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Terahertz technology has attracted extensive attention because of its unique applications in environmental monitoring, space explorations, chemical identification, material characterization, security screening, medical imaging, and biological sensing. In the meantime, the practical feasibility of many terahertz systems is still limited by the low power, low efficiency, and bulky nature of existing terahertz sources. Among various techniques for terahertz generation, photoconduction has demonstrated very promising performance for generating both pulsed and continuous-wave (CW) terahertz radiation. Compared to other optically driven terahertz emitters based on nonlinear optical processes, performance of photoconductive terahertz emitters is not constrained by the Manley–Rowe limit and, therefore, can offer significantly higher optical-to-terahertz conversion efficiencies. In spite of their great promise, the performance of existing photoconductive terahertz emitters is severely limited by poor quantum efficiency of ultrafast photoconductors. This limitation is mainly caused by inefficient collection of the majority of the photocarriers in a sub-picosecond time scale. To address this limitation, we introduce novel photoconductive terahertz source designs that incorporate plasmonic contact electrodes to offer significantly higher efficiencies compared to conventional designs. By utilizing plasmonic contact electrodes, a large portion of the incident optical pump beam is concentrated and absorbed in close proximity to the plasmonic contact electrodes. Therefore, the average transport path length of photo-generated carriers to the contact electrodes is greatly reduced. As a result, higher photocurrent levels are fed to the terahertz antenna within the oscillation cycle of the terahertz radiation and higher optical-to-terahertz conversion efficiencies are achieved. We demonstrate record-high optical-to-terahertz conversion efficiencies as high as 7.5%, exhibiting three orders of magnitude higher efficiencies compared to conventional designs. We also demonstrate CW terahertz generation with a radiation frequency tuning range of more than 2 THz and a radiation power of 17 μW at 1 THz, exhibiting a 3-fold higher radiation power level compared to the state-of-the art. Moreover, we demonstrate an integrated photoconductive terahertz source with compact optical pump sources, which offers a very promising solution for future high-performance and compact terahertz imaging and sensing systems.

This dissertation investigates the application of the frequency-scanned beam of a microstrip leaky wave antenna (LWA) to track humans in the two-dimensional (2-D) range-azimuth plane. The history, operating principles and frequency-scanned properties of a microstrip LWA are first reviewed. The basic concept of using a microstrip LWA to track humans is verified by designing, building and testing a broadband microstrip LWA, developing the necessary processing algorithm, and collecting data using a vector network analyzer. A number of topics are then investigated to further advance the concept. First, the idea of combining the frequency-scanned antenna with a short-pulse ultra-wideband (UWB) radar is developed to realize a portable, real-time system for human tracking. The radar concept and the components of the system are discussed in detail. Line-of-sight and through-wall measurements of a human subject are carried out to demonstrate the performance. Second, a new LWA structure is proposed to achieve a narrower azimuth beam, which requires both a small leaky-wave attenuation constant and a long aperture. The transverse resonance method (TRM) is applied to analyze the proposed structure and the results are verified with measurements of a built prototype. Third, a new signal processing technique, compressive sensing, is applied to further improve the resolution in both the azimuth and down range dimensions. The technique is tested with simulation and measurement data and is shown to produce sharper target responses in both the down range and azimuth dimensions. Lastly, the radar cross-section (RCS) of a microstrip LWA is studied. The antenna mode scattering and structural mode scattering are modeled separately. A ray picture is provided to explain the observed time-domain features using the group delay of the leaky wave.

Food safety scares affect consumption behavior, and food safety and animal health issues are increasingly impacting international agricultural trade. Foot-and-mouth disease (FMD) is a highly contagious viral-type disease, and has raised not only the concerns of animal health issue but also food safety issue. Over 58 countries in the world have experienced FMD outbreaks, and pork exports and imports among these countries are largely impacted. This dissertation focuses on how global pork trade is affected by FMD. This dissertation consists of three parts: first, this study specifically focuses on the market of U.S. pork exports. Results show that disease-affected pork importers are potential traders with the U.S., and only importing countries with a vaccination policy are more likely to increase pork imports from the U.S. rather than those importers with a slaughter policy. Second, a further investigation focuses basic hypothesis on import demand of FMD-affected importers by using a gravity model with fixed-effects to show how pork trade is affected by FMD among 186 countries. Results confirm that pork export falls when an exporting country develops FMD. Exporters with a vaccination policy have larger negative impacts than those with a slaughter policy. Further, pork importers that develop FMD and institute a slaughter policy will import more pork, but importers with a vaccination policy import the same level of pork. Third, the findings of part one and two reveal that FMD-free pork exporters face different market opportunities when pork importers have FMD outbreaks. Hence, four major FMD-free pork exporters, such as Canada, U.S., Germany, and Spain, are further investigated. Results confirm that the impacts of foreign FMD have altered pork exporters differently. Germany has gained the most exports during foreign FMD outbreaks in pork importers; the U.S. is second; Spain is third; and Canada is fourth. In sum, this dissertation contributes to the literature of gravity model when endogeneity and heteroskedasticity may coexist, when an extremely large number of zero observations are included, when single commodity for one specific exporter is analyzed, when a spatial econometric approach is compared, and when pork export market has been altered by foreign FMD outbreaks.

In this work, a low-complexity two-dimensional (2D) frequency-scanned antenna is proposed to image a human using a Doppler radar. It consists of two back-to-back, air-filled microstrip leaky wave antennas (LWAs). The frequency-scanned pattern of the microstrip LWA is used to determine the target bearing in one dimension. Two such elements are used as an interferometer to determine the target bearing in the other dimension. In order to pack two LWAs closely, a design is proposed to achieve a minimal disturbance on the azimuth and elevation beam patterns. The design is measured with both static and Doppler targets to demonstrate the capability to form 2D frontal images. To investigate the potential performance of using the proposed antenna to image a human, a simulator that includes a dynamic human signature model and the frequency-scanned antenna pattern is developed. A radar waveform that is different from that used for the measurement conducted with simple Doppler targets is proposed. A simple five-point human model is tested first to understand the capability of the antenna to image a human. Next, the antenna design and the radar processing parameters are studied to improve the image quality. Simulated frontal images of a walking human are generated and discussed. With a redesigned antenna and new radar processing steps, simulation shows that frontal imagery of a human undergoing motion can be generated.

Previous authors have combined tests for pairs and unpaired data so that population means can be compared using a paired study design with incomplete data. The primary object of my thesis is to determine the appropriate sample size and the appropriate proportion and configuration of complete data and incomplete data so that a normal approximation can be used to calculate p-values. The test statistic studied is one due to Wilson (1992) in which the sign test and rank sum test are combined to form of composite test statistic. To fulfill these objectives, the following approach is adopted:(1)Choose different data scenarios in terms of different sample sizes of paired data and different proportions of complete data.(2)Obtain the exact sampling distribution of the test statistic under each data scenario we study.(3)Obtain the normal approximation distribution under each data scenario we study.(4)Compare the exact and approximate cumulate distribution by their difference on each possible test statistic value. The results show that when the study groups are approximately balanced with respect to incomplete data, and have at least 9 observations in each group, the normal approximation appears to be useful when the number of complete pairs is as low as 5. However, when the groups are highly unbalanced with respect to incomplete data, using the normal approximation seems not to be appropriate, at least when the total sample size is 70 or less. These results may make public health studies easier to carry out when the data include both complete and incomplete pairs.