This research investigated the characteristics of single-mode optical pulses generated with orthogonal-polarization optical feedback (OPF) in Fabry-Perot type semiconductor lasers. Single-mode pulse trains with a pulse frequency of 470 MHz and 3.76 GHz were observed. A modified model was proposed to solve the inconsistency between the experimental results and the computer simulations of Otsuka and Chern¡¦s model. These results also solve the problem of a round-trip feedback distance that is too short to enable the feedback system to be implemented, making this technology accomplishable in currently working systems. Furthermore, this investigation recovered and maintained a stable oscillation of every missing longitudinal mode in a hysteresis type mode-hopping gap of a semiconductor laser. A special feature of this method is that both the laser power and spectral purity are preserved during mode recovery and mode switching. The experimental results also reveal that the OPF effectively suppressed mode-hopping in semiconductor lasers and drove the laser into a stable single-mode state. Finally, this research employed OPF to suppress the intensity noise stimulated by coherent optical feedback in a semiconductor laser. At a coherent-feedback level as strong as ¡V14 dB, an OPF ratio of ¡V29 dB could return the laser to its primitive single mode from the multimode, yielding a spectral purity and relative intensity noise (RIN) even better than the solitary values. These discoveries constitute an important contribution to our understanding of applications of semiconductor lasers.

Ultra-Wideband impulse radio (UWB-IR) technology is an attractive method on multi-user for high data rate transmitting structures. In this thesis, we use the ultra wideband (UWB) signal that is modulated by the time-hopping spread spectrum technique in a wireless multiple access environments, and discuss the influences of multiple access interference. We discuss two parts of the influences of multiple access interference in this thesis. The first, we analyze the multiple access interferences on the conventional correlation receiver, and discuss the influences by using the time hopping code on different multiple access structures. The second, we know that the performances of user detection and system capacity would be degraded by the conventional correlation receiver in the multiple access channels. The Probabilistic Data Association(PDA) multi-user detection technology can eliminate multiple access interferences in this part. We will use this method to verify the system performance through the computer simulations, and compare to other multi-user detectors with convention correlation receivers. Finally, the simulation results show that the performance of the PDA multi-user detections is improved when the system is full loaded.

Type III secretion is a virulence system utilized by several clinically important Gram-negative pathogens. Computational methods have been used to develop two classes of type III secretion inhibitors, the salicylidene acylhydrazides and the acetylated salicylanilides. For these classes of compounds, quantitative structure-activity relationship models have been constructed with data from focused libraries obtained by statistical molecular design. The models have been validated and shown to provide useful predictions of untested compounds belonging to these classes. Scaffold hopping of the salicylidene acylhydrazides have resulted in a number of synthetic targets that might mimic the scaffold of the compounds. The synthesis of two libraries of analogs to two of these scaffolds and the biological evaluation of them is presented.

In this thesis quantum-classical dynamics is applied to the study of quantum condensed phase processes. This approach is based on the quantum-classical Liouville equation where the dynamics of a small subset of the degrees of freedom are treated quantum mechanically while the remaining degrees of freedom are treated by classical mechanics to a good approximation. We use this approach as it is computationally tractable, and the resulting equation of motion accurately accounts for the quantum and classical dynamics, as well as the coupling between these two components of the system. By recasting the quantum-classical Liouville equation into the form of a generalized master equation we investigate connections to surface-hopping. The link between these approaches is decoherence arising from interaction of the subsystem with the environment. We derive an evolution equation for the subsystem which contains terms accounting for the effects of the environment. One of these terms involves a memory kernel that accounts for the coherent dynamics. If this term decays rapidly, a Markovian approximation can be made. By lifting the resulting subsystem master equation into the full phase space, we obtain a Markovian master equation that prescribes surface-hopping-like dynamics. Our analysis outlines the conditions under which such a description is valid. Next, we consider the calculation of the rate constant for a quantum mechanical barrier crossing process. Starting from the reactive-flux autocorrelation function, we derive a quantum-classical expression for the rate kernel. This expression involves quantum-classical evolution of a species operator averaged over the initial quantum equilibrium structure of the system making it possible to compute the rate constant via computer simulation. Using a simple model for a proton transfer reaction we compare the results of the rate calculation obtained by quantum-classical Liouville dynamics with that of master equation dynamics. The master equation provides a good approximation to the full quantum-classical Liouville calculation for our model and a more stable algorithm results due to the elimination of oscillating phase factors in the simulation. Finally, we make use of the theoretical framework established in this thesis to analyze some aspects of decoherence used in popular surface-hopping techniques.

GaSe thin films were deposited by thermal evaporation technique with and without Cd doping. X-ray analysis showed that the crystallinity increases in (1014) preferred orientation direction with annealing for doped and undoped films. The room temperature conductivity and mobility values of the samples were found to be for doped and undoped films in between 1.3&times / 101 - 3.4&times / 102 (&amp / #8486 / -cm)-1, 1.2&times / 10-6 - 1.5&times / 10-6 (&amp / #8486 / -cm)-1 and 5.9 &amp / #8211 / 20.9 (cm2/V.s) (for doped samples only), respectively. Due to the high resistivity of the undoped samples mobility measurements could not be performed. The dominant conduction mechanisms were determined to be thermionic emission in the high temperature region (250-400 K), tunneling in the range 160-250 K and between 100-150 K variable range hopping mechanism for the doped films. For the undoped films above 250 K thermionic emission was the dominant conduction mechanism. Space charge limited currents in parallel and perpendicular directions of the film surface showed two different localized energy levels with different concentrations for each case, namely, 99.8 meV with concentration 3.5&times / 1012 cm-3 and 418.3 meV with the concentration 2.2&times / 105 cm-3 for parallel direction and for perpendicular direction 58.3 meV with concentration 6.2&times / 1025 cm-3 and 486.1 meV with concentration 3.3&times / 1022 cm-3. Photocurrentillumination intensity dependences indicated that power exponent of illumination intensity with values n&gt / 1 implied two recombination centers exist in studied samples.

This work explores two very different structural systems: n-type hydrogenated amorphous silicon (a-Si:H) and gold nanoparticles (AuNPs) suspended in a matrix of organic ligands. For a-Si:H, examination of the gas-phase concentration of dopant (1-6% PH3/SiH4) and argon diluent effects includes the temperature dependent conductivity, low-frequency electronic noise, and Raman spectroscopy to examine structure. It is found that a-Si:H samples grown with high dopant concentration or with argon dilution exhibit an anomalous hopping conduction mechanism with an exponent of p=0.75. An experimental approach is used to determine correlations between conduction parameters, such as the pre-exponential factor and the characteristic temperature, rather than an analysis of existing models to explain the anomalous conduction. From these results, the anomalous conduction is a result of a change in the shape of the density of states and not a shift of the Fermi level with dopant. Additionally, it is found that argon dilution increases the carrier mobility, reduces the doping efficiency, and causes a degradation of the short-range order. With AuNPs, a comparison of temperature dependent conductivity and low-frequency noise shows that the temperature coefficient of resistance (TCR) is independent of the length of interparticle distance while the noise magnitude decreases.

Στην εργασία αυτή μελετάται η ηλεκτρική απόκριση νανοσύνθετων υλικών πολυαιθυλενοξειδίου (poly(ethylene oxide)) – τροποποιημένων νανοσωλήνων άνθρακα πολλαπλού τοιχίου (MWCNT), ως συνάρτηση της περιεκτικότητας σε νανοσωλήνες άνθρακα. Συγκεκριμένα εξετάζεται η διηλεκτρική απόκριση και η ειδική αγωγιμότητα των νανοσύνθετων με παραμέτρους την περιεκτικότητα σε MWCNT, τη θερμοκρασία και τη συχνότητα του εφαρμοζόμενου ηλεκτρικού πεδίου. Από τη διηλεκτρική απόκριση εξάγονται πληροφορίες σχετικά με την μοριακή κινητικότητα του συστήματος και διεπιφανειακά φαινόμενα. Η εξάρτηση της ειδικής αγωγιμότητας από την περιεκτικότητα σε MWCNT δίνει τη δυνατότητα προσδιορισμού του κατωφλιού μετάβασης από τη μονωτική στην αγώγιμη συμπεριφορά, ενώ η εξάρτηση της ειδικής αγωγιμότητας από τη θερμοκρασία και τη συχνότητα προσφέρει τη δυνατότητα μελέτης των μηχανισμών μεταφοράς ηλεκτρικού φορτίου. Για τον καλύτερο προσδιορισμό των μηχανισμών ηλεκτρικής αγωγής εξετάζεται η δυνατότητα περιγραφής των πειραματικών αποτελεσμάτων μέσω των θεωρητικών προσεγγίσεων Variable Range Hopping model και Symmetric Hopping model. Για λόγους αναφοράς εξετάζεται και η ηλεκτρική απόκριση του καθαρού πολυμερούς και νανοσύνθετων, της ίδιας πολυμερικής μήτρας, που περιλαμβάνουν μη τροποποιημένους νανοσωλήνες άνθρακα. Τέλος, το σύνολο των αποτελεσμάτων γίνεται αντικείμενο εκτεταμένης συζήτησης. / In the present work the electric response of nanocomposites material consisted of poly (ethylene oxide) as matrix and modified multiwall carbon nanotubes (MWCNT) as reinforcing phase, is studied. Specifically, the dielectric response and the conductivity of nanocomposites is examined with parameters the content in MWCNT, the temperature and the frequency of the applied electric field. From the dielectric response valuable information concerning the molecular mobility of the systems and the interfacial phenomena can be extracted. The dependence of conductivity on the MWCNT content allows the determination of percolation threshold or critical concentration, which governs the transition from the insulating to the conductive behaviour. Moreover, the dependence of conductivity on temperature and frequency offers the possibility of studying the occurring charge transport mechanisms. Aiming to study further the type of the conduction mechanisms, the applicability of Variable Range Hopping model and Symmetric Hopping model is tested. For reasons of reference the electric response of pure polymer matrix and nanocomposites incorporating unmodified MWCNT is also examined. Finally, results become the object of an extensive discussion.

In this work, we have investigated temperature dependent electrical transport properties of carbon based two-dimensional (2D) nanomaterials. Various techniques were employed to synthesize the samples. For instance, high quality large area graphene and boron, nitrogen doped graphene (BNC) were grown using thermal catalytic chemical vapor deposition (CVD) method. Liquid phase exfoliation technique was utilized to exfoliate graphene and graphitic carbon nitride samples in isopropyl alcohol. Chemical reduction technique was used to reduce graphene oxide (rGO) by utilizing ascorbic acid (a green chemical) as a reducing agent. Detailed structural and morphology characterization of these samples was performed using state of the art microscopy as well as spectroscopic techniques (for example; Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), UV-Visible spectroscopy as well as Raman Spectroscopy). The low temperature (5 K< T <400 K) electrical transport properties of these materials show substantial difference from sample to sample studied. For instance, CVD grown graphene film has displayed metallic behavior over a wide range of temperature (5 K < T <300 K). At higher temperatures, resistivity followed linearly with the temperature (ρ(T) ~T). A power law dependence (ρ(T) ~ T4) observed at lower temperatures. Where as liquid phase exfoliated graphene and graphitic carbon nitride samples displayed nonmetallic nature: increasing resistance with decrease in temperature over a wide range (8 K < T < 270 K) of temperature. Electrical transport behavior in these samples was governed by two different Arrhenius behaviors in the studied temperature range. In the case of rGO and BNC layers, electrical conduction show two different transport mechanisms in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, Mott's two dimensional-Variable Range Hopping (2D-VRH) behavior was observed.

abstract: In-situ exploration of planetary bodies such as Mars or the Moon have provided geologists and planetary scientists a detailed understanding of how these bodies formed and evolved. In-situ exploration has aided in the quest for water and life-supporting chemicals. In-situ exploration of Mars carried out by large SUV-sized rovers that travel long distance, carry sophisticated onboard laboratories to perform soil analysis and sample collection. But their large size and mobility method prevents them from accessing or exploring extreme environments, particularly caves, canyons, cliffs and craters. This work presents sub- 2 kg ball robots that can roll and hop in low gravity environments. These robots are low-cost enabling for one or more to be deployed in the field. These small robots can be deployed from a larger rover or lander and complement their capabilities by performing scouting and identifying potential targets of interest. Their small size and ball shape allow them to tumble freely, preventing them from getting stuck. Hopping enables the robot to overcome obstacles larger than the size of the robot. The proposed ball-robot design consists of a spherical core with two hemispherical shells with grouser which act as wheels for small movements. These robots have two cameras for stereovision which can be used for localization. Inertial Measurement Unit (IMU) and wheel encoder are used for dead reckoning. Communication is performed using Zigbee radio. This enables communication between a robot and a lander/rover or for inter-robot communication. The robots have been designed to have a payload with a 300 gram capacity. These may include chemical analysis sensors, spectrometers and other small sensors. The performance of the robot has been evaluated in a laboratory environment using Low-gravity Offset and Motion Assistance Simulation System (LOMASS). An evaluation was done to understand the effect of grouser height and grouser separation angle on the performance of the robot in different terrains. The experiments show with higher grouser height and optimal separation angle the power requirement increases but an increase in average robot speed and traction is also observed. The robot was observed to perform hops of approximately 20 cm in simulated lunar condition. Based on theoretical calculations, the robot would be able to perform 208 hops with single charge and will operate for 35 minutes. The study will be extended to operate multiple robots in a network to perform exploration. Their small size and cost makes it possible to deploy dozens in a region of interest. Multiple ball robots can cooperatively perform unique in-situ science measurements and analyze a larger surface area than a single robot alone on a planet surface. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

abstract: The formation of dendrites in materials is usually seen as a failure-inducing defect in devices. Naturally, most research views dendrites as a problem needing a solution while focusing on process control techniques and post-mortem analysis of various stress patterns with the ultimate goal of total suppression of the structures. However, programmable metallization cell (PMC) technology embraces dendrite formation in chalcogenide glasses by utilizing the nascent conductive filaments as its core operative element. Furthermore, exciting More-than-Moore capabilities in the realms of device watermarking and hardware encryption schema are made possible by the random nature of dendritic branch growth. While dendritic structures have been observed and are well-documented in solid state materials, there is still no satisfactory theoretical model that can provide insight and a better understanding of how dendrites form. Ultimately, what is desired is the capability to predict the final structure of the conductive filament in a PMC device so that exciting new applications can be developed with PMC technology. This thesis details the results of an effort to create a first-principles MATLAB simulation model that uses configurable physical parameters to generate images of dendritic structures. Generated images are compared against real-world samples. While growth has a significant random component, there are several reliable characteristics that form under similar parameter sets that can be monitored such as the relative length of major dendrite arms, common branching angles, and overall growth directionality. The first simulation model that was constructed takes a Newtonian perspective of the problem and is implemented using the Euler numerical method. This model has several shortcomings stemming majorly from the simplistic treatment of the problem, but is highly performant. The model is then revised to use the Verlet numerical method, which increases the simulation accuracy, but still does not fully resolve the issues with the theoretical background. The final simulation model returns to the Euler method, but is a stochastic model based on Mott-Gurney’s ion hopping theory applied to solids. The results from this model are seen to match real samples the closest of all simulations. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2016