A scaled direct energy conversion device was built to convert kinetic energy of singly ionized helium ions into an electric potential by the process of direct conversion. The experiments in this paper aimed to achieve higher potentials and higher efficiencies than ever before. The predicted maximum potential that could be produced by the 150 kV accelerator at the Texas A&M Ion Beam Lab was 150 kV, which was achieved with 92% collection efficiency. Also, an investigation into factors affecting collection efficiency was made. It was concluded that charge was being lost due to charge exchange occurring near the surface of the target which caused positive target atoms to be ejected from the face and accelerated away. Introducing a wire mesh near the face of the target with an electric potential, positive or negative, which aimed to control secondary ion emissions, did not have an effect on the collection efficiency of the system. Also, it was found that the gas pressure inside the chamber did not have an effect on the collection efficiency. The goal of achieving higher electric potentials and higher efficiencies than previous direct conversion work was met.

This study evaluated the potential use of an antimicrobial packaging system in combination with electron beam irradiation to enhance quality of fresh produce. Irradiated romaine lettuce up to 3.2 kGy showed negligible (p > 0.05) changes in color, but texture and sensory attributes were less acceptable with increased dose. We established the antimicrobial effectiveness of various active compounds incorporated into the low-density polyethylene (LDPE)/polyamide films to increase radiation sensitivity of surrogate bacteria (Listeria innocua and Escherichia coli). All films showed inhibition zones in an agar diffusion test. In the liquid culture test, the active compounds reduced the specific growth rate and decreased final cell concentration of strains. Films incorporated with active compounds increased the radiation sensitivity of the tested strains, demonstrating their potential to reduce the dose required to control microbial contamination using electron beam technology. The active compounds maintained their antimicrobial activity by exposure to ionizing radiation up to 3 kGy. Antimicrobial activity of LDPE/polyamide films incorporated with transcinnamaldehyde was tested with fresh-cut romaine lettuce. Total aerobic plate counts (APC) and yeast and mold counts (YMC) were determined as a function of dose (0, 0.5, and 1.0 kGy) for 14 days of storage at 4°C. Irradiation exposure significantly lowered APCs of lettuce samples by 1-log CFU/g compared to the non-irradiated controls; however, it only slightly reduced YMCs. The effectiveness of using irradiation with antimicrobial films was enhanced with increased radiation dose and transcinnamaldehyde concentration. Electron beam irradiation up to 20 kGy did not affect the tensile strength and toughness of the polymeric films. The film’s flexibility and barrier properties were significantly improved by exposure to 20 kGy. The addition of an active compound did not affect the tensile strength and barrier properties of the films, but decreased the percent elongation-at-break and toughness, making them slightly more brittle. Ionizing radiation affected the release kinetics of the antimicrobial agent from the packaging material into a model food system. Irradiated films exhibited slower release rates than non-irradiated film by 69%. In addition, release rate was lower at 4ºC by 62.6% than at 21-35ºC. The pH of the simulant solution affected release rate with pH 4 yielding higher rates than pH 7 and 10.

Optics has the inherent advantages of parallelism and wide bandwidths in processing information. However, the need to interface with electronics creates a bottleneck that eliminates many of these advantages. The proposed research explores novel optical devices and techniques to overcome some of these bottlenecks. To address parallelism issues we take a specific example of a content-addressable memory that can recognize images. Image recognition is an important task that in principle can be done rapidly using the natural parallelism of optics. However in practice, when presented with incomplete or erroneous information, image recognition often fails to give the correct answer. To address this problem we examine a scheme based on free-space interconnects implemented with diffractive optics. For bandwidth issues, we study possible ways to eliminate the electronic conversion bottleneck by exploring all-optical buffer memories and all-optical processing elements. For buffer memories we examine the specific example of slow light delay lines. Although this is currently a popular research topic, there are fundamental issues of the delay-time-bandwidth product that must be solved before slow light delay lines can find practical applications. For all-optical processing we examine the feasibility of constructing circuit elements that operate directly at optical frequencies to perform simple processing tasks. Here we concentrate on the simplest element, a sub-wavelength optical wire, along with a grating coupler to interface with conventional optical elements such as lenses and fibers. Even such a simple element as a wire has numerous potential applications. In conclusion, information processing by all-optical devices are demonstrated with an associative memory using diffractive optics, an all-optical delay line using room temperature slow light in photorefractive crystals, and a subwavelength optical circuit by surface plasmon effects.

A collinear fast ion beam laser apparatus was constructed and tested. It will be used on-line to the SLOW RI radioactive beam facility in RIKEN (Japan) and as in the present experiment for precision absolute frequency measurements of astrophysically important reference lines. In the current work we conducted absolute measurements of spectral lines of Ar ions using parallel and antiparallel geometries. To provide a reference for the laser wavelength iodine saturation spectroscopy was used. The precision of this reference was enhanced by simultaneously observing the beat node between the spectroscopy laser and the corresponding mode of a femtosecond laser frequency comb. When performing collinear and anticollinear measurements simultaneously for the laser induced fluorescence, the exact relativistic formula for the transition frequency v0 = pvcoll � vanticoll can be applied. In this geometry ion source instabilities due to pressure and anode voltage fluctuation are minimized. The procedure of fluorescence lineshapes fitting is discussed and the errors in the measurements are estimated. The result is v0 = 485, 573, 619.7 � 0.3MHz corresponding to (delta v)/v = 6 � 10?10 and is an improvement of two orders of magnitude over the NIST published value.

Optical beam induced current (OBIC) mapping has found wide-spread applications in characterizing semiconductor devices and integrated circuitry. In this study, we have used a two-photon scanning microscope to investigate InGaN light emitting diodes (LED). The defects induced by electrostatic discharge (ESD) can be clearly identified by DC-OBIC images. Additionally, we have combined an E-O modulator and a high frequency phase sensitive lock-in amplifier to conduct time-resolved study on the dynamical properties of the LEDs. The defects also exhibit different delay time when compared with the normal parts.

The strong electromagnetic field that is induced at the surface of a plasmonic nanoparticle can be utilized for many important applications, including spectroscopic enhancement and electromagnetic waveguides. The focus of this thesis is to study some of the properties of induced plasmonic fields around metal nanoparticles. Current methodologies for fabricating nanoparticles are discussed, including lithography and colloidal synthesis. This dissertation includes studies on plasmonic driven nanoparticle motion of surface supported gold nanoprisms from a substrate into solution via a femtosecond pulse. The mechanism of particle motion is discussed and the stability of the unprotected nanoprisms in solution is studied. Fundamental plasmonic near-field coupling between two plasmonic nanoparticles is also examined. Experimental results using electron beam lithography fabricated samples are used to explicitly describe the plasmonic coupling between dimers as a function of the nanoparticle size, shape, and orientation. These variables are systematically studied and the dependence is compared to mathematically derived functional dependencies in order to model and predict the effects of plasmonic coupling. As an extension, the coupling between plasmonic nanoparticles is shown in a common application, surface enhanced Raman scattering. The final chapter is devoted to an investigation of the nature of nanocatalysis, homogeneous and heterogeneous, for several reactions using metal nanoparticles.

III-nitrides are a promising material system with unique material properties, which allows them to be utilized in a variety of semiconductor devices. III-nitrides grown by NH3-MOMBE are typically grown with high carbon levels (> 1021 cm-3) as a result of the incomplete surface pyrolysis of the metal-organic sources. Recent research has involved the compensating nature of carbon in III-nitrides to produce semi-insulating films, which can provide low-leakage buffer layers in transistor devices. The aim of this work is to investigate the possibility of forming a 2DEG, which utilizes the highly carbon-doped GaN layers grown by NH3-MOMBE to produce low-leakage buffer layers in the fabrication of HEMTs. These low leakage GaN buffers would provide increased HEMT performance, with better pinch-off, higher breakdown voltages and increased power densities. Additionally, methods of controlling and/or reducing the incorporation of carbon will be undertaken in an attempt to broaden the range of possible device applications for NH3-MOMBE. To realize these transistor devices, optimization and improved understanding of the growth conditions for both GaN and AlGaN will be explored with the ultimate goal of determining the feasibility of III-nitride transistors grown by NH3-MOMBE.

The work presented in this thesis shows how tunneling spectroscopy techniques can be applied to metallic nanoparticles to obtain useful information about fundamental physical processes in nanoscopic length scales. At low temperatures, the discrete character of the energy spectrum of these particles, allows the study of spin-polarized current via resolved "electron-in-a-box" energy levels. In samples consisting of two ferromagnetic electrodes tunnel coupled to single aluminum nanoparticles, spin accumulation mechanisms are responsible for the observed spin-polarized current. The observed effect of an applied perpendicular magnetic field, relative to the magnetization orientation of the electrodes, indicates the suppression of spin precession in such small particles. More generally, in the presence of an external non-collinear magnetic field, it is the local field "felt" by the particle that determines the character of the tunnel current. This effect is also observed in the case where only one of the electrodes is ferromagnetic. In contrast to the non-magnetic case, ferromagnetic nanoparticles exhibit a much more complex energy spectrum, which cannot be accounted for, using the simple free-electron picture. It will be shown that interactions between quasi-particle excitations due to sequential electron tunneling and spin excitations in the particle are likely to play an important role in the observed temperature/voltage dependence of magnetic hysteresis loops.

The recent improvement of additive manufacturing has allowed designers to achieve a level of complexity and customizability that is difficult or impossible to accomplish using traditional manufacturing processes. As a result, much research has been conducted on developing new methods to utilize the larger design space brought by additive manufacturing. One such research area is in the design of mesoscale lattice structures. Mesoscale lattice structures are a type of cellular structure with support element sizes on the order of magnitude of centimeters. These types of structures are engineered for high performance and have applications in industries where both low weight and high strength are desired. However, due to the small size of their struts, these structures can easily have hundreds to thousands of individual struts. As a result, design poses a unique challenge. Current methods approach design of mesoscale lattice structures as a topological optimization problem, treating each strut diameter in the structure as a design variable. For structures with a fewer number struts, these optimization methods can converge, but will generally be very time-consuming. For structures with a large number of struts, the optimization problem becomes too large for current algorithms to solve. In previous research, a new, highly efficient design method for mesoscale lattice structures was presented that eliminates the need for global size or topological optimization. This method, termed the Size, Matching and Scaling method, used a unique combination of a solid-body finite element analysis and a library of pre-defined lattice configurations, termed the "unit-cell library," to generate lattice topologies. The results from this method were highly promising: design time was significantly reduced when compared to optimization methods. Furthermore, lattices designed using the SMS method had performance results that were either comparable or better than their optimized counterparts. However, the method developed was highly conceptual, lacking a true systematic methodology for generating topologies and suffering from some gaps in implementation. In this research, we present a modified Size Matching and Scaling (SMS) design method. Firstly, we introduce and outline the modified methodology. This methodology particularly includes an optimization step for determining strut diameters that replaces the manual search used in the original method. Secondly, we expand and explore the unit-cell library in an attempt to improve the performance of lattices generated using the SMS method. In particular, we optimize several unit-cell configurations and compare their performance in the context of the SMS method. Finally, we test the updated SMS methodology and unit-cell library using various design examples. Results from the various example problems indicate that optimization is not only a viable systematic method for determining diameter values, but is actually preferred to the manual, iterative process used in the original method. Furthermore, various optimization algorithms and approaches yield different results. Between the two optimization algorithms utilized in this method: constrained optimization and least-squares minimization, constrained minimization converges faster, but least-squares minimization yields slightly improved performance results. In addition to these algorithms, a one-variable approach using an untested, simplifying assumption, dubbed the "28% approach," was tested. Results indicate that this assumption was incorrect and cannot be utilized. Finally, results from the expanded unit-cell library indicate that the best unit-cell configuration is still the same original unit-cell configuration utilized in the first SMS method. The addition of more unit-cell does not improve the performance of structures generated using the SMS method. In fact, both performance and design time worsen when additional configurations are utilized.

Beams are structural members with one dimension much larger than the other two. Examples of beams include propeller blades, helicopter rotor blades, and high aspect-ratio aircraft wings in aerospace engineering; shafts and wind turbine blades in mechanical engineering; towers, highways and bridges in civil engineering; and DNA modeling in biomedical engineering. Beam analysis includes two sets of equations: a generally linear two-dimensional problem over the cross-sectional plane and a nonlinear, global one-dimensional analysis. This research work deals with a relatively new set of equations for one-dimensional beam analysis, namely the so-called fully intrinsic equations. Fully intrinsic equations comprise a set of geometrically exact, nonlinear, first-order partial differential equations that is suitable for analyzing initially curved and twisted anisotropic beams. A fully intrinsic formulation is devoid of displacement and rotation variables, making it especially attractive because of the absence of singularities, infinite-degree nonlinearities, and other undesirable features associated with finite rotation variables. In spite of the advantages of these equations, using them with certain boundary conditions presents significant challenges. This research work will take a broad look at these challenges of modeling various boundary conditions when using the fully intrinsic equations. Hopefully it will clear the path for wider and easier use of the fully intrinsic equations in future research. This work also includes application of fully intrinsic equations in structural analysis of joined-wing aircraft, different rotor blade configuration and LCO analysis of HALE aircraft.