Browsing by Author "Rocca, Jorge, committee member"
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Item Open Access A study of the influence of process parameter variations on the material properties and laser damage performance of ion beam sputtered Sc2O3 and HfO2 thin films(Colorado State University. Libraries, 2016) Langston, Peter F., author; Menoni, Carmen, advisor; Rocca, Jorge, committee member; Marconi, Mario, committee member; Yalin, Azer, committee memberThis work is a study of the influence of process parameter variations on the material properties and laser damage performance of ion beam sputtered Sc2O3 and HfO2 thin films using a Vecco Spector ion deposition system. These parameters were explored for the purpose of identifying optically sensitive defects in these high index materials after the deposition process. Using a host of optical metrology and materials analysis techniques we report on the relationship between oxygen partial pressure in the deposition chamber during film growth and optical absorption in the grown material at 1 μm. These materials were found to be prone to excess oxygen incorporation. Positive identification of this excess oxygen is made and exactly how this oxygen is bound in the different materials is discussed. The influence of this defect type on the optical and mechanical properties of the material is also given and discussed. Laser damage results for these single layers are presented. The influence of higher and lower deposition energy was also studied to determine the potential for defect creation both at the surface and in the bulk of the material grown. Optimized thin films of HfO2, Sc2O3 and Ta2O5 were grown and tested for laser damage with a 1030 nm laser having a pulse width of ~375 ps and a nominal spot size of ~100 um FWHM. The laser damage threshold ranking of these materials followed fairly well with the band gap of the material when tested in air. When these same materials were tested in vacuum Sc2O3 was found to be very susceptible to vacuum mediated laser induced surface defect creation resulting in a greatly reduced LIDT performance. Ta2O5 showed much the same trend in that its in vacuum performance was significantly reduced from its in air performance but there was not as great of a difference between the in air and in vacuum performance as there was for Sc2O3. HfO2 also showed a large reduction in its in vacuum LIDT results compared with its in air LIDT values however, this material showed the smallest decrease of the three high index materials tested. A second contribution of this work is in the investigation of the impact of capping layers on the in air and in vacuum LIDT performance of single layer films. Ultra thin capping layers composed of different metal oxides were applied to 100 nm thick single layers of the same high index materials already tested, HfO2, Sc2O3 and Ta2O5. These capped samples were then LIDT tested in air and in vacuum. These ultra thin capping layers were shown to greatly influence the in air and in vacuum damage performance of the uncapped single layers. Damage probability curves were analyzed to retrieve surface and bulk defect densities as a function of local fluence. Methods for maximizing the LIDT performance of metal oxides based on our studied materials for use in air and in vacuum are discussed.Item Open Access Characterization of plasma conductivity by laser Thomson scattering in a high-voltage laser-triggered switch(Colorado State University. Libraries, 2023) Gottfried, Jacob A., author; Yalin, Azer P., advisor; Dumitrache, Ciprian, committee member; Rocca, Jorge, committee memberHigh-voltage laser-triggered switches (HV-LTSs) are used in pulsed-power applications where low jitter and high current are required. The switches allow operation in the mega-ampere, megavolt regime while maintaining low insertion losses. Low inductance HV-LTS designs have shown discrepancies between modeled and experimental behavior, reinvigorating interest in the physics of HV-LTS operation. Detailed spatially- and temporally- resolved measurements of plasma properties within the switches could contribute to validating and advancing numeric models of these systems by checking the assumptions used in their derivation. To date, there is minimal experimental data detailing the evolution of plasma properties during switch operation. This work investigates HV-LTS plasma channel conductivity (the assumption within current models drawing the most critique) during the rising edge of the current pulse through both derivative (V-Dot) electrical probes and electron temperature measurements via laser Thomson scattering. A HV-LTS testbed utilizing an aqueous (variable impedance) resistive load was designed to produce experimental conditions found in larger pulsed power applications. This work describes the design of the load and experimental results under a variety of load conditions and operating voltages of 5 - 6 kV. The results indicate the electron temperature increases during the rising edge of the current pulse, suggesting that the plasma conductivity is temporally evolving. Further, electrical measurements show an increase in plasma conductivity during the rising edge of the current pulse. Evidence from both optical and electrical measurements calls into question the assumption of a temporally constant plasma conductivity as both the optical and electrical diagnostics show a temporally increasing plasma conductivity during the rising edge of the current pulse.Item Open Access Development and testing of a solid core fiber optic delivery system and ultraviolet preionization for laser ignition(Colorado State University. Libraries, 2012) Wilvert, Hurley Nicholas, author; Yalin, Azer, advisor; Marchese, Anthony, committee member; Rocca, Jorge, committee memberLaser ignition of natural gas engines has shown potential to improve many facets of engine performance including brake thermal efficiency, exhaust emissions, and durability as compared with traditional spark ignition. Laser ignition technology has yet to transition to industry primarily because no system for reliably and safely delivering the laser pulse to the combustion chamber exists. This thesis presents a novel fiber optic delivery approach using solid core multimode step index silica fibers with large cladding diameters (400 μm core, 720 μm cladding). Testing was done on the fibers to determine their response to bending, vibration, high power input, and long duration beam transmission. It was found that in configurations representative of what is required on a real engine, and in the presence of vibration, reliable spark formation could be achieved in pressures as low as 3.4 bar using a specially designed optical spark plug. Comparative tests between the fiber delivered laser ignition system and a traditional J-gap spark plug were performed on a single cylinder Waukesha Cooperative Fuel Research (CFR) engine running on bottled methane. Tests were run at three different Net Mean Effective Pressures (NMEP) of 6, 8, and 12 bar at various air-fuel ratios. Results indicate reliable performance of the fiber and improved engine performance at high NMEP and lean conditions. Thesis research also includes initial studies into the use of dual laser pulses for plasma formation and ignition. In this approach, a first ultraviolet pulse preionizes a volume of air while a second overlapped pulse adds additional energy. Electron density measurements reveal the ultraviolet beam generates substantial preionization even with no visual breakdown, and Schlieren images are used to study the interaction between the two beams at atmospheric and lower pressures.Item Open Access Extreme ultraviolet laser ablation mass spectrometer for molecular imaging at the nanoscale(Colorado State University. Libraries, 2018) Kuznetsov, Ilya, author; Menoni, Carmen, advisor; Rocca, Jorge, committee member; Crick, Dean, committee member; Bernstein, Elliot, committee member; Krapf, Diego, committee memberThe demand for high-precision analytical instrumentation in modern science and technology is exploding. The quality of questions to be answered sets the requirements for a given piece of technology. The type of analytical instrumentation that enables users to unambiguously identify, quantify, and map the chemical structure of a solid is imaging mass spectrometry (IMS). Most common commercially available instruments include desorption electrospray ionization (DESI), matrix-assisted laser desorption and ionization (MALDI), and secondary ion mass spectrometry (SIMS) as well as their derivatives. Each of these methods possesses a set of capabilities that define its use for one or another research task. None of them, however, enables scientists to map a solid's molecular composition in three dimensions at the nanoscale. We have developed an extreme ultraviolet laser ablation time-of-flight mass spectrometer (EUV TOF) that relies on sample probing by a 46.9 nm wavelength laser. In this work, the unique interaction of EUV light with matter was experimentally assessed and compared to SIMS TOF. It was found that the spatial resolution can be as small as 80 nm in molecular and atomic analysis in organic and inorganic materials respectively. Depth resolution is as high as 20 nm as measured on an organic bilayer. Sensitivity of the EUV TOF reaches ~0.02 amol, which is estimated to be 20× better than that of SIMS TOF in the sample of the amino acid alanine. Sensitivity in other units—sample utilization efficiency (SUE)—was found to be similar to SIMS TOF when assessed by means of detecting trace actinides in a glass matrix. It was shown that it can be further improved by means of post-ablation ionization (PI) with a secondary UV laser source. Using vacuum ultraviolet (VUV) laser light can increase the mass range of molecular detection. For instance, an intact cholesterol molecule was first detected by EUV TOF operating in VUV PI mode. This approach opens a range of opportunities to use the technique for biological studies. EUV TOF is capable to image chemical composition. This capability is demonstrated by imaging the 3D nanoscale spatial distribution of low mass fragments in a single mycobacterium. With additional instrumental modifications, it will be possible to achieve sub-cellular imaging of the molecular structure of a single microorganism without the need for using externally applied ionization-promoting matrix. Such capabilities may help to steer the development of new drugs in pharmacology and identify the signature isotope pattern of the miniscule bits of material examined by nuclear scientists.Item Open Access Investigation on the structural, mechanical and optical properties of amorphous oxide thin films for gravitational wave detectors(Colorado State University. Libraries, 2024) Castro Lucas, Samuel, author; Menoni, Carmen, advisor; Rocca, Jorge, committee member; Sambur, Justin, committee memberAmorphous oxide thin films grown through physical vapor deposition methods like ion beam sputtering, play a crucial role in optical interference coatings for high finesse optical cavities, such as those used in gravitational wave detectors. The stability of these atomically disordered solids is significantly influenced by both deposition conditions and composition. Consequently, these enable the tuning of structural, mechanical, or optical properties. The sensitivity of current gravitational wave interferometric detectors at the frequency range of around 100 Hz is currently limited by a combination of quantum and coating thermal noise (CTN). CTN is associated with thermally driven random displacement fluctuations in the high reflectance amorphous oxide coatings of the end-test masses in the interferometer. These fluctuations cause internal friction, acting as an anelastic relaxation mechanism by dissipating elastic energy. The dissipated internal elastic energy can be quantified through the mechanical loss angle (Q-1). These unwanted fluctuations associated with mechanical loss can be reduced through modifications of the atomic network in the amorphous oxides. Specifically, the combination of two or more metal cations in a mixed amorphous thin film and post-deposition annealing are known to favorably impact the network organization and hence reduce internal friction. The first study of this thesis reports on the structural modifications between amorphous TiO2 with GeO2 and with SiO2. High-index materials for gravitational wave detectors such as amorphous TiO2:GeO2 (44% Ti), have been found to exhibit low mechanical loss post-annealing at 600°C. Reaffirming annealing to be a major contributor to reducing mechanical loss this thesis examines: a) cation interdiffusion between amorphous oxides of TiO2 with GeO2 and with SiO2 and b) the modifications to the structural properties, both after annealing. The annealing temperature, at which this interdiffusion mechanism occurs, is key for pinpointing structural rearrangements that are favorable for reducing internal friction. Furthermore, to determine whether diffusion occurs into SiO2 after annealing is also important, given that the multi-layer mirrors of gravitational wave detectors utilize SiO2 as a low-index layer. The study of cation interdiffusion used nanolaminates of TiO2, SiO2 and GeO2 to identify cation diffusion across the interface. The results show Ge and Ti cation interfacial diffusion, at temperatures above 500°C. Instead, Si cations diffuse into TiO2 at a temperature around 850°C and Ti into SiO2 at around 950°C. These temperatures correspond to an average of 0.8 of the glass transition temperature (Tg), with Tg=606°C for GeO2 and Tg=1187°C for SiO2. These findings support previous research by our group in amorphous GeO2, which showed that elevated temperature deposition and annealing at 0.8 Tg, leads to favorable organization of the atomic network which is associated with low mechanical loss. The second study of this thesis investigates the structural, mechanical, and optical properties of amorphous ternary oxide mixtures following post-annealing. These mixtures consist of TiO2:GeO2 combined with SiO2 and ZrO2, as well as TiO2:SiO2 combined with ZrO2. Candidate high index layers, such as amorphous TiO2:GeO2 (44% Ti), and TiO2:SiO2 (69.5% Ti) exhibit low mechanical loss after post-annealing at 600°C, and 850°C, respectively. The inclusion of a third metal cation is shown to delay the onset of crystallization to temperatures around 800°C. The addition of a third metal cation also modifies the residual stress of the ternary compared to the binary materials. There is an indication of densification when annealing past 600°C. The reduction in residual tensile stress, combined with the higher crystallization temperature of the ternary mixtures, present attractive properties. These properties will expand the parameter space for post-deposition processing, mainly of the TiO2:GeO2 -based mixtures, to further reduce mechanical loss. This advancement paves the way for amorphous oxide coatings for gravitational wave detectors with lower mechanical loss, aligning with plans for future detectors.Item Open Access Off site and boundary external radiation exposures from the Cotter Uranium Mill located in Cañon City, Colorado(Colorado State University. Libraries, 2017) Townsend, Amanda, author; Johnson, Thomas, advisor; Brandl, Alexander, committee member; Rocca, Jorge, committee memberAlthough many of the uranium mills in the United States have been decommissioned, the mill tailings remain and can pose health threats to those living nearby. Many studies have been done showing the relationship between radon exposure and lung cancer development for those living near a uranium mill, but it seems that little attention has been paid to the possible threat posed by exposure to gamma radiation from these tailings piles. Since 1979 the Cotter Uranium Mill in Cañon City, Colorado has been measuring external gamma exposure rates at the fence line, and at several offsite locations including the closest residence and the Shadow Hills Golf Course. These exposure rate measurements were tested against background and it has been shown that exposure rates above background exist at all locations except for the nearest residence. Assuming full time occupancy, the excess dose received by members of the public from these exposure rates do not exceed regulatory limits of 100 mrem/yr except at the entrance road of the mill, which was remediated in 2009. For a hypothetical person living in the area of highest exposure rate above background, their risk of developing a fatal cancer is only increased by 0.43%. These exposure rates were compared against the background values measured by the Cotter Corporation and published in their 2010 Environmental and Occupational Performance Report, ALARA Review and Annual Report on Remedial Action Plan Activities. It was later discovered that the background values published in this report were read from a dosimeter that was kept in lead shielding at an offsite location. This means that the background values are only transit values, and are not representative of the actual background. As such, a discussion of what justifies an appropriate background measurement as well as its effect on the results of this study are outlined. Lastly, correlation analysis was performed on the exposure rate data to determine if there was an underlying factor effecting all the exposure rates. It was found that a single factor is responsible for 60.28% of the variation in the exposure rates, but the factor affecting the data could not be determined. It was suspected that either precipitation values, cosmic radiation fluctuations, or radium-226 air concentrations may have affected the exposure rates, and, as such, correlation analysis was conducted. It was determined that no correlation exists between any of these variables and the exposure rates measured. The inability to determine the factor contributing to the fluctuation in exposure rates over the years provides opportunity for continued research.