No description available.

Terahertz technology is rapidly growing for applications in various fields such as medical sciences, remote sensing, material characterization, and security. This accelerated growth has motivated engineers to develop compact, portable, and cost-effective terahertz sources and detectors. Terahertz generation and detection can be achieved using photoconductive antennas (PCAs), which have unique advantages. Notably, they do not require a vacuum or cryogenic cooling to function. PCAs operate on the principle of photoconductivity, which allows for compact integration with a fiber optic laser. It is also possible to launch THz radiation to a waveguide, which can be used for making a robust THz spectroscopy system. Ultra-short laser pulses are available in both 800 nm and 1550 nm wavelengths. However, the 1550 nm window has distinctive advantages such as availability of fiber amplifiers and fiber based electro-optical components at a relatively lower cost. The goal of this research is to introduce cost-effective and state-of-the-art solutions to develop THz transceivers for use in terahertz time-domain spectroscopy (THz-TDS) at 1550 nm wavelength. In this thesis we explore three approaches for enhancing THz emission and reception using PCAs. First, an array of hexagonal shape plasmonic nano-structures was used to increase the optical field coupling to the minimum depth of the substrate. Next, nano-structures also helped with enhancing the local electric field inside a low-cost semi-insulating GaAs substrate. This technique resulted in a 60% enhancement of the THz emission compared to a commercial LT-GaAs based PCA with antireflection coating. Moreover, the plasmonic nano-structures efficiently remove heat from the gap area allowing for operation at higher bias voltages. Plasmonic structures on LT-GaAs were investigated, which use a mid-gap Arsenic defect state to absorb 1550 nm light. The plasmonic devices were found to outperform existing InGaAs substrate based THz devices by factor of two. Finally, optimization of the LT-GaAs growth and annealing conditions was investigated to maximize the THz signal at 1550 nm. Outcomes of this research pave the way for designing cost-effective THz transceivers for time domain Terahertz spectroscopy systems at 1550 nm wavelength. / Graduate

Thesis (Ph. D.)--University of Wisconsin--Madison, 1957. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 89-93).

none

No description available.

Thesis (Ph.D.) - University of Glasgow, 1979. / Ph.D. thesis submitted to the Faculty of Engineering, University of Glasgow, 1979. Print version also available.

Cadmium sulphide (CdS) films were electrodeposited from an aqueous electrolytic solution onto indium tin oxide and fluorine doped tin oxide substrates, commonly used in photovoltaic applications. The CdS films were deposited for between 1 minute and 4 hours at temperatures ranging from 30°C to 90°C and a deposition potential of 600 mV with respect to a Ag/AgCI reference electrode. Some films were annealed in an argon atmosphere at temperatures of 300°C, 400°C and 460°C for 15 minutes and 30 minutes with and without CdCh2 treatment. The cleaning efficiency of the substrates by several solvents was determined using atomic force microscopy (AFM), surface roughness and scanning electron microscopy (SEM). It was found that the cleaning of the substrates could be monitored by the RMS roughness of the surfaces, and that the efficiency of the cleaning methods was substrate dependent. The microstructure, composition and optical properties of the films were determined using SEM, transmission electron microscopy, transmission electron diffraction, AFM, x-ray photoelectron spectroscopy (XPS), transmittance and Raman spectroscopy. All films, including those formed at the lowest deposition temperature of 30°C, were crystalline with the hexagonal (wurtzite) structure. At deposition temperatures ≤50°C the crystallites were extremely small as evidenced by broad diffraction rings and Raman peaks. At higher deposition temperatures (≥60°C) the deposition rate increased and the crystalline quality improved significantly. At the very early stages of deposition the deposits consisted of isolated, three-dimensional particles. Rapid lateral growth of these particles dominated the initial stages of deposition. Field emission SEM and AFM showed that ostensibly single particles consisted of agglomerates of elongated and plate-like grains with no preferred orientation. The final films had a columnar structure with good contact between the substrate and the film. XPS results showed that there was no significant compositional change in the deposition temperature range used and the films were all slightly cadmium rich. Carbon, oxygen and chlorine were detected mainly on the surface of the films. After annealing, the composition of the CdS films were close to stoichiometric once the surface layer had been removed. There was an improvement in the crystallinity of the films after annealing. / Thesis (PhD (Physics))--University of Pretoria, 2006. / Physics / unrestricted

The idea of terahertz-time-domain spectroscopy (THz-TDS) is to exploit a single cycle, spectrally broad THz radiation pulse to gain insight into the response of matter. Photoconductive devices and nonlinear crystals are utilized in both the generation as well as the coherent detection of THz radiation. The relatively high cost and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems for commercial applications. Er-doped femtosecond fiber lasers operating at 1.55 μm could offer a viable alternative. In this thesis nonlinear crystals and photoconductive emitters are discussed for excitation in the near infrared (NIR) window of between 800 nm to 1550 nm. The main focus of this thesis is a detailed study of substrate materials for an interdigitated photoconductive antenna. Photoconductive antennas with microstructured electrodes provide high electric acceleration fields at moderate voltages because of small electrode separations. The scalability of these devices allows for large active areas in the mm^2 range, which are sufficient for excitation at large optical powers. In comparison with conventional emitter structures, these antennas have more favourable characteristics regarding THz power, spectral properties, and ease of handling. Depending on the utilized substrate material, photoconductive antennas can then be operated using different excitation wavelengths. By employing substrates with short carrier trapping times these antennas can be operated as THz-detectors. Moreover the design of electrode structures for generating radially and azimuthally polarized THz waves are presented. A second topic deals with the signal analysis and signal interpretation of THz pulses transmitted through several material systems. These experiments show the potential for tomographic and spectroscopic applications. The third part deals with THz emission by frequency mixing in nonlinear organic and inorganic crystals. Hereby the focus is on polaritonic phase matching in GaAs. Furthermore, indications of THz tunability by the excitation wavelength were found by utilizing waveguide structures. However, the observed tuning range is much lower then theoretically predicted. Specific reasons for this are discussed.

Thesis (M.S.)--University of Nevada, Reno, 2008. / "December, 2008." Includes bibliographical references (leaves 69-76). Online version available on the World Wide Web.

Terahertz (THz) has become a strong area for scientific research and commercial application in recent years. This research group has redesigned and optimized a THz photoconductive antenna, which currently operates with approximately 10x the power of a commercial antenna. It has been determined by this research that the THz signal emitted from a photoconductive antenna consists of coherent and incoherent signals. In addition to the improvement of the THz photoconductive antenna, I have optimized an electro optic THz detection system by characterizing the field dependency of an electro optic crystal, which enabled me to estimate the THz electric field strength. The high power THz source and optimized detection system were combined into a high power, high resolution time domain THz spectrometer. This spectrometer was used to conduct original measurements of the THz spectrum of water vapor, ionized air, and various chemical vapor including explosives. Most of these measurements were only possible with our improved THz spectrometer. In order to understand ionized air, an additional study was carried out to explore the ionization of several gases (e.g. N2, O2, Ar, CO2, and water vapor) which were ionized by radioactive isotopes. This unique study found that in addition to dose rate, the gamma energy of the radioactive isotopes and the sequential ionization levels of gases affect the equilibrium ion densities of these gases. This effect was especially pronounced for argon gas. The study of ion dynamics in gases has lead to the development of a prototype for stand-off detection and identification of radioactive isotopes. This prototype, despite being simple in design, can detect isotopes faster and more cheaply than a conventional gamma ray spectrometer. Throughout this thesis research I have successfully developed a high power, high resolution terahertz spectrometer and demonstrated that with the spectrometer I could identify characteristic resonances of water vapor, some chemicals including explosives, and even ionized air produced by nuclear isotopes. From the characteristic resonance frequencies one can understand the underlying physics or chemistry of molecules or atoms. / Physics