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Item Numerical modelling of nanoparticle diffusion and microstructure formation during selective laser melting process(Montana State University - Bozeman, College of Engineering, 2022) Alam, Taosif; Chairperson, Graduate Committee: M. Ruhul AminSelective laser melting (SLM) is a popular metal additive manufacturing technique that has a wide range of industrial applications lately. This additive process allows the development of new metal matrix nanocomposites by fusing metallic powders with nanoparticles. However, the molten pool flow generated by a moving laser heat source has complex fluid dynamics which redistribute the nanoparticles. Consequently, the microstructures of the solidified molten pool are affected by the local distribution of nanoparticles, which is reflected in their mechanical properties. Smaller grains can increase the strength and isotropic behavior of the solid layers. Therefore, the current research aims to numerically investigate the relationships among the SLM process parameters, nanoparticle transport, and microstructure evolution to explore the formation of nanocomposites. The current study formulated a three-dimensional computational fluid dynamics (CFD) model of the SLM process in a commercial software package, ANSYS FLUENT. A volumetric laser heat source model melted the aluminum alloy powders and the underlying solid substrate. The difference between the powder and the solid or liquid state of the metal alloy was defined using an effective thermal conductivity model. Lagrangian particle transport calculation was performed to track TiB 2 nanoparticles in the molten pool. This model was coupled with a 2D Cellular Automata (CA) model to simulate the solidified microstructure using MATLAB. Finally, a detailed parametric analysis was conducted to study the effects of varying laser power, scanning speed, and preheating temperature. The numerical results showed that the maximum temperature and Marangoni convection in the molten pool increased at higher laser powers, higher preheating temperatures, and lower scanning speeds. The particle-voided region was significantly large with high Marangoni convection but decreased with weaker Marangoni convection. The simulated microstructure was dominated by large columnar grains when nanoparticles were not considered. The introduction of nanoparticles disrupted the columnar grain growth by promoting small, randomly oriented, equiaxed grains. A decrease of 30%-40% in average grain diameter was measured at the cross-section of the solidified layer when nanoparticles were present. The qualitative comparison of the microstructures showed that the grains were smaller in the uniformly distributed particle region compared to the particle-voided region.Item Transient passive mode-locked ND:YAG laser using a semiconductor saturable absorber mirror(Montana State University - Bozeman, College of Letters & Science, 2022) Shaffer, Heather Rose; Chairperson, Graduate Committee: Joseph A. ShawQ-switched-mode-locking in a Nd:YAG bulk resonator was demonstrated using a semiconductor saturable absorber mirror (SESAM). A 10-W-pulsed-diode-pumped Nd:YAG laser system at Quantel USA by Lumibird, Inc. was adapted for mode-locking operation in a breadboard setup. Three SESAM mirrors were tested with initial reflectivities R 0=85%, 90%, and 95% in several cavity configurations to show enhanced sub-nanosecond pulse modulation at the free spectral range of each resonator. Transient Q-switched and long-pulse envelopes are shown with underlying mode-locked pulse modulation.Item Digitally automated alignment of a phase-shifting point diffraction interferometer(Montana State University - Bozeman, College of Engineering, 2020) Field, Nathaniel James; Chairperson, Graduate Committee: Joseph A. ShawReal-time sensing of wavefront error in laser instruments is an exceptionally useful tool for fine-tuning of laser systems during fabrication. Measurement and correction for potential wavefront aberrations are especially important for high-energy laser system applications, such as defense and industrial manufacturing. The self-referencing Mach-Zehnder interferometer and the Shack-Hartmann wavefront sensor are two common methods used to achieve real-time wavefront aberration measurements for laser system output quality; however, the former requires a precise and arduous alignment procedure for each operation and the latter exchanges spatial resolution for phase resolution and is highly sensitive to global tilt. The use of electronically controlled spatial light modulators has been shown as a method of quickly retrieving wavefront reconstructions from phase-shifting point diffraction interferometers. In this paper, the development of an algorithm that automates the selection of the point diffractor position and size was added to the phase-shifting point diffraction method with a purely reflective spatial light modulator. Computer simulations and laboratory tests were conducted as proofs of concept using a few simple optical elements. The results of these simulations and lab measurements show promise for continually automated alignment of a point diffraction interferometer to greatly reduce alignment time and almost entirely remove sensitivity to global tilt. With further development, this method can be applied to increase the efficiency of a wide variety of optical system measurement scenarios.Item Development of an active/adaptive laser scanning microscope(Montana State University - Bozeman, College of Engineering, 2018) Archer-Zhang, Christian Chunzi; Chairperson, Graduate Committee: David L. DickensheetsLaser scanning techniques such as confocal microscopy and two-photon excitation fluorescence microscopy (TPM) are powerful tools for imaging biological samples with high resolution, offering three-dimensional (3D) visualization of the behavior of cells in their natural environment. Traditionally, the 3D images are acquired from 2D image stacks with focusing depth controlled through mechanical movement of the specimen relative to the objective lens. The slow mechanical movement (~<20Hz) does not allow the spot of light to be scanned axially sufficiently fast to monitor cell:cell and cell:environment interactions in real time over hundreds of microns in all three dimensions. A fast focus control mirror supports agile scan patterns such as vertical or oblique planes or even arbitrary surfaces, minimizing the time and photo damage required to monitor features of interest within the 3D volume. Because aberrations cause image quality to decrease as the focal point of the beam penetrates deeper into the sample, adaptive optics can enhance resolution and contrast at depth for confocal microscopy and TPM. Combining a fast focus control mirror with a fast aberration correcting mirror leads to a flexible platform called the active/adaptive laser scanning microscope, capable of aberration-corrected beam scanning throughout a 3D volume of tissue. This opens up the possibility of fully corrected, variable-depth imaging along oblique sections or more complex user-defined surfaces within a single image frame.Item Photonic applications of functionalized gold nanorods: progress towards nonlinear metamaterials(Montana State University - Bozeman, College of Letters & Science, 2016) Latterman, Ryan Eric Michael; Chairperson, Graduate Committee: Robert Walker; This dissertation contains one article of which Ryan Eric Michael Latterman is not the main author.Nonlinear processes are used to convert one color of laser light into another and are very useful to any research who needs multiple colors of light for basic research or commercial purposes. However, nonlinear processes are inherently very weak and require high input energy to utilize them. Using modeling, it was predicted that ordered arrays of gold nanorods (GNRs), acting as a metamaterial, could be used to fabricate a material that would exhibit field enhancement. Because nonlinear processes depend on the strength of the field in which they interact, enhancing the field strength while using the same amount of input power would make these processes much more efficient and useful. Progress towards these materials included the synthesis of GNRs and gold reactive polymers. These polymers were used to solubilize GNRs in organic solvents while also introducing attachment points for nonlinear chromophores. To study a material which resembled the one in our modeling predictions, flat gold and two dimensional arrays of gold nanopillars were functionalized with gold-reactive nonlinear organic chromophores and studied using sum frequency generation. It was found that flat gold samples functionalized with a nonlinear chromophore exhibited a tremendous SFG signal, but only in one input polarization configuration. However, gold nanopillar samples exhibited a significant SFG signal in both PPP and SSP polarization configurations. These data agree with our modeling results and indicate that the materials produced here have the potential to be used as a mirror-less optical parametric oscillator. Organic-soluble GNRs produced in this thesis were then used in an adjacent project to improve the efficiency of a diode-pumped solid state laser design. Nd:YAG lasers are routinely used to produced 1064 nm light by pumping at 808 nm with a semi-conductor diode laser. However, when 1064 nm light is reflected back into the laser cavity, a parasitic phenomenon called amplified spontaneous emission (ASE) occurs. ASE can be combatted by applying a material to the laser rod that absorbs at 1064 nm. GNRs were synthesized at a specific size to absorb at 1064 nm, solubilized in epoxy and applied to a Nd:YAG laser, increasing efficiency by almost two fold.Item Laser frequency stabilization to spectral hole burning frequency references in erbium-doped crystals : material and device optimization(Montana State University - Bozeman, College of Letters & Science, 2002) Bottger, ThomasItem Silicon nitride deformable mirrors for focus and spherical aberration correction in micro-optical systems(Montana State University - Bozeman, College of Engineering, 2002) Himmer, Phillip Alexander; Chairperson, Graduate Committee: David Dickensheets.Laser beam scanning systems benefit from dynamic focus control and aberration correction using a deformable mirror, enabling 3D real-time scanning. Designed for focus and spherical aberration control in optical beam scanning systems, these mirrors are capable of video rate bandwidths allowing real-time 3D imaging in micro-optical scanning systems. Field curvature aberration can also be corrected with the same mirror. Previous work with polysilicon deformable mirrors validated the concept of using specialized deformable optics in beam scanning systems. Fabrication of micro-optics in this dissertation was achieved using a modified surface micromachining technique with silicon nitride replacing polysilicon as the structural material. A bulk anisotropic silicon etch following a PSG release etch allows the creation of a variable cavity depth, overcoming the typical deformation limitations of standard surface micromachining. To perform as desired, parabolic curvature of the mirror surface is required with the ability to introduce positive and negative quartic curvature. Analytical theory showed that deformed simply supported plates have curvature closer to parabolic than deformed clamped plates. Segmentation and thinning of the perimeter was done to in order to emulate this simply supported boundary. Two annular actuation zones were implemented to give independent control over the second and fourth order curvatures. It was found that residual stress present in the silicon nitride structural plate improved the surface curvature and mechanical resonance at the expense of a larger actuation pressure. Mirrors with diameters of 1500, 1000, 750, and 300 microns were built and tested. Zonal actuation with annular electrodes proved successful in providing sufficient correction of the mirror surface curvature, allowing spherical aberration free focus control. Perimeter segmentation greatly reduced required actuation loads allowing improved focal ranges. It was found that the sacrificial layer thickness has a significant impact on the initial curvature of the mirror. Sacrificial layers 0.2 microns thick proved sufficient for release, improved device yield, and resulted in an initially flat mirror.Item The diode-pumped continuous-wave Raman laser : classical, Quantum, and thermo-optic fundamentals(Montana State University - Bozeman, College of Letters & Science, 2002) Roos, Peter AaronItem Two-photon absorption and two-photon-resonant four-wave mixing for the Tb^3+ ion in insulators(Montana State University - Bozeman, College of Letters & Science, 1987) Huang, JinItem Oxidation of N:Al(110)(Montana State University - Bozeman, College of Letters & Science, 1990) Brown, Wade William