The energy absorbing behavior of stainless steel 304L during the pulsed Nd:YAG laser welding is investigated in this thesis. The equivalent absorptivity is estimated from the comparison of measured and finite element method (FEM) results simulated melting pool shape parameters, e.g. pool width, pool depth, cross-section area and total volume of the pool. To simulate the actual pulsed laser beam, the energy density of heating source is performed as a Guassian distribution in the transection of a circular laser beam. For evaluating the feasibility and the accuracy of the estimated equivalent absorptivity, the multi-pulsed Nd:YAG laser welding is simulated by using the estimated absorptivities. A good agreement between this simulated and measured melting pool shapes are found in the multi-pulsed laser welding. The equivalent absorptivity can be interpolated from different parameters of the molten pool. However, absorptivity curve fitted from the cross-section area and total volume of the melting pool provide a more stable value. Results also indicate that the absorptivity and the pulse energy are in inverse proportion. The thermal-elastic-plastic FEM model is employed to simulate the fusion and solidification process of the pulsed laser welding. A complicate residual stress distribution introduced from the shrinkage in the solidification process is also calculated and presented. The distribution of post-weld-deformation near the melting pool has also been studied in this thesis. This post-weld-deformation may be a key factor in high precision laser welding, e.g. laser packaging for the optoelectronic components. The absorptivity estimated in this thesis may be helpful to simulate the laser welding process accurately.

<p> This thesis reports findings from three series of experiments related to ultrashort laser pulse interactions with materials. The first series investigates the formation of laser induced ripples that have spatial periods much shorter than the irradiation wavelength after laser irradiation. The second series of experiments explores the capabilities of ultrashort pulse laser micromachining on optical fiber modifications for niche applications. Lastly, preliminary work in establishing a double-pulse ablation technique is reported. </p> <p> The first set of experiments reported in this thesis investigates the morphology of surface ripples that are generated when irradiated with multiple ultrashort laser pulses. Two types of surface ripples can form after irradiation. The fust type has spatial periods near the wavelength of the irradiation pulses and the second has spatial periods substantially below the irradiation wavelength (typically 114 to 115 of the free-space irradiation wavelength are observed in our studies). These substantially subwavelength ripples form when the irradiation wavelength corresponds to a photon energy that is below the bandgap of the target material. The Ti:Sapphire laser systems used in this series of experiments provides pulses centered around 800 nm. Gallium phosphide (GaP) was chosen to be the main material for investigation since 800 nm corresponds to a photon energy that is below the bandgap of this material; no frequency conversion needs to be carried out when GaP is the material of choice for subwavelength ripples studies. In this series of experiments substantially different irradiation conditions were investigated: pulse durations varied from 150 fs to 7 ns, laser energies ranges from well above the ablation and modification threshold to well below, both 800 nm and 400 nm wavelengths, and "scrambled" (where polarization was rotated between each successive pulse) polarization as well as circular polarization were used. Microscopy techniques employed to study these ripples include optical microscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Cross-sectional studies with transmission electron microscopy were also carried out by using focused ion beam milling to prepare thin specimens across irradiated regions. Sapphire was also used as the irradiation target for 800 nm and 400 nm pulses since it has a large bandgap and even 400 nm corresponds to an energy that is below its bandgap. Irradiation conditions where the two types of ripples are observed are determined. Also, microscopy of the ripple features provided insights in to the formation mechanism of the subwavelength ripples. </p> <p> In the second series of experiments, preliminary work was performed to investigate the capabilities of ultrashort laser micromachining in fiber optic applications. This series of experiments can be subdivided in to two categories. </p> <p> The goal of the first fiber investigation was to create a slit in a metallic coating deposited on a fiber facet. Such a feature might eliminate the use of external slits (e.g. for spectrometers), especially ifthe output of the fiber depends on its geometry (e.g. polarization-maintaining fiber). The first experiment carried out was micromachining of a ~ 180 nm layer of gold that was deposited on a glass substrate, in order to determine irradiation conditions where the gold layer can be removed while the glass is not damaged. Once the irradiation condition was established by studying the micromachined gold layer on glass substrate, gold layers were deposited on fiber facets for micromachining experiments. The results showed promising potential, but fme tuning of the irradiation parameters, and processing as well as microscopy techniques are needed before useful applications can be realized. </p> <p> The second set of fiber experiments investigates irradiation conditions that are appropriate to micromachine features into fibers such as v-grooves and beveled ends. Preliminary work was carried out to determine a suitable focusing scheme for this application. Different pulse durations and a pulse train were also employed in hope of minimize chipping and cracking. This investigation did not reach a conclusion on whether micromachining with ultrashort laser pulses are in fact suitable for processing of optical fibers, where high quality facets are required. Future investigation could provide further information on the feasibility of laser micromachining on fabricating features in optical fibers. </p> <p> Lastly, a double-pulse ablation scheme was established and explored. Double-pulse ablation had been reported in the literature to improve material removal rate and the appearance of the fmal morphology. However, this setup can be adapted to investigate the ablation mechanisms and provide insight into the state of the material at different time frames of ablation. While the experimental results are preliminary, this technique showed potential, along with possible extensions of this technique, to further investigate the ablation mechanisms. </p> / Thesis / Master of Applied Science (MASc)

Microwave spectroscopy is a gas phase technique typically geared toward measuring the rotational transitions of Molecules. The information contained in this type of spectroscopy pertains to a molecules structure, both geometric and electronic, which give insight into a molecule's chemistry. Typically this type of spectroscopy is high resolution, but narrowband ≤1 MHz in frequency. This is achieved by tuning a cavity, exciting a molecule with electromagnetic radiation in the microwave region, turning the electromagnetic radiation o, and measuring a signal from the molecular relaxation in the form of a free induction decay (FID). The FID is then Fourier transformed to give a frequency of the transition. "Fast passage" is defined as a sweeping of frequencies through a transition at a time much shorter (≤10 s) than the molecular relaxation (≈100 s). Recent advancements in technology have allowed for the creation of these fast frequency sweeps, known as "chirps", which allow for broadband capabilities. This work presents the design, construction, and implementation of one such novel, high-resolution microwave spectrometer with broadband capabilities. The manuscript also provides the theory, technique, and motivations behind building of such an instrument. In this manuscript it is demonstrated that, although a gas phase technique, solids, liquids, and transient species may be studied with the spectrometer with high sensitivity, making it a viable option for many molecules wanting to be rotationally studied. The spectrometer has a relative correct intensity feature that, when coupled with theory, may ease the difficulty in transition assignment and facilitate dynamic chemical studies of the experiment. Molecules studied on this spectrometer have, in turn, been analyzed and assigned using common rotational spectroscopic analysis. Detailed theory on the analysis of these molecules has been provided. Structural parameters such as rotational constants and centrifugal distortion constants have been determined and reported for most molecules in the document. Where possible, comparisons have been made amongst groups of similar molecules to try and get insight into the nature of the bonds those molecules are forming. This has been achieved the the comparisons of nuclear electric quadrupole and nuclear magnetic coupling constants, and the results therein have been determined and reported.

Azo-Dye Doped Liquid Crystal (DDLC) is a developed material which can be used to fabricate optical shutter, displays, etc. In this thesis, we presents of the transient grating on a planar aligned DDLC. The effect of various polarizations of writing and probing beams, and of temperature on the transient grating are examined. Then, we propose a model to explain the result. Through this study, we understand the factors that determine the light-induced aligning Dye effect on nematic liquid crystals.

Recently, lifetime imaging has become a subject of intensive research. Lifetime is an important parameter to understand the dynamics of targeted objects and its applications ranges from fluorescence decay of biological objects to relaxation of semiconductor materials and devices. Many methods, such as time-correlated single photon counting (TCSPC) and phase detection in frequency domain, were developed to measure the characteristic lifetime. These methods are now rather matured and widely applied in various studies. However, these methods are only effective for lifetime longer than 100 picoseconds due to the bandwidth limitation of high-speed electronics. For even faster temporal resolution, novel techniques that do not rely on high-speed electronics will be required. In this study, we are integrating an autocorrelator with a galvo-based laser scanning microscope to enable imaging with very high temporal resolution. The principle and technique of pump-probe is implemented through the autocorrelator. In this way, imaging based pump-probe measurements can be realized. Specifically, we have applied the experimental setup so developed in measuring fluorescent dyes and semiconductor devices.

The characterization of work functions and field emission stability for molybdenum and iridium oxide coatings was examined. Single emission tips and flat samples of molybdenum and iridium oxide were prepared for characterization. The flat samples were characterized using X-ray Photoelectron Spectroscopy and X-ray diffraction to determine elemental composition, chemical shift, and crystal structure. Flat coatings of iridium oxide were also scanned by Atomic Force Microscopy to examine topography. Work functions were characterized by Ultraviolet Photoelectron Spectroscopy from the flat samples and by Field Emission Electron Distributions from the field emission tips. Field emission characterization was conducted in a custom build analytical chamber capable of measuring Field Emission Electron Distribution and Fowler-Nordheim I-V plots simultaneously to independently evaluate geometric and work function changes. Scanning Electron Microscope pictures were taken of the emission tips before and after field emission characterization to confirm geometric changes. Measurement of emission stability and work functions were the emphasis of this research. In addition, use of iridium oxide coatings to enhance emission stability was evaluated. Molybdenum and iridium oxide, IrO2, were characterized and found to have a work function of 4.6 eV and 4.2 eV by both characterization techniques, with the molybdenum value in agreement with previous research. The analytic chamber used in the field emission analysis demonstrated the ability to independently determine the value and changes in work function and emitter geometry by simultaneous measurement of the Field Emission Energy Distribution and Fowler-Nordheim I-V plots from single emitters. Iridium oxide coating was found to enhance the stability of molybdenum emission tips with a relatively low work function of 4.2 eV and inhibited the formation of high work function molybdenum oxides. However, the method of deposition of iridium and annealing in oxygen to form iridium oxide on molybdenum emitters left rather severe cracking in the protective oxide coating exposing the molybdenum substrate.

A texturização por microusinagem a laser é uma importante alternativa para que se consiga melhorar a ligação de aderência entre materiais compósitos e o titânio, aplicados em componentes estruturais na indústria aeronáutica. A execução de texturização em chapas de titânio deve-se ao fato de que a técnica de junção preferida para muitos materiais compósitos é a ligação adesiva. Neste trabalho foram realizadas texturizações em chapas de titânio utilizando laser com pulsos ultracurtos de largura temporal da ordem de femtossegundos. Tal processo resultou em mínima transferência de calor para o material, evitando assim deformação superficial da chapa de titânio bem como a formação de material ressolidificado na região ablacionada. Estes inconvenientes ocorreram na utilização do laser chaveado com pulsos de nanossegundos. Foram executadas três tipos de texturizações utilizando laser com pulsos de femtossegundos, com variações nas distâncias entre as linhas usinadas. Pela análise das superfícies obtidas, detectou-se que a molhabilidade aumenta quando há o aumento da distância entre as linhas da texturização. Avançando nas análises, pela perfilometria óptica das superfícies texturizadas observou-se que há sensível aumento do volume disponível para penetração do adesivo estrutural quando são diminuídas as distâncias entre as linhas texturizadas. Nos ensaios de tração realizados observou-se que há o aumento da resistência ao cisalhamento da junta adesiva com a diminuição da distância entre as linhas texturizadas. / The texturing laser micromachining is an important alternative to improve the bonding adhesion between composites and titanium, which are applied to structural components in the aerospace industry. The texturing running on titanium plates is due to the fact that the preferred joining technique for many composite materials is the adhesive bonding. In this work, titanium plates were texturized using laser ultrashort pulses temporal widths of femtoseconds. This process resulted in minimal heat transfer to the material, avoiding deformation of the titanium plate surface as well as the formation of resolidified material in the ablated region. These drawbacks have occurred with the use of nanoseconds pulses. Were performed three types of texturing using laser with femtosecond pulses, with variations in the distances between the machined lines. The analysis of the obtained surfaces found that the wettability increases when there is the increased distance between the texturing lines. Advancing in the analysis by optical profilometry of textured surfaces was observed that there is substantial increase in the volume available for penetration of structural adhesive when the distances between the textured lines are diminished. In tensile tests conducted it was observed that there is an increase in shear strength of the adhesive joint by reducing the distance between the textured lines.

A texturização por microusinagem a laser é uma importante alternativa para que se consiga melhorar a ligação de aderência entre materiais compósitos e o titânio, aplicados em componentes estruturais na indústria aeronáutica. A execução de texturização em chapas de titânio deve-se ao fato de que a técnica de junção preferida para muitos materiais compósitos é a ligação adesiva. Neste trabalho foram realizadas texturizações em chapas de titânio utilizando laser com pulsos ultracurtos de largura temporal da ordem de femtossegundos. Tal processo resultou em mínima transferência de calor para o material, evitando assim deformação superficial da chapa de titânio bem como a formação de material ressolidificado na região ablacionada. Estes inconvenientes ocorreram na utilização do laser chaveado com pulsos de nanossegundos. Foram executadas três tipos de texturizações utilizando laser com pulsos de femtossegundos, com variações nas distâncias entre as linhas usinadas. Pela análise das superfícies obtidas, detectou-se que a molhabilidade aumenta quando há o aumento da distância entre as linhas da texturização. Avançando nas análises, pela perfilometria óptica das superfícies texturizadas observou-se que há sensível aumento do volume disponível para penetração do adesivo estrutural quando são diminuídas as distâncias entre as linhas texturizadas. Nos ensaios de tração realizados observou-se que há o aumento da resistência ao cisalhamento da junta adesiva com a diminuição da distância entre as linhas texturizadas. / The texturing laser micromachining is an important alternative to improve the bonding adhesion between composites and titanium, which are applied to structural components in the aerospace industry. The texturing running on titanium plates is due to the fact that the preferred joining technique for many composite materials is the adhesive bonding. In this work, titanium plates were texturized using laser ultrashort pulses temporal widths of femtoseconds. This process resulted in minimal heat transfer to the material, avoiding deformation of the titanium plate surface as well as the formation of resolidified material in the ablated region. These drawbacks have occurred with the use of nanoseconds pulses. Were performed three types of texturing using laser with femtosecond pulses, with variations in the distances between the machined lines. The analysis of the obtained surfaces found that the wettability increases when there is the increased distance between the texturing lines. Advancing in the analysis by optical profilometry of textured surfaces was observed that there is substantial increase in the volume available for penetration of structural adhesive when the distances between the textured lines are diminished. In tensile tests conducted it was observed that there is an increase in shear strength of the adhesive joint by reducing the distance between the textured lines.

The dissertation is split into two parts. The first part will be focused on changes in material properties found at the nanoscale, as miscibility and electronic structure can change significantly with size. The formation of classically-immiscible bimetallic nanoparticles (BNPs) becomes favorable at the nanoscale and novel catalytic properties can emerge from the bimetallic alloying. The formation of alloyed and non-alloyed BNPs is achieved through pulse laser ablation (PLA) and a significant increase in catalytic activity is observed for both. Recently discovered, the increased activity in the non-alloyed BNPs, deemed multicomponent photocatalysis, is examined and the proposed mechanism discussed. The second part of the talk will focus on thermal barrier coatings (TBCs), which are advanced, multi-layered coatings used to protect materials in high temperature environments. MCrAlY (M=Ni, Co) bond coats deposited via atmospheric plasma spray (APS) are intrinsically rough and initially the roughness provides a high surface area platform for the mechanical interlocking of the yttria stabilized zirconia (YSZ) top coat, which provides the bulk of the thermal insulation. After high temperature exposure, a protective oxide scale forms at the top coat/bond coat interface however the convex asperities of the bond coat can grow non-α-Al2O3 type oxides that can be detrimental for coating lifetime. A surface modification technique that removes the asperities while leaving intact the concavities is used to examine the role that roughness distribution has on 1100°C APS coating lifetime. Lastly, recent work validating a modelling strategy for evaluating 900°C TBC lifetimes, which can typically surpass 25 kh, is presented. Differences in coating-substrate interdiffusion behavior over 5-20 kh of 900°C exposure are discussed and reproduced with Thermo- Calc/DICTRA for three superalloys (1483, 247, X4) deposited with high velocity oxy fuel (HVOF) NiCoCrAlY coatings.

Carbon fiber-reinforced plastics (CFRPs) have gained widespread popularity as a lightweight, high-strength alternative to traditional materials. The unique anisotropic properties of CFRP make processing difficult, especially using conventional methods. This study investigates laser cutting by ablation as an alternative by comparing two near-infrared laser systems to a typical mechanical machining process. This research has potential applications in the automotive and aerospace industries, where CFRPs are particularly desirable for weight savings and fuel efficiency. First, a CNC mill was used to study the effects of process parameters and tool design on machining quality. Despite high productivity and flexible tooling, mechanical drilling suffers from machining defects that could compromise structural performance of a CFRP component. Rotational feed rate was shown to be the primary factor in determining the axial thrust force, which correlated with the extent of delamination and peeling. Experimental results concluded that machining quality could be improved using a non-contact laser-based material removal mechanism. Laser machining was investigated first with a Yb:YAG fiber laser system, operated in either continuous wave or pulse-modulated mode, for both cross-ply and woven CFRP. For the first time, energy density was used as a control variable to account for changes in process parameters, predicting a logarithmic relationship with machining results attributable to plasma shielding effects. Relevant process parameters included operation mode, laser power, pulse overlap, and cross-ply surface fiber orientation, all of which showed a significant impact on single-pass machining quality. High pulse frequency was required to successfully ablate woven CFRP at the weave boundaries, possibly due to matrix absorption dynamics. Overall, the Yb:YAG fiber laser system showed improved performance over mechanical machining. However, microsecond pulses cause extensive thermal damage and low ablation rates due to long laser-material interaction time and low power intensity. Next, laser machining was investigated using a high-energy nanosecond-pulsed Nd:YAG NIR laser operating in either Q-Switch or Long Pulse mode. This research demonstrates for the first time that keyhole-mode cutting can be achieved for CFRP materials using a high-energy nanosecond laser with long-duration pulsing. It is also shown that short-duration Q-Switch mode results in an ineffective cutting performance for CFRP, likely due to laser-induced optical breakdown. At sufficiently high power intensity, it is hypothesized that the resulting plasma absorbs a significant portion of the incoming laser energy by the inverse Bremsstrahlung mechanism. In Long Pulse mode, multi-pass line and contour cutting experiments are further performed to investigate the effect of laser processing parameters on thermal damage and machined surface integrity. A logarithmic trend was observed for machining results, attributable to plasma shielding similar to microsecond fiber laser results. Cutting depth data was used to estimate the ablation threshold of Hexcel IM7 and AS4 fiber types. Drilling results show that a 2.2 mm thick cross-ply CFRP panel can be cut through using about 6 laser passes, and a high-quality machined surface can be produced with a limited heat-affected zone and little fiber pull-out using inert assist gas. In general, high-energy Long Pulse laser machining achieved superior performance due to shorter pulse duration and higher power intensity, resulting in significantly higher ablation rates. The successful outcomes from this work provide the key to enable an efficient high-quality laser machining process for CFRP materials.