Reversed-phase liquid chromatography (RPLC) is a commonly used separation technique in chemistry. Nevertheless, the mechanistic interactions at the molecular level among the eluent, analyte, and the stationary phase are not fully understood. Because of this limited understanding, optimization of the separation must be done experimentally. Learning more about molecular interactions should aid in improving separations. We are currently using second-harmonic generation (SHG) spectroscopy to investigate how analytes adsorb to the surface. SHG is a spectroscopic technique that produces signal only at places of non-isotropic symmetry; this typically occurs at surfaces. SHG can be used to produce surface isotherms of test analytes adsorbed to a model C18 stationary phase surface. Fitting these isotherms with a Langmuir model produces an adsorption equilibrium constant. However, the equilibrium constant can only be accurately determined if the true bulk concentration is known; this thesis describes an approach to ensure this. The equilibrium constant relates to Gibbs free energy and is the start to obtaining other thermodynamic information. The long equilibration times of analytes with the stationary phase observed in this study emphasize the importance of both thermodynamic information and kinetic values for understanding retention. Once equilibrium constants and other parameters are accurately obtained, this information can be used to improve predictions and calculations from numerical models.

The rapid progress on developing new and improved multifunctional materials, for optoelectronic and spin based phenomena/devices, have increased the importance of the fundamental understanding of their coherent states and nonlinear optical properties. This study is aimed at characterizing, modeling, and controlling the fundamental electronic, phononic, and spin properties of several classes of materials through nonequilibrium and nonlinear light-matter interactions, coupled with a novel design of the material phases, interfaces, and heterostructures. This research directly addresses the Grand Challenges identified in the Basic Energy Sciences Advisory Committee report "Directing Matter and Energy: Five Challenges for Science and the Imagination" (Hemminger, 2007) [1], in particular, the area: "Matter far beyond equilibrium" and addresses the questions, "How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties?" and "How do we design and perfect atom- and energy-efficient synthesis of revolutionary new forms of matter with tailored properties?". The knowledge gained from these fundamental studies can provide new information for a broad community to provide concepts for the next generation of multifunctional materials and devices, and resulted in several publications and conference presentations. The materials studied in this dissertation included multiferroic BaTiO3-BiFeO3 [2], ferroelectric Pb0.52Zr0.48TiO3 (PZT), InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7]. Probing multiferroics, which are materials that can exhibit ferromagnetic, ferroelectric, and ferroelastic orders simultaneously in a single phase, was a main focus of this study. BiFeO3 (BFO) is the most widely investigated multiferroic due to its high Neel and Curie temperatures and has antiferromagnetic and ferroelectric properties [8]. An inherent drawback of BFO is its large leakage currents. In this project, (1 − x)BaTiO3-(x)BiFeO3, x = 0.725 (BTO-BFO) heterostructures were investigated [9], where the conductivity of the solid solution can be reduced by adding another perovskite material, BaTiO3 [2]. We aimed to study optically induced coherent states in our BTO-BFO structures. Time resolved pumpprobe spectroscopic measurements were performed at room temperature as well as at low temperature (100 K) up to 10 T. Coherent acoustic phonons were observed both in a film and nanorods, resulting in coherent phonon frequencies of 27 and 33 GHz, respectively [2]. Coherent phonon spectroscopy is a sensitive tool to characterize the interfaces and can be employed as an effective ultrasensitive quantum sensor [10]. Furthermore, in the nanorods arrays of BTO-BFO, an additional oscillation with frequency in the range of 8.1 GHz was observed. This frequency is close to a theoretically predicted magnon frequency which could indicate the coexistence of coherent phonons and magnons in the nanorods arrays [2]. In an analogy to photonics which relies on electromagnetic waves, magnonics utilizes spin waves to carry and process information, offering several advantages such as an operation frequency in the THz range. Recently, "a quantum tango" [11] was reported where coupled coherent magnon and phonons modes were formed on a surface patterned ferromagnet. Furthermore, BTO-BFO heterostructures were probed using transient birefringence and magneto-optical Kerr effect spectroscopy. The results demonstrated that the magnetic field dependence of coherent phonons, measured by these two techniques, exhibits more sensitivity to the external magnetic fields compared to the differential reflectivity technique [2]. Moreover, nonlinear optical properties of this structure were investigated via second harmonic generation spectroscopy, where wavelength and polarization dependence of this nonlinear observation will be discussed in this dissertation. As part of this study, another class of multiferroic materials, with strong ferroelectric and piezoelectric properties, Pb0.52Zr0.48TiO3 (PZT) was studied [12]. In this project, the nonlinear optical properties of PZT nanorod arrays were investigated. Clear signatures of second harmonic generations from 490-525 nm (2.38-2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties in the range of 690-1000 nm where multiphoton processes were responsible for the photoexcitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼ 3.1 eV) was close to the bandgap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications. Furthermore, the established techniques during my research provided new insights on optical properties of InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7], and the results will be briefly presented in this dissertation. / Doctor of Philosophy / My research activities have explored multifunctional materials and heterostructures with strongly enhanced coupled electric and magnetic orders and optical properties. In particular, pursuing novel heterostructure designs such as multiferroics can provide control over electric and magnetic ordering in mixed dimensionality. This, together with control at the level from lattice structure to electron spin states can give rise to improved or even qualitatively new and robust materials properties. For example, a better understanding of the phenomena associated with the spin degree of freedom of electrons allows for advancement in spintronic device applications such as storage, logic, and sensors, which are associated with quantum computers and quantum communications [13, 14, 15]. Overarching questions and goals of my activities included: What are the microscopic origins and mechanisms of nonlinear response in strongly coupled nanostructured materials and its relationship to electronic, spin, and lattice degrees of freedom? (2) What are the effects of dimensionality and quantum confinement on optical properties? (3) How do we control and manipulate the coherent states, such as coherent phonons and magnons using external and internal fields, material composition, and morphology to achieve maximal efficiency and tunability? Addressing many of the challenges in the fast-paced technological world requires continued developments of new materials with enhanced optical properties. The knowledge gained from my fundamental studies can provide new information for the next generation of multifunctional materials and devices with advanced optical properties and resulted in several publications and conference presentations.

Semiconductor based micro- and nano-structures grown in a systematic and controlled way using selective area growth are emerging as a promising route toward devices for integrated optical circuitry in optoelectronics and photonics field. This dissertation focuses on the experimental investigation of the nonlinear optical effects in selectively grown gallium nitride micro-pyramids that act as optical cavities, zinc oxide submicron rods and indium gallium nitride multiple quantum well core shell submicron tubes on the apex of GaN micro pyramids that act as optical antennae. Localized spatial excitation of these low dimensional semiconductor structures was optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multi-photon luminescence (MPL). The evolution of both processes are mapped along the symmetric axis of the individual structures for multiple fundamental input frequencies of light. Effects such as cavity formation of generated light, electron-hole plasma generation and coherent emission are observed. The efficiency and tunability of the frequency conversion that can be achieved in the individual structures of various geometries are estimated. By controlling the local excitation cross-section within the structures along with modulation of optical excitation intensity, the nonlinear optical process generated in these structures can be manipulated to generate coherent light in the UV-Blue region via SHG process or green emission via MPL process. The results show that these unique structures hold the potential to convert red input pulsed light into blue output pulsed light which is highly directional.

Battery energy storage system (BESS) is promising to be implemented in residential applications for supporting PV integration, load shifting, and backup power purposes. For this application, 48V second-life battery draws more and more attentions for their cost-effectiveness, safe voltage level, reliability, and potential large market. This thesis proposes the comprehensive control and design of an isolated battery-driven grid interface (IBDGI) with the dual-active-bridge (DAB) converter for residential applications with 48V battery pack. The three-phase DAB converter is a promising candidate as the front-end DC/DC converter in the two-stage IBDGI due to its high efficiency, high power density, and low capacitance requirement. An effective design strategy for the three-phase DAB converter is proposed based on the zero-voltage-switching (ZVS) zone and back-ow power to achieve high efficiency for a wide operating voltage range and different load conditions. Based on the power loss model, an easily-implemented variable switching frequency operating method is proposed to further increase the efficiency at light load conditions. The dead-time effect is observed in the three-phase DAB converter. To avoid the dead-time effect and better understand the phenomena, a comprehensive analysis is proposed. All the cases of the dead-time effect in the three-phase DAB converter are analyzed in terms of the buck, boost, and matching states. The expressions of the transmission power, constraint conditions, and key time of the dead-time effect are derived for each state. The operation waveforms of the dead-time effect are also presented. The hybrid capacitor bank composed by the LC resonant lter with electrolytic capacitor and lm capacitor is utilized for the DC bus of the IBGDI. The electrolytic capacitors work as passive decoupling purpose while the lm capacitor is responsible for high switching harmonic ltering. Moreover, a current sharing method between the hybrid capacitor bank is proposed to extend the electrolytic capacitor's life. The LCL single-phase inverter is applied for the downstream of the IBDGI. A step-by-step design procedure of the LCL lter with passive damping is proposed for the 120V/240V dual grid-tied and standalone modes. The PR controllers are also designed for the LCL inverter for standalone and grid-tied modes. At the system level, a novel second harmonic current (SHC) reduction strategy is proposed for the IBDGI with the three-phase DAB converter by adding a load current feedforward (LCFF) path to the DAB voltage closed-loop controller. This method will suppress the SHC without modi cations of the original controller's bandwidth, which make it easy to be implemented. The small-signal model of the three-phase DAB converter is provided and veri ed by the step response. The parameter sensitivity analysis for the LCFF method is proposed to show that the SHC is well suppressed within ±20% parameter error. The proposed converter and control methods are veri ed by simulation and experimental results. / Thesis / Doctor of Philosophy (PhD)

Around 80% of women experience vaginal tears during labor when the diameter of the vagina must increase from ~2.5 cm to ~9.5 cm to allow the passage of a full-term baby. Vaginal tears vary from superficial cuts of the mucosal lining to tears propagating through the entire vaginal wall and into the surrounding tissues and organs. Complications associated with vaginal tears include postpartum hemorrhaging, fecal incontinence, urinary incontinence, and dyspareunia. Beyond the agonizing pain, these complications are emotionally and psychologically traumatic for women. Prevention, evaluation, and treatment of vaginal tears and subsequent conditions are limited due to the lack of studies examining the mechanical behavior of the pelvic floor tissues. Therefore, the mechanical response of healthy and torn vaginal tissue is investigated here to establish quantitative metrics for maternal healthcare. Toward this end, swine and rat vaginal tissue is subjected to biaxial loads until tearing to reveal its mechanical properties. The resulting large inhomogeneous deformations are measured by the digital image correlation optical method to calculate material strain. The influence of these strains near to and far from the immediate vicinity of the tears on the tearing behavior is studied. Coupling mechanisms of the mechanical properties in the circumferential and axial directions as well as their effect on the nature of tear resistance is studied. Collagen fibers, the component within tissue responsible for its strength under tension, are imaged using a multiphoton microscopy technique known as second-harmonic generation imaging to investigate the change in organization with mechanical loading. Furthermore, imaging is performed in the near-regions of tears to reveal the relationship between collagen fibers and tearing resistance. The data collected through these studies provide new knowledge on the nonlinear elastic behavior of vaginal tissue, the geometrical and micro-structural characteristics of tears, and the mechanisms that contribute to the formation and propagation of tears. The mechanical properties and tearing mechanisms of vaginal tissue will be crucial in developing new prevention and treatment methods for maternal trauma following childbirth. Episiotomy, late-term stretching, surgical treatment with graft materials and other protocols will all benefit from a mechanically-informed perspective. It is our hope that this work will raise awareness to the serious complexities of pelvic floor trauma and encourage a more refined and systematic approach to the inspection, imaging, and treatment of all vaginal tears following delivery. This work was supported in part by the National Science Foundation fund #1511603 and the Institute for Critical Technology and Sciences at Virginia Tech. / Doctor of Philosophy / Every year nearly three million women give birth vaginally with 80% experiencing vaginal tears. These injuries sustained during delivery vary with severity and are associated with several conditions, including pelvic floor disorders. These disorders are a set of long-term conditions of the pelvic region presently affecting one-fourth of adult women in the United States. Pelvic floor disorders are: pelvic organ prolapse - a pelvic organ such as the uterus "falls" from its natural position, urinary incontinence - difficulty controlling urination, and fecal incontinence - difficulty controlling bowel movements. Pelvic floor disorders lower the quality of life for women not only physically due to pain and daily discomfort, but also mentally as the disorders are generally perceived as an embarassing and private matter. The pelvic floor represents a complex system of muscles, organs, and support structures that work together to ensure everything stays in place and is functioning properly. Injury to any of these structures poses the risk of developing a disorder. As a central supporting organ, injuries to the vagina may be particularly worrisome. Surprisingly, little is known about the magnitude of forces and/or stretching that is placed on the pelvic floor during delivery, how much force and/or stretching is required for an injury, or how various injuries relate to future complications. The goal of this research is to describe how much the normal, healthy vagina stretches to various forces including forces that will result in injuries. The research further examines the stretching of injured vaginas to quantify any observable differences due to this injury. Finally, the relationships between the biological components of the vagina, such as collagen, and the forces placed on the vagina are examined. The result of this work will provide doctors and engineers with guidelines for understanding the conditions that produce vaginal injuries. The relationships examined between the tissue makeup and forces exerted onto the tissue may also aid in identifying any irregularities that would place a woman at risk for injury. Many of the medical procedures surrounding childbirth as well as surgical treatment for pelvic floor disorders will benefit from knowing how far the vagina can stretch before being injured. It is our hope that this work will raise awareness to the serious complexities of pelvic floor injuries and encourage a more refined and systematic approach to the inspection, imaging, and treatment of all vaginal tears following delivery. This work was supported in part by the National Science Foundation fund #1511603 and the Institute for Critical Technology and Sciences at Virginia Tech.

Plasmonic nanostructures can enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices are optimized to operate in a single wavelength band, which is not efficient for wavelength-multiplexed operations and quantum optical applications involving multi-photon nonlinear processes at multiple wavelength bands. Overcoming the limitations of single-resonant plasmonics requires development of plasmonic devices that can enhance the optical interactions at the same locations but at different resonance wavelengths. This dissertation comprehensively studies the theory, design, and applications of such devices, called "multiresonant plasmonic systems with spatial mode overlap". We start by a literature review to elucidate the importance of this topic as well as its current and potential applications. Then, we briefly discuss the fundamentals of plasmonic resonances and mode hybridization to thoroughly explore, classify, and compare the different architectures of the multiresonant plasmonic systems with spatial mode overlap. Also, we establish the black-box coupled mode theory to quantify the coupling of optical modes and analyze the complicated dynamics of optical interactions in multiresonant plasmonic systems. Next, we introduce the nanolaminate plasmonic crystals (NPCs), wafer-scale metamaterials structures that support many (>10) highly-excitable plasmonic modes with spatial overlap across the visible and near-infrared optical bands. The enabling factors behind the NPC's superior performance as multiresonant systems are also theoretically and experimentally investigated. After that, we experimentally demonstrate the NPCs application in simultaneous second harmonic generation and anti-Stokes photoluminescence (ASPL) with controllable nonlinear emission properties. By designing specific non-linear optical experiments and developing advanced ASPL models, this work addresses some important but previously unresolved questions on the ASPL mechanism as well. Finally, we conclude the dissertation by discussing the potential applications of out-of-plane plasmonic systems with spatial mode overlap in wavelength-multiplexed devices and presenting some preliminary results. / Doctor of Philosophy / Emergence of electronic devices such as cellphones and computers has revolutionized our lifestyles over the past century. By manipulating the flow/storage of electrons at the nanometer scale, electronic components can be very compact, but their speed and energy performance is ultimately limited due to ohmic losses and finite velocity of the electrons. In parallel, photonic devices and circuits have been proposed that by molding the flow of light can overcome the mentioned limitations but are not as integrable as their electronic counterparts. Plasmonics is an emerging research field that combines electronics and photonics using nanostructures that can couple the light waves to the free electrons in metals. By confining the light at deep subwavelength scales, plasmonic devices can highly enhance the light-matter interactions, with applications in ultrafast optical communications, energy-harvesting, optical sensing, and biodetection. Conventionally, plasmonic devices are optimized to operate with a single light color, which limits their performance in wavelength-multiplexed operations and ultrafast non-linear optics. For such applications, it is far more efficient to use the more advanced "multiresonant plasmonic systems with spatial mode overlap" that can enhance the optical interactions at the same locations but for multiple light colors. This dissertation comprehensively studies these systems in terms of the fundamental concepts, design ideas, and applications. Our work advances the plasmonic field from both science and technology perspectives. In particular, we explore and classify the strategies of building multiresonant plasmonic systems with spatial mode overlap for the first time. Also, we establish the black-box coupled mode theory, a novel framework for analysis and design of complicated plasmonic structures with optimized performance. Furthermore, we introduce the "nanolaminate plasmonic crystals" (NPCs), large area and cost-effective devices that can enhance the optical processes for both visible and near-infrared lights. Finally, we demonstrate NPCs ability in simultaneous frequency-doubling and broadband emission of light and come up with advanced theoretical models that can explain the light generation and color conversion in plasmonic devices.

The increase in the magnitude of local electric fields through resonances of plasmonic excitations in metallic nanoparticles is a major area of current optical research. This dissertation is focused on plasmon-enhanced second harmonic generation of organic ionic self-assembled films via localized surface plasmon resonance of gold nanorods. By matching the plasmon resonance of the gold nanorods to the wavelength of the fundamental light, it is possible to greatly enhance the SHG efficiency. To demonstrate this, the surface of the gold nanorods was functionalized with a nonlinear-optical (NLO) polymer, PCBS, via the layer-by-layer method and deposited on a polymer thin film created on a glass substrate using the ionic self-assembled multilayer (ISAM) method. The sample fabrication is divided into two parts: gold nanorod synthesis and functionalization. The gold nanorods were synthesized by the seed-mediated method with varying amounts of silver ions to control their LSPR wavelengths. The functionalization started by replacing the original thick CTAB bilayer on the surface of the gold nanorods by a thin PAH-DTC layer via dialysis. The nanorods were then alternately coated with PAH (polycation) and PCBS (NLO polyanion) up to three bilayers of PAH/PCBS. The number of polymer layers on the nanorods was chosen in consideration of the LSPR decay length (a few nm). The functionalized gold nanorods were then deposited on either PAH/PCBS or PAH/PSS ISAM films. Characterization was performed via optical spectral measurement, zeta potential measurement, and field-emission scanning electron microscopy (FESEM). The LSPR wavelength shifted when the surrounding medium changed. It was red-shifted for each added polymer layer on the nanorod surface. However, when the functionalized nanorods were deposited on the ISAM film, the resonance peak blue-shifted. The zeta potential confirmed the proper electric charge of each polymer layer coated on the nanorods. Finally, FESEM was performed on the samples for visual inspection of the nanorod deposition and distribution after the SHG measurement was complete. The SHG from the functionalized gold nanorods was measured using a Maker-like fringe method. In this method, second harmonic waves generated from the front and rear sides of the substrate interfere constructively and destructively when the sample is rotated with respect to the incoming pump wave. Electrical noise reduction techniques were implemented to improve the SHG signal readings. Signal processing was implemented using LabVIEW software in order to read a reliable SHG signal from the setup. The maximum tolerable fluence of the gold nanorods was determined in order to prevent optical damage. The interference fringe pattern was observed from the functionalized gold nanorods and compared with that from the conventional ISAM film. The enhancement from the gold nanorods was as high as 600 times compared to the bare films. Polarization dependent SHG measurements were conducted to ascertain the effect of coupling between p- or s-polarized fundamental incident light to the SH light. To further improve the SHG enhancement, the self-assembly method herein can be extended from a monolayer to multilayers of functionalized gold nanorods. / Ph. D. / The field of optics examines the interactions of light and matter. The most commonly observed optical phenomena are the reflection and refraction of light where the frequency of light remains unchanged. However, when light becomes intense, interesting optical phenomena occur where the frequency of the outgoing light differs from that of the incoming light. With the invention of the first working laser in 1960, many interesting nonlinear phenomena were experimentally confirmed, including second harmonic generation (SHG) which was the first nonlinear optical process to be observed. In the original SHG experiment, a visible ruby laser was illuminated into a quartz crystal which produced UV light. This demonstrated light frequency doubling corresponding to wavelength halving from 694 nm (ruby laser) to 347 nm (UV light). Following progress in molecular engineering, many organic materials and polymers have been employed to study nonlinear optics for applications such as optical frequency conversion, electro-optic modulation, and second harmonic generation imaging microscopy. Nonetheless, the SHG conversion efficiency is very low due to phase-mismatch. This stems from frequency dispersion in a medium, where the incoming light and the generated light travel at different velocities. In the past, efforts toward enhancing the SHG conversion efficiency was focused on selecting specific crystals in which the incoming light and the second harmonic light pass through different effective path lengths in the medium. Although the phase-matching method is the most effective method to achieve high conversion efficiency it is also important to increase the intrinsic nonlinearity of a material. A new multidisciplinary approach using the surface plasmon resonance has become an important technique for improving the conversion efficiency. Plasmons are the collective oscillation of electrons on a metal surface. At the resonant optical frequency, the amplitude of the plasmon oscillation becomes maximized. When metallic nanoparticles are resonantly illuminated with light, the electric field can be locally intensified at the sharp boundaries of the nanoparticle. Since the intensity of SHG increases by the square of the incoming light intensity, the SHG efficiency can be greatly enhanced via surface plasmons on the metal nanoparticles. In this dissertation, the fabrication of new optical materials incorporating gold plasmonic nanoparticles for SHG enhancement was demonstrated. The plasmonic nanomaterials were fabricated by coating the surface of gold nanorods with nonlinear polymer films and depositing them on another polymer thin film on a flat glass substrate. The enhanced SHG intensity was measured and compared with that of the conventional nonlinear polymer films alone. It was observed that the enhancement from the gold nanorods was as high as 600 times. To further improve the SHG enhancement, the surface modified gold nanorods can be extended from a single layer to multilayers on the polymer film substrate.

This thesis examines second-order optical nonlinear wave mixing processes in domain-disordered quasi-phase matching waveguides and evaluates their potential use in compact, monolithically integrated wavelength conversion devices. The devices are based on a GaAs/AlGaAs superlattice-core waveguide structure with an improved design over previous generations. Quantum-well intermixing by ion-implantation is used to create the quasi-phase matching gratings in which the nonlinear susceptibility is periodically suppressed. Photoluminescence experiments showed a large band gap energy blue shift around 70 nm after intermixing. Measured two-photon absorption coefficients showed a significant polarization dependence and suppression of up to 80% after intermixing. Similar polarization dependencies and suppression were observed in three-photon absorption and nonlinear refraction. Advanced modeling of second-harmonic generation showed reductions of over 50% in efficiency due to linear losses alone. Self-phase modulation was found to be the dominant parasitic nonlinear effect on the conversion efficiency, with reductions of over 60%. Simulations of group velocity mismatch showed modest reductions in efficiency of less than 10%. Experiments on second-harmonic generation showed improvements in efficiency over previous generations due to low linear loss and improved intermixing. The improvements permitted demonstration of continuous wave second-harmonic generation for the first time in such structures with output power exceeding 1 µW. Also, Type-II phase matching was demonstrated for the first time. Saturation was observed as the power was increased, which, as predicted, was the result of self-phase modulation when using 2 ps pulses. By using 20 ps pulses instead, saturation effects were avoided. Thermo-optically induced bistability was observed in continuous wave experiments. Difference frequency generation was demonstrated with wavelengths from the optical C-band being converted to the L- and U-bands with continuous waves. Conversion for Type-I phase matching was demonstrated over 20 nm with signal and idler wavelengths being separated by over 100 nm. Type-II phase matched conversion was also observed. Using the experimental data for analysis, self-pumped conversion devices were found to require external amplification to reach practical output powers. Threshold pump powers for optical parametric oscillators were calculated to be impractically large. Proposed improvements to the device design are predicted to allow more practical operation of integrated conversion devices based on quasi-phase matching superlattice waveguides.

Radiation damage occurs in reactor pressure vessel (RPV) steel, causing microstructural changes such as point defect clusters, interstitial loops, vacancy-solute clusters, and precipitates, that cause material embrittlement. Radiation damage is a crucial concern in the nuclear industry since many nuclear plants throughout the US are entering the first period of life extension and older plants are currently undergoing assessment of technical basis to operate beyond 60 years. The result of extended operation is that the RPV and other components will be exposed to higher levels of neutron radiation than they were originally designed to withstand. There is currently no nondestructive evaluation technique that can unambiguously assess the amount of radiation damage in RPV steels. Nonlinear ultrasound (NLU) is a nondestructive evaluation technique that is sensitive to microstructural features such as dislocations, precipitates, and their interactions in metallic materials. The physical effect monitored by NLU is the generation of higher harmonic frequencies in an initially monochromatic ultrasonic wave, arising from the interaction of the ultrasonic wave with microstructural features. This effect is quantified with the measurable acoustic nonlinearity parameter, beta. In this work, nonlinear ultrasound is used to characterize radiation damage in reactor pressure vessel steels over a range of fluence levels, irradiation temperatures, and material composition. Experimental results are presented and interpreted with newly developed analytical models that combine different irradiation-induced microstructural contributions to the acoustic nonlinearity parameter.

Le développement des lasers de puissance engendre un intérêt pour la recherche de matériaux présentant des propriétés optiques non linéaires (ONL). Les matériaux vitreux sont de très bons candidats puisqu’ils peuvent être transparents et élaborés en grandes dimensions. La précipitation de particules non centro-symétrique dans un verre permet d’engendrer cette propriété en volume, et d’ingérer facilement ce matériau dans les dispositifs lasers. Dans ce contexte, cette thèse présente les résultats obtenus sur la précipitation de la phase LiNO3 dans la matrice vitreuse 35 Li2O- 25 Nb2O5- 40 SiO2. Le mécanisme de cristallisation de cette phase est étudié par analyse thermique, imagerie optique et électronique ainsi que par une analyse in-situ. Ces analyses mettent en évidence une cristallisation sphérolitique du niobate de lithium dans ce verre, conduisant à l’obtention de vitrocéramiques. Les propriétés optiques non linéaires d’ordre deux sont mesurés sur ces matériaux. Un signal original et isotrope de Génération de Second Harmonique a été mesuré. Une analyse multi-échelle permet une meilleure compréhension et une corrélation entre la structure des sphérolites et l’origine de la génération d’un tel signal. Le modèle développé suite à ces analyses permet d’entrevoir le développement de nouveaux matériaux micro-composites à propriétés ONL isotropes. / The high power laser development required the need of materials with nonlinear properties. Glass materials can be considered as ideal materials as they can be transparent and elaborated in very large dimension. Precipitation of non-centro symmetric crystalline particles in bulk glass leads to a material with bulk nonlinear properties. This glass-ceramic should be then easily integrated in such laser facilities. In this thesis, the results concerning the precipitation of the LiNO3 phase in the glassy-matrix 35 Li2O- 25 Nb2O5- 40 SiO2 are detailed. The crystallization mechanism of this phase is studied through thermal analysis, optical and electronic microscopy as well as in-situ analyses. These studies reveal glass-ceramics are obtained through a precipitation of the lithium niobate crystalline phase in spherulite shape. The nonlinear optical properties are investigated on this materials and an original, isotropic Second Harmonic Generation signal (SHG) is registered in the bulk glass-ceramic. A complete study using a multi-scale approach allows the correlation between the spherulite structure and the nonlinear optical properties. A mechanism at the origin of the SHG signal is proposed. This leads to a new approach for transparent inorganic materials development for isotropic SHG conversion.