive. The same test phantom has been used to determine X-ray On-Board Imager and Terahertz Broadband Confocal Imaged resolutions of 1.25 lp/mm and 0.56 lp/mm respectively. X-rays have a higher spatial resolution than the Terahertz techniques used here. X-rays and THz have different contrast mechanisms. Despite the resolution difference Terahertz radiation may image lower atomic number samples better than X-rays, including water and plastics. Terahertz imaging applications include security screening, medical imaging and material imaging for quality assurance.