Browsing by Author "Brewer, Samuel, committee member"

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Open Access
Brillouin light scattering spectroscopy of phonons, magnons, and magnetoelastic waves
(Colorado State University. Libraries, 2022) Nygren, Katherine Elise, author; Buchanan, Kristen S., advisor; Field, Stuart, committee member; Brewer, Samuel, committee member; Shores, Matthew, committee member
This thesis discusses three projects that involve the propagation of waves through the utilization of an optical measurement technique known as Brillouin light scattering (BLS) spectroscopy. BLS spectroscopy measurements were completed using a six pass tandem Fabry-Pérot interferometer to detect light that has inelastically scattered from vibrational, spin, or magnetoelastic waves in a sample. This measurement method is noncontact, so wires do not need to be connected to the sample, nondamaging (unless the laser power is too high, and only for sensitive samples), and can detect nonlinear signals. The first project uses an antenna called an interdigital transducer to produce a surface acoustic wave. This wave travels across a piezoelectric substrate and couples to a spin wave in a nickel film. The coupled wave known as a magnetoelastic wave is then studied using BLS as a function of the external applied magnetic field. These results are used to help us understand how the magnetic resonance contributes to the coupled wave. Further BLS measurements as a function of distance across the nickel film are used to calculate a decay length of the magnetoelastic wave two orders of magnitude larger than the decay length for a pure spin wave in nickel. Second, we explore a device using a thin film of an organic ferrimagnet called vanadium tetracyanoethylene (VTCNE) that is magnetic at room temperature and has low damping, which rivals damping in high quality YIG films commonly used in microwave applications. Because VTCNE is oxygen sensitive it is encapsulated between two pieces of glass using an epoxy. The encapsulation does not change the damping, however due to magnetostriction, the strain of the epoxy may change the magnetic properties of the film. To understand how the epoxy strain can effect this device and others with similar encapsulation, we study thermal phonons in the encapsulation materials using Brillouin light scattering. The thermal phonon measurements along with phonon simulations allow us to calculate both the wave speeds and the elastic properties of the materials. These calculated properties can then be used to model future VTCNE devices. The final major project uses BLS spectroscopy to study spin waves in a Y-shaped structure of an iron nickel alloy. Using an in-plane externally applied magnetic field and an antenna across the top of the Y, we excite magnons in each arm of the Y, which then propagate into the base of the Y. BLS measurements are taken in each arm and the base of the Y, as a function of the driving frequency, and a 2D spatial map of the spin waves in the Y-structure was obtained to gain additional information on the modes that propagate past the junction of the Y. The BLS data in conjunction with simulations, demonstrate an indirect way to efficiently excite Damon-Eshbach spin waves as well as convert low wavevector spin waves in the arms of the Y into higher wavevector spin waves as they propagate into the base of the Y. The wavevector conversion and more efficient method of generating Damon-Eshbach spin waves are tools that can be exploited in magnonic device designs. Three additional spin wave projects are also discussed briefly. The projects include a yttrium iron garnet (YIG) confined structure, a VO2 film with a metal-insulator-transition near room temperature, and a heavy metal-ferrimagnet-heavy metal sample that should have a strong interfacial Dzyaloshinskii-Moriya interaction.
Embargo
Charge carrier dynamics of 2-dimensional photoelectrodes probed via ultrafast spectroelectrochemistry
(Colorado State University. Libraries, 2024) Austin, Rachelle, author; Sambur, Justin, advisor; Krummel, Amber, advisor; Rappe, Anthony, committee member; Prieto, Amy, committee member; McNally, Andrew, committee member; Brewer, Samuel, committee member
The integration of hot charge carrier-based energy conversion systems with two-dimensional (2D) semiconductors holds immense promise for enhancing the efficiency of solar energy technologies and enabling novel photochemical reactions. Current approaches, however, often rely on costly multijunction architectures. In this dissertation, I present research that combines spectroelectrochemical and in-operando transient absorption spectroscopy measurements to unveil ultrafast (<50 fs) hot exciton and free charge carrier extraction in a proof-of-concept photoelectrochemical solar cell constructed from earth-abundant monolayer (ML) MoS2. Theoretical analyses of exciton states reveal enhanced electronic coupling between hot exciton states and neighboring contacts, facilitating rapid charge transfer. Additionally, I discuss insights into the physical interpretation of transient absorption (TA) spectroscopy data in 2D semiconductors, comparing historical perspectives from physical chemistry and solid-state physics literature. My perspective encompasses various physical explanations for spectral features and experimental trends, particularly focusing on the contribution of trions to TA spectra. Furthermore, I examine how different physical interpretations and data analysis procedures can yield distinct timescales and mechanisms from the same experimental results, providing a comprehensive framework for understanding charge carrier dynamics in 2D semiconductor-based optoelectronic devices.
Open Access
Demonstration of filament-guided electrical discharges from a high average power 1 kHz picosecond laser
(Colorado State University. Libraries, 2023) Dehne, Kristian A., author; Rocca, Jorge, advisor; Marconi, Mario, committee member; Brewer, Samuel, committee member
The atmospheric propagation of ultrashort, high energy laser pulses is of interest for applications including remote sensing, directed energy, and the guiding of lightning. In this thesis, the filamentation of high energy picosecond laser pulses at repetition rates up to 1 kHz is demonstrated and the guiding of electrical discharges in air at high repetition rates is studied. The design and performance of the diode-pumped Yb:YAG chirped pulse amplification (CPA) system utilized for this experiment is also described. Diode-pumped solid state lasers in a CPA layout have emerged as the modern choice for the generation of high pulse energies at high repetition rates. For the work presented in this thesis, a high average power diode-pumped Yb:YAG laser system utilized for filament formation is de- tailed. The compact CPA system, which combines a room temperature regenerative amplifier and cryogenically cooled Yb:YAG amplifiers, results in compressed pulses of < 5 ps duration with up to 1.1 J of energy at 1 kHz repetition rate. This record Joule-level 1 kHz repetition rate picosecond laser (average power output of more than 1 kW) has enabled the results described herein. The application of this high average power Yb:YAG system for producing laser guided electrical discharges is the main focus of this thesis. The compressed output pulses from the Yb:YAG laser induce filamentation in air, resulting from the counterbalance between Kerr self-focusing and plasma refraction defocusing. The hydrodynamic response of the atmospheric air results in a density depression of similar geometry to the filament. The result is a preferential path which both triggers and guides electrical discharges. The majority of previous laser-guided discharge studies have been conducted at repetition rates of 10 Hz, where the medium completely recovers before the next laser pulse arrives. This thesis reports on the physics of laser filament-guided electric discharges in air initiated by high energy (up to 250 mJ) 1030 nm wavelength laser pulses of ∼7 ps duration at repetition rates up to 1 kHz. A breakdown voltage reduction of up to 4.2 X was measured and determined to result primarily from the perturbation caused by a single laser pulse, with cumulative effects playing only a secondary role. A current proportional to the laser pulse energy arises as soon as the laser pulse arrives, initiating a high impedance phase of the discharge channel evolution. Full breakdown, characterized by impedance collapse and the onset of high current conduction, occurs 100s of ns to a few μs later. The gaps between the filamentary plasma channel and the electrodes are observed to play a role in the delay between arrival of the laser pulse and the onset of a discharge. The breakdown voltages measured for 100 Hz and 1 kHz repetition rates are shown to be nearly equivalent. This is consistent with the results of interferometric analysis which shows that the filament formed by a single laser shot causes a deep density depression up to 75%, compared with the 20% density depression measured 10 μs prior to the arrival of a laser pulse in a sustained 1 kHz sequence. The physical insight gained from this work on the formation of laser filament-guided discharges in air at 1 kHz repetition rate can be expected to contribute to their use in applications.
Open Access
Design exploration and optimization of silicon photonic integrated circuits under fabrication-process variations
(Colorado State University. Libraries, 2024) Mirza, Asif Anwar Baig, author; Nikdast, Mahdi, advisor; Pasricha, Sudeep, advisor; Wilson, Jesse, committee member; Brewer, Samuel, committee member
Silicon photonic integrated circuits (PICs) have become a key solution to handle the growing demands of large data transmission in emerging applications by consuming less power and low heat dissipation while offering ultra-high data bandwidth than electronic circuits. With Moore's Law slowing down and the end of Dennard scaling, PICs offer a logical step to improve data movement and processing performance in future computing systems. On PICs, light is processed and routed by means of optical waveguides. Silicon has a unique feature of high refractive index contrast in the silicon-on-insulator (SOI) platform which allows for tight confinement of light in nanometer waveguide cores and bends with a radius of only a few microns. PICs comprise of a diverse set of elements such as waveguide splitters, combiners, crossings, and couplers which help with distribution, routing, and computation of optical signals. Optical signals are converted to electrical signals with the help of photodiodes which in silicon photonics are implemented using Germanium. To enable PICs for wavelength-division multiplexing (WDM), there is a need for efficient wavelength filters consisting of optical delay lines or resonators. Optical delay lines are usually built using Mach-Zehnder Interferometers (MZIs) which consists of a splitter, two waveguides with a given group delay, and a combiner. Other devices such as microring resonators (MRRs) can be used as wavelength filters when the input wavelength matches a whole multiple times in the circumference of the ring. Other components such as grating coupler help couple the light into and out of a PIC. PICs can be fabricated on the infrastructure developed for complimentary metal–oxide–semiconductor (CMOS) electronics. This technology now enables deep submicron features with unprecedented accuracy in large volumes along with close integration of photonics and electronic circuits. The use of silicon as a base material makes reuse of these manufacturing tools possible, but photonics imposes different demands on the processes. Although silicon photonics offers data transmission and computation at light speed with high bandwidth and low power consumption, the fundamental building blocks in PICs (e.g., optical waveguides) are extremely sensitive to nanometer-scale fabrication-process variations (FPVs) caused due to slight randomness in optical lithography processes. Active compensation by means of electronic circuits (a.k.a. tuning) is necessary to compensate for FPVs. Tunable microheaters can be used for active compensation which affect the material properties of silicon to improve PIC's performance under FPVs. However, the total power consumed due to tuning in a working PIC can be drastically high. For example, variations as small as 1 nm in an MRR can deviate the optical frequency response of the device by 2 nm that leads to approximately 25% increase in the tuning power consumption to compensate for variations of a single MRR. Additionally, a system can have thousands of such MRRs that can easily add up the total power consumption of the system. In order to address FPVs we need to observe the reliability not just at a system level but down to the device level by enabling reliable, FPV-aware devices to enable FPV-resilient PICs and photonic systems. Designing more reliable and FPV-tolerant photonic devices should not only help us with reducing the total power consumption but also build more reliable circuits with fault-free operational behavior for data transmission and computation in future computing systems. This PhD thesis covers the impact of process variations on photonic devices primarily MRRs. We take a bottom-up approach in improving the reliability of an MRR towards FPVs. We propose an improved and optimized MRR designs which can be used in any PIC to reduce the overall shift in resonant wavelength of the device due to FPVs, further reducing the total power consumption required to tune the device. We confirmed our findings by further fabricating such MRRs and comparing the improved and optimized designs against conventional MRRs. Furthermore, we study the impact these improved MRRs have in photonic artificial intelligence (AI) accelerators and how they can further improve the network accuracy and overall power consumption. Finally, we also compile our work into a device exploration tool that allows photonic designer to set design parameters in an MRR and study its behavior under different FPV profiles. With this tool we aim to give the designer the ability to determine desired MRR designs based on desired design and performance requirements and budget constraints set on a photonic system.
Open Access
Muon neutrino reconstruction with machine-learning techniques at the ICARUS detector
(Colorado State University. Libraries, 2024) Mueller, Justin J., author; Mooney, Michael, advisor; Harton, John, committee member; Brandl, Alexander, committee member; Brewer, Samuel, committee member; Terao, Kazuhiro, committee member
The ICARUS T600 LArTPC detector successfully ran for three years at the underground LNGS laboratories, providing a first sensitive search for LSND-like anomalous electron neutrino appearance in the CNGS beam. After a significant overhauling at CERN, the T600 detector has been placed in its experimental hall at Fermilab, fully commissioned, and the first events observed with full detector readout. Regular data-taking began in May 2021 with neutrinos from the Booster Neutrino Beam (BNB) and neutrinos six degrees off-axis from the Neutrinos at the Main Injector (NuMI). Modern developments in machine learning have allowed for the development of an end-to-end machine-learning-based event reconstruction for ICARUS data. This reconstruction folds in 3D voxel-level feature extraction using sparse convolutional neural networks and particle clustering using graph neural networks to produce outputs suitable for physics analyses. The analysis presented in this thesis demonstrates a high-purity and high-efficiency selection of muon neutrino interactions in the BNB suitable for the physics goals of the ICARUS experiment and the Short-Baseline Neutrino Program.