Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2006. / Title from title screen. Includes bibliographical references.

We present the results of a numerical simulation tool for modeling the second-harmonic generation (SHG) interaction experienced by a diffracting beam. This code is used to study the simultaneous frequency and spatial profile conversion of a truncated Bessel beam that closely resembles a higher-order mode (HOM) of an optical fiber. SHG with Bessel beams has been investigated in the past and was determined have limited value because it is less efficient than SHG with a Gaussian beam in the undepleted pump regime. This thesis considers, for the first time to the best of our knowledge, whether most of the power from a Bessel-like beam could be converted into a second-harmonic beam (full depletion), as is the case with a Gaussian beam. We study this problem because using HOMs for fiber lasers and amplifiers allows reduced optical intensities, which mitigates nonlinearities, and is one possible way to increase the available output powers of fiber laser systems. The chief disadvantage of using HOM fiber amplifiers is the spatial profile of the output, but this can be transformed as part of the SHG interaction, most notably to a quasi-Gaussian profile when the phase mismatch meets the noncollinear criteria. We predict, based on numerical simulation, that noncollinear SHG (NC-SHG) can simultaneously perform highly efficient (90%) wavelength conversion from 1064 nm to 532 nm, as well as concurrent mode transformation from a truncated Bessel beam to a Gaussian-like beam (94% overlap with a Gaussian) at modest input powers (250 W, peak power or continuous-wave operation). These simulated results reveal two attractive features – the feasibility of efficiently converting HOMs of fibers into Gaussian-like beams, and the ability to simultaneously perform frequency conversion. Combining the high powers that are possible with HOM fiber amplifiers with access to non-traditional wavelengths may offer significant advantages over the state of the art for many important applications, including underwater communications, laser guide stars, and theater projectors.

No description available.

Skin is the largest organ of the body. Its integrity plays a crucial role in maintaining physiological homeostasis, protects against mechanical forces and infections, fluid imbalance, and thermal dysregulation. Numerous pathological states, such as diabetes mellitus, peripheral vascular disease, thermal injuries, or degloving lead to inadequate wound healing, necessitating medical intervention. Established wound healing techniques such as autologous and allogeneic skin grafts are inefficient due to the limited availability of donor tissues or probable immunogenic reactions. Current research in the field of tissue engineering aims to facilitate wound healing and restore skin functionality, focusing on key aspects of wound healing, such as extracellular matrix (ECM) reorganization, cell growth, and collagen synthesis/deposition. The research aims at developing and characterizing an in-vitro fibrin gel culture model system that stimulates the process of wound healing. The specific goal of this research is to investigate how the varied chemical composition of fibrin hydrogels can enhance fibroblast proliferation and promote accelerated collagen matrix formation, which is a significant step in tissue repair and regeneration.The fibrin gels are optimized by modulating the primary gel constituents (i.e. the concentrations of fibrin and thrombin). The ensuing hydrogels are characterized using Scanning Electron Microscope and compression testing to test for fiber size, porosity, elasticity, and mechanical properties. Cultured fibroblasts are used to investigate the effects of varying fibrin concentrations on cell-biomaterial interactions, including cell proliferation, cellular infiltration, and network formation. Furthermore, matrix formation and maturation as a function of fibrinogen concentration as defined by collagen matrix deposition, are also studied. Increasing the fibrinogen concentration, lead to an increase in elasticity and Young’s modulus, while a decrease in thrombin concentration generated a stronger fiber structure. Additionally, a decrease in fibrinogen concentration resulted in an increased proliferation rate of fibroblast cells, suggesting better cell adhesion and network formation within the gel substrate. These results were consistent and confirmed by quantifying a mature collagen matrix deposited by fibroblasts when subjected to ascorbic acid. In summary, this research investigates how the varied chemical composition of fibrin hydrogels can enhance fibroblast proliferation and promote accelerated collagen matrix formation, which is a significant step in tissue repair and regeneration. / Bioengineering

No description available.

Quadratic spatial soliton interactions were investigated in this Dissertation. The first part deals with characterizing the principal features of multi-soliton generation and soliton self-reflection. The second deals with two beam processes leading to soliton interactions and collisions. These subjects were investigated both theoretically and experimentally. The experiments were performed by using potassium niobate (KNBO3) and periodically poled potassium titanyl phosphate (KTP) crystals. These particular crystals were desirable for these experiments because of their large nonlinear coefficients and, more importantly, because the experiments could be performed under non-critical-phase-matching (NCPM) conditions. The single soliton generation measurements, performed on KNBO3 by launching the fundamental component only, showed a broad angular acceptance bandwidth which was important for the soliton collisions performed later. Furthermore, at high input intensities multi-soliton generation was observed for the first time. The influence on the multi-soliton patterns generated of the input intensity and beam symmetry was investigated. The combined experimental and theoretical efforts indicated that spatial and temporal noise on the input laser beam induced multi-soliton patterns. Another research direction pursued was intensity dependent soliton routing by using of a specially engineered quadratically nonlinear interface within a periodically poled KTP sample. This was the first time demonstration of the self-reflection phenomenon in a system with a quadratic nonlinearity. The feature investigated is believed to have a great potential for soliton routing and manipulation by engineered structures. A detailed investigation was conducted on two soliton interaction and collision processes. Birth of an additional soliton resulting from a two soliton collision was observed and characterized for the special case of a non-planar geometry. A small amount of spiraling, up to 30 degrees rotation, was measured in the experiments performed. The parameters relevant for characterizing soliton collision processes were also studied in detail. Measurements were performed for various collision angles (from 0.2 to 4 degrees), phase mismatch, relative phase between the solitons and the distance to the collision point within the sample (which affects soliton formation). Both the individual and combined effects of these collision variables were investigated. Based on the research conducted, several all-optical switching scenarios were proposed.

Single-pass frequency conversion by a nonlinear optical crystal is an attractive method to generate coherent radiation in various spectral domains from ultraviolet to mid-infrared. Wavelength converters based on quasi-phase matched (QPM) periodically poled lithium niobate (PPLN) have proved to be important wavelength conversion devices for many useful applications. This thesis develops a novel integration design for temperature controlling and temperature wavelength tuning of a QPM-PPLN waveguide wavelength converter. A Cr/Pt/Au thin film alloy layer is deposited on a PPLN device with a polymer buffer layer to work as a micro-heater and a temperature sensor at the same time. The temperature of the device can be tailored by applying current to the micro-heater layer, which changes the effective period of the QPM grating and thus the QPM wavelength through the thermal optical effect (TOE). The device's temperature can be monitored by measuring the resistance change of the alloy layer. Micro-heater design and mode profile simulation are involved in the thesis. The entire device fabrication process is introduced. Both electrical and optical features of the device are characterized and discussed. In contrast to the conventional temperature tuning method based on a bulky oven, the proposed design has some excellent characteristics such as compact package size and low power consumption. / Thesis / Master of Applied Science (MASc)

In this thesis, detailed studies are presented into self-assembled, noncentrosymmetric, optically active films. Second harmonic generation (SHG) is used to measure the second order nonlinear optical susceptibility (?(2)) of ionically self-assembled monolayer (ISAM) thin films. Conventional ISAM films are fabricated by alternately immersing a substrate into oppositely-charged polyelectrolyte solutions. The polyelectrolytes bind electrostatically to the oppositely-charged substrate, and thus reverse the charge of the substrate. The charge reversal limits the amount of adsorbed material and primes the substrate for the next layer. During the deposition of the nonlinear optical (NLO) active layer, the chromophores are attracted to the oppositely-charged surface, which results in net orientation of the chromophores. Some of the net orientation is lost during the deposition of the next NLO-inactive layer as this layer orients some of the chromophores away from the substrate. A disadvantage of the polymer ISAM deposition method is that although there is a net orientation toward the substrate, a large number of chromophores are randomly or oppositely oriented. This reduces the nonlinear optical response. To overcome this problem, two alternative methods with a better net orientation are discussed: hybrid covalent / ionic deposition and multivalent monomer deposition. In both deposition methods, the NLO-active material is a monomer instead of a polymer. In hybrid covalent / ionic deposition, the NLO-inactive polymer is deposited using electrostatic attraction while the NLO-active monomer is deposited covalently. This forces alignment of the chromophores. The multivalent method uses chromophores with multiple charges on one side of the molecule and one charge (same sign) on the other. The difference in electrostatic attraction causes a preferential orientation of the chromophores during deposition. Attempts have been made to further improve the net orientation by complexation of the monomers with cyclodextrins (cone shaped organic molecules), so far with only limited success. The SHG response of NLO-active layers near the glass and air interfaces is much stronger than the SHG response of layers in the bulk of the film for all deposition methods and NLO-active materials investigated in this thesis. For larger number of bilayers (the bulk regime), the square root of the SHG signal increases linearly with the number of bilayers as expected for a uniform chromophore orientation. We isolated the interface effects through use of buffer layers of NLO-inactive polymers. The glass interface effect extends roughly one bilayer deep for all investigated materials. The air interface effect is different for polymers and monomers. For monomers, this effect extends only one bilayer deep, while it extends multiple layers deep for polymers. Using glass cells to contain the polyelectrolyte solutions, we were able to measure the SHG signal in situ, which proved to be a powerful tool to monitor the deposition rate as a function of chosen parameters. All depositions were rapid, on the order of one minute or less. Provided that a minimum concentration is met, the deposition rate and final SHG values are independent of concentration. Bulk layers deposit at the same rate as layers near the interface. For polymer NLO-active layers a secondary, slower growth of SHG is observed that is presumably due to reorganization of the adsorbed polymer layer. This secondary growth is not observed in the deposition of NLO-active monomers. / Ph. D.

Detailed studies are presented of thin films that self-assemble into the noncentrosymmetric structure required for second order nonlinear optical responses. Second harmonic generation is used as a probe of the second-order nonlinear optical susceptibility (c(2)) of ionic self-assembled monolayer (ISAM) films. Films produced from the ISAM technique are shown to possess significant c(2), with values presently comparable to quartz (c(2)=1.53*10⁹ esu). These films show exceptional stability over time, with negligible decrease in c(2) after 26 months. ISAM films self-assemble from polyelectrolyte solutions due to coulombic interactions between a charged substrate and the charged polymer in solution. This process is self-limiting since charge overcompensation at the surface restricts further deposition as like charges accumulate at the surface. We have found that this 'kinetically hindered equilibrium' occurs quickly for the samples studied, after approximately 45 seconds immersion. Non-centrosymmetry is obtained during deposition as chromophores orient towards the substrate as a strong, localized collection of opposite charge. This net orientation is partially diminished as some amount of chromophore extends in the opposite direction at the film/solution interface. Second harmonic measurements suggest that chromophores at the outermost interface collapse against the film surface when dried, resulting in a larger c(2) than other 'capped' layers. Any polymer which is not located at the interfaces is thought to possess random orientation, and therefore does not contribute to c(2). We have investigated how ionic strength and solution pH affect the structure of ISAM films. These parameters serve to control the electrostatic screening in solution. Low salt concentrations result in low or no electrostatic screening. As a result, charges on a polymer strongly feel one another's presence, and decrease the net electrostatic energy by maximizing their distance from each other. This results in a rod-like conformation, which when adsorbed onto the film surface produces thin layers. Large salt concentrations serve to screen the electrostatic interaction. Because charges do not experience the strong repulsion from their neighbors, the polymer backbone is more likely to loop and coil. If the polymer is weakly soluble (pH near the solubility edge), the polymer will loop about itself and other polymer chains in order to reduce the number of polymer/water contacts. Increased screening results in adsorption of thicker films. We show that this also results in a marginal increase in film density, likely due to an increase in polymer interpenetration of adjacent layers. We can associate an increase in chromophore population at the interface with this increase in density. The reduction in screening also is shown to decrease the chromophore orientation angle, presumably by decreasing the repulsion between charges located on the chromophore ends. The improved orientation leads to an increased non-centrosymmetry in the layer. c(2) is decreased, however, as film thickness (and therefore the population of randomly oriented chromophore between interfaces) increases faster than the improvements to non-centrosymmetry at the interface. We have investigated the thermal stability of ISAM films at elevated temperatures, and have found that these films do not exhibit a permanent decay of c(2) with increased temperature as do poled guest-host polymers. A temperature-dependent decrease is observed for temperatures up to 250°C. This decrease is completely reversible (for films heated to 150C), with c(2) recovering its initial value upon cooling in spite of a glass transition temperature measured as Tg=140°C The decrease in c(2) is thought to be due to a combination of effects. Predominant decrease is thought to be due to disassociation of ionic bonds, which serve to provide noncentrosymmetry in films. A slower, smaller decay due to decrease in moisture content of the films at high temperature is also thought to be present. / Ph. D.

The interaction of a high power, high frequency (HF) pump/electromagnetic (EM) wave transmitted from a ground-based station with the ionosphere, experiments which have been termed "ionospheric heating", produces secondary radiation known as stimulated electromagnetic emissions (SEEs). SEEs have been developed into powerful diagnostics yielding information such as electron temperature, ion species and hydrodynamic evolution of the modified ionospheric plasma. Classic SEEs which exist outside ±1 kHz of the pump wave frequency (ω0) have recently been classified into wideband SEEs (PW-WSEEs) and distinguished from narrowband SEEs (PW-NSEEs) which exist within ±1 kHz of ω0, where the "PW" prefix has been used to indicate that the frequency regimes in the aforementioned classification are relative to the pump wave (PW) frequency. The occurrence of SEEs near 2ω0 is known as second harmonic generation (SHG). SHG is longstanding and well-established in the field of Laser Plasma Interactions (LPI) where SHG has been harnessed to yield diagnostics such as the velocity of the critical region of the plasma, inference of the region in the plasma where the interaction that results in SHG occurs, plasma turbulence and density scale lengths. Past studies of ionospheric heating SHG were limited by the effective radiated power (ERP) available at ionospheric heating facilities and the frequency resolution of receivers/spectrum analyzers of the time. Experimental observations from these past studies reported either SEEs produced as a result of SHG in isolation or compared these SEEs with PW- WSEEs. Moreover, these experiments did not evaluate effects such as transmit ERP, tilt of the transmit antenna beam from the geomagnetic field (B0) and the offset of ω0 from harmonics of the electron gyrofrequency (ωce) on SEEs within a narrowband of twice the pump wave frequency produced as a result of SHG. Also, these studies did not attempt to draw from the knowledge-base on SHG from LPI. The novelty of the experimental observations in this dissertation is the juxtaposition of PW-NSEEs and second harmonic narrowband SEEs (SH-NSEEs), which are SEEs within kHz of 2ω0, measured at the same time. The heating experiments were all performed at HAARP using an O-mode polarized EM pump wave. Additionally, these measurements evaluate the effects on SHG of the transmit ERP, tilt of the transmit station antenna beam from the geomagnetic field (B0) and the offset of ω0 from nωce, n = 2, 3. The experimental observations show, for the first time, a clear association between PW-NSEEs and SH-NSEEs. This association is subsequently used, in conjunction with theories from LPI to propose the non-linear wave-mixing mechanisms responsible for the SH-NSEEs. As a prelude to harnessing the wealth of diagnostics that can be obtained from SHG, initial diagnostics of the velocity of the critical region and the interaction region where SHG occurs are determined using theories from LPI. With the association between PW-NSEEs and SH-NSEEs established, Particle- In-Cell (PIC) simulations are used to investigate the characteristics of a PW- NSEE herein referred to as the "SBS line", produced as a result of stimulated Brillouin scatter (SBS) instability in which the pump EM wave decays into a backscattered EM wave and an ion acoustic wave. The PIC simulations reveal that for high pump powers, the SBS line, which is intense at the onset of the heating experiment, is suppressed within 3 seconds due to the development of cavities in the ionospheric plasma (density) in which the pump wave depletes its energy in heating up electrons. Although, no PIC simulation results of SHG have been presented in this work, the association between PW-NSEEs and SH-NSEEs shown in this work is used to propose that similar mechanisms are responsible for the suppression the SBS line and its associated SH-NSEE for high pump powers. Results from ionospheric heating experiments presented in this dissertation show a rapid suppression of both the SBS line and its associated SH-NSEE for high pump powers. The attribution of the suppression of SH-NSEEs to the development of artificial field-aligned irregularities (AFAIs) in a past study fails to explain the rapid suppression in the experimental observations contained herein since the suppression occurs on a much faster timescale than the development of AFAIs. Thus, the PIC model results have led to a more feasible interpretation of the observed rapid suppression. To re-iterate, the contributions of this dissertation are as follows: 1. First observations of an SH-NSEE named "SH decay line" within 2ω0±30 Hz. The SH decay line occurs at the same transmit power as the SBS line within ω0±30 Hz and both of these SEEs are suppressed for ω0 ≈ 3ωce. Offset of the SH decay line from 2ω0 is twice the offset of the SBS line from ω0. 2. First experimental evaluation of the impact of B0 assessed by stepping the transmit beam offset from B0 and stepping ω0 near 2ωce shows contemporaneous SH-NSEEs and PW-NSEEs both ordered by the O+ ion cyclotron frequency. 3. First experimental observations of suppression of SBS line and SH decay line for high pump powers, which unlike a past study cannot be attributed to AFAIs. 4. First PIC simulation investigation of suppression of SBS line observed during high pump power ionospheric heating, revealing depletion of pump energy in heating electrons in cavities created in the plasma (density) as the mechanism responsible for the suppression. Broadening of SBS line observed in ionospheric heating with high power is also observed in PIC simulation results. This work has laid the foundations to develop SHG into powerful ionospheric diagnostics. / Doctor of Philosophy / When a high power, high frequency radio wave is injected from a ground-based transmit station into the ionosphere, a region of Earth's atmosphere containing charged particles in addition some neutral atoms and molecules, the frequency spectrum measured at a location removed from the transmit station shows emissions at other frequencies in addition to an emission at the transmit frequency. The emissions at these other frequencies are known as stimulated electromagnetic emissions (SEEs). The frequency offsets of SEEs contain information such as the average kinetic energy associated with random motion of electrons, a parameter known as electron temperature and the ion species present in the region of the ionosphere the radio wave is injected into. The occurrence of SEEs near twice the pump wave frequency is known as second harmonic generation. This dissertation presents experimental observations that compare SEEs which exist within ±1 kHz of the transmit frequency with SEEs which exist within a similar frequency range of twice the transmit frequency unlike past studies. This dissertation also investigates effects of varying the transmit frequency, power and the direction of the transmit station antenna beam relative to the local direction of the magnetic field of the Earth. These new studies reveal, for the first time, a similarity in characteristics of the SEEs near the transmit frequency and two times the transmit frequency. This similarity is used in conjunction with theories from studies of Laser Plasma Interaction (LPI), which have corollaries with high power radio wave-ionosphere interaction, to propose the processes that underlie the occurrence of SEEs near twice the transmit frequency. Methods from LPI have also been used for the first time to obtain measurements of some parameters of the ionosphere. High power radio wave-ionosphere interaction experiments are very expensive and moreover, direct measurement of ionospheric parameters/processes require radar facilities which may not be available or sounding rockets or satellites which increase the cost of experiments. Computer simulations offer a facile and an inexpensive means to investigate SEEs and processes internal to the ionosphere. Computer simulations have been used for the first time in this dissertation to investigate the mechanisms responsible for the characteristics of SEEs near the transmit frequency for low and high transmit powers. Since an association has been established in this dissertation between SEEs near the transmit frequency and SEEs near twice the transmit frequency, the mechanisms responsible for the characteristics for the SEEs near the transmit frequency for high transmit power, have been proposed to be the same mechanisms responsible for the characteristics of SEEs near twice the transmit frequency for a similar transmit power regime. The experimental results, computer simulation results and the corollaries drawn between high power radio wave-ionosphere interaction and LPI detailed in this dissertation have opened new doors to develop SEEs near twice the transmit frequency into a powerful tool to study the ionosphere.