A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Impact of Polymeric Drops on Drops and Films of a Different but Miscible Polymer(2024) Bera, Arka; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fluid mechanics of a liquid drop impacting on another stationery (or spreading) liquid drop or on a liquid film (of thickness comparable, or smaller, or larger than the impacting drop) has attracted significant attention over the past several years. Such problems represent interesting deviations from the more widely studied problems of liquid drops impacting on solid surfaces having different wettabilities with respect to the impacting drops. These deviations stem from the fact that the resting liquid (in the form of the drop or the film) itself undergoes deformation on account of the drop impact and can significantly affect the overall combined drop-drop or drop-film dynamics. The problem becomes even more intriguing depending on the rheology of the drop(s) and the film as well as the (im)miscibility of the impacting drop with the underlying drop or the film. Interestingly, the majority of such drop-impact-on-drop or drop-impact-on-film studies have considered Newtonian drop(s) and films, with little attention to polymeric drop(s) and films. This thesis aims to bridge that void by studying, using Direct Numerical Simulation (DNS) based computational methods, the impact-driven dynamics of one polymeric drop on another (different but miscible) polymeric drop or film. As specific examples, we consider two separate problems. In the first problem, we consider the impact of a PMMA (poly-methyl methacrylate) drop on a resting PVAc (polyvinyl acetate) drop as well as the impact of a PVAc drop on a resting PMMA drop. In the second problem, we consider the impact of a PMMA drop on a PVAc film as well as the impact of a PVAc drop on a PMMA film. For the first problem, the wettability of the resting drop (on the resting surface), the Weber number of the impacting drop, the relative surface tension values of the two polymeric liquids (PVAc and PMMA), and the miscibility (or how fast the two liquids mix) dictate the overall dynamics. PVAc has a large wettability on silicon (considered as the underlying solid substrate); as a result, during the problem of the PMMA drop impacting on the PVAc drop, the PVAc drop spreads significantly and the slow mixing of the two liquids ensures that the PMMA drop spreads as a thin film on top of the PVAc film (formed as the PVAc drop spreads quickly on silicon). Depending on the Weber number, such a scenario leads to the formation of transient liquid films (of multitudes of shapes) with stratified layers of PMMA (on top) and PVAc (on bottom) liquids. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, a combination of the weaker spreading of the PMMA drop on silicon and the “engulfing” of the PMMA drop by the PVAc drop (stemming from the PVAc having a smaller surface tension than PMMA) ensures that the impacting PVAc drop covers the entire PMMA drop and itself interacts with the substrate giving rise to highly intriguing transient and stratified multi-polymeric liquid-liquid structures (such as core-shell structure with PMMA core and PVAc shell). For both these cases, we thoroughly discuss the dynamics by studying the velocity field, the concentration profiles (characterizing the mixing), the progression of the mixing front, and the capillary waves (resulting from the impact-driven imposition of the disturbance). In the second problem, we consider a drop of the PMMA (PVAc) impacting on a film of the PVAc (PMMA). In addition to the factors dictating the previous problem, the film thickness (considered to be either identical or smaller than the drop diameter) also governs the overall droplet-impact-driven dynamics. Here, the impact, being on the film, the dynamics is governed by the formation of crown (signifying the pre-splashing stage) and a deep cavity (the depth of which is dictated by the film thickness) on the resting film. In addition to quantifying these facets, we further quantify the problem by studying the velocity and the concentration fields, the capillary waves, and the progression of the mixing front. For the PMMA drop impacting on the thin film, a noticeable effect is the quick thinning of the PMMA drop on the PVAc film (or the impact-driven cavity formed on the PVAc film), which gives rise to a situation similar to the previous study (development of transient multi-polymeric-liquid structures with stratified polymeric liquid layers). For the case of the PVAc drop impacting on the PMMA film, the PVAc liquid “engulfs” the deforming PMMA film, and this in turn, reduces the depth of the cavity formed, the extent of thinning, and the amplitude of the generated capillary waves. All these fascinating phenomena get captured through the detailed DNS results that are provided. The specific problems considered in this thesis have been motivated by the situations often experienced during the droplet-based 3D printing processes (e.g., Aerosol jet printing or inkjet printing). In such printing applications, it is commonplace to find one polymeric drop interacting with an already deposited polymeric drop or a polymeric film (e.g., through the co-deposition of multiple materials during multi-material printing). The scientific background for explaining these specific scenarios routinely encountered in 3D printing problems, unfortunately, has been very limited. Our study aims to fill this gap. Also, the prospect of rapidly solidifying these polymeric systems (via methods such as in-situ curing) can enable us to visualize the formation of solidified multi-polymeric structures of different shapes (by rapidly solidifying the different transient multi-polymeric-liquid structures described above). Specifically, both PMMA and PVAc are polymers well-known to be curable using in-situ ultraviolet curing, thereby establishing the case where the present thesis also raises the potential of developing PMMA-PVAc multi-polymeric solid structures of various shapes and morphologies.Item A CAUSAL INFORMATION FUSION MODEL FOR ASSESSING PIPELINE INTEGRITY IN THE PRESENCE OF GROUND MOVEMENT(2024) Schell, Colin Andrew; Groth, Katrina M; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pipelines are the primary transportation method for natural gas and oil in the United States making them critical infrastructure to maintain. However, ground movement hazards, such as landslides and ground subsidence, can deform pipelines and potentially lead to the release of hazardous materials. According to the Pipeline and Hazardous Materials Safety Administration (PHMSA), from 2004 to 2023, ground movement related pipeline failures resulted in $413M USD in damages. The dynamic nature of ground movement makes it necessary to collect pipeline and ground monitoring data and to actively model and predict pipeline integrity. Conventional stress-based methods struggle to predict pipeline failure in the presence of large longitudinal strains that result from ground movement. This has prompted many industry analysts to use strain-based design and assessment (SBDA) methods to manage pipeline integrity in the presence of ground movement. However, due to the complexity of ground movement hazards and their variable effects on pipeline deformation, current strain-based pipeline integrity models are only applicable in specific ground movement scenarios and cannot synthesize complementary data sources. This makes it costly and time-consuming for pipeline companies to protect their pipeline network from ground movement hazards. To close these gaps, this research made significant steps towards the development of a causal information fusion model for assessing pipeline integrity in a variety of ground movement scenarios that result in permanent ground deformation. We developed a causal framework that categorizes and describes how different risk-influencing factors (RIFs) affect pipeline reliability using academic literature, joint industry projects, PHMSA projects, pipeline data, and input from engineering experts. This framework was the foundation of the information fusion model which leverages SBDA methods, Bayesian network (BN) models, pipeline monitoring data, and ground monitoring data to calculate the probability of failure and the additional longitudinal strain needed to fail the pipeline. The information fusion model was then applied to several case studies with different contexts and data to compare model-based recommendations to the actions taken by decision makers. In these case studies, the proposed model leveraged the full extent of data available at each site and produced similar conclusions to those made by decision makers. These results demonstrate that the model could be used in a variety of ground movement scenarios that result in permanent ground deformation and exemplified the comprehensive insights that come from using an information fusion approach for assessing pipeline integrity. The proposed model lays the foundation for the development of advanced decision making tools that can enable operators to identify at-risk pipeline segments that require site specific integrity assessments and efficiently manage the reliability of their pipelines in the presence of ground movement.Item Automated Management of Network Slices with Service Guarantees(2024) Nikolaidis, Panagiotis; Baras, John; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Future mobile networks are expected to support a diverse set of applications including high-throughput video streaming, delay-sensitive augmented reality applications, and critical control traffic for autonomous driving. Unfortunately, existing networks do not have the required management mechanisms to handle this complex mix of traffic efficiently. At the same time, however, there is a significant effort from both industry and academia to make networks more open and programmable, leading to the emergence of software-defined networking, network function virtualization, and packet-forwarding programming languages. Moreover, several organisations such as the Open Networking Foundation were founded to facilitate innovation and lower the entry barriers in the mobile networking industry. In this setting, the concept of network slicing emerged which involves the partitioning of the mobile network into virtual networks that are tailored for specific applications. Each network slice needs to provide premium service to its users as specified in a service level agreement between the mobile network operator and the customer. The deployment of network slices has been largely realized thanks to network function virtualization. However, little progress has been made on mechanisms to efficiently share the network resources among them. In this dissertation, we develop such mechanisms for the licensed spectrum at the base station, a scarce resource that operators obtain through competitive auctions. We propose a system architecture composed of two new network functions; the bandwidth demand estimator and the network slice multiplexer. The bandwidth demand estimator monitors the traffic of the network slice and outputs the amount of bandwidth currently needed to deliver the desired quality of service. The network slice multiplexer decides which bandwidth demands to accept when the available bandwidth does not suffice for all the network slices. A key feature of this architecture is the separation of the demand estimation task from the contention resolution task. This separation makes the architecture scalable for a large number of network slices. It also allows the mobile network operator to charge fairly each customer based on their bandwidth demands. In contrast, the most common approach in the literature is to learn online how to split the available resources among the slices to maximize a total network utility. However, this approach is neither scalable nor suitable for service level agreements. The dissertation contributes several algorithms to realize the proposed architecture and provisioning methods to guarantee the fulfillment of the service level agreements. To satisfypacket delay requirements, we develop a bandwidth demand estimator based on queueing theory and online learning. To share resources efficiently even in the presence of traffic anomalies, we develop a network slice multiplexer based on the Max-Weight algorithm and hypothesis testing. We implement and test the proposed algorithms on network simulators and 5G testbeds to showcase their efficiency in realistic settings. Overall, we present a scalable architecture that is robust to traffic anomalies and reduces the bandwidth needed to serve multiple network slices.Item ENGINEERING TARGETED LIGHT ACTIVATABLE NANOPLATFORMS TO MANAGE RECURRENT CANCERS(2024) Pang, Sumiao; Huang, Huang Chiao HH; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cancer recurrence poses a significant challenge in various malignancies that adverselyaffect long-term survival and quality of life. Glioblastoma (GBM) and ovarian cancer exhibit particularly high recurrence rates. For GBM, tumor recurrence is nearly universal (90%) within 10 months post initial treatment due to its invasive characteristics, limited delivery of therapeutic agents, and persistent drug resistance, resulting in a 5-year survival rate of <10%. While standard chemotherapy and surgery can temporarily alleviate symptoms for both diseases, there has been no significant improvement in long-term disease management or survival extension over several decades. Therefore, it is critical to develop targeted therapies that integrates well with current standards of care strategies. Photomedicine is a promising treatment modality, and the two main phototherapies are photodynamic therapy (PDT) which involves photosensitizer administration followed by light activation resulting in non-thermal chemical damage and photothermal therapy (PTT) which involves exogenous or endogenous sensitizing agents followed by light activation resulting in thermal damage. Clinical applications of both modalities have shown its feasibility and safety; however, they face challenges due to (i) limited cancer selectivity, (ii) heterogenous treatment response, and (iii) low monotherapy treatment efficacy. Leveraging strategic therapeutic targets to advance the current sensitizing agents for targeted delivery is a potential solution to overcome these limitations. The overall objective of this dissertation is to advance and evaluate targeted light-activatable nanoplatforms for phototherapy delivery with considerations for the current clinical workflow of GBM and advanced ovarian cancer. This is achieved through the following goals, (1) engineering a novel Fn14 receptor-directed gold nanorods (DART-GNRs) to assess selectivity and PTT efficacy for GBM, and (2) evaluate safety and long-term efficacy of targeted light-activatable multi-agent nanoplatform (tLAMP) to deliver targeted PDT for peritoneal carcinomatosis. First, this work establishes a reproducible synthesis protocol for DART-GNRs, characterizes its photothermal properties, and demonstrate high selectivity towards the Fn14 receptor of cancer cells. Second half of this dissertation established and investigated a two-fiber tissue optical property (TOP) monitoring method for liquid phantoms and for peritoneal carcinomatosis mouse model to enable safer light dosimetry during PDT, established an irinotecan active loading method to reproducibly synthesize tLAMP, and determined tLAMP tumor nodule penetration depth for enhanced targeted PDT combination therapy with adjuvant chemotherapy to enhance long-term survival for ovarian cancer.Item LENGTH-SCALE DEPENDENCE OF VISCOPLASTIC PROPERTIES OF SILVER SINTER REVEALED BY INDENTATION TESTING AND MODELING(2024) Leslie, David; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This doctoral dissertation research focuses on using a combination of indentation testing and modeling to characterize the creep behavior of heterogeneous silver sinter at different temperatures, using multiple indenter sizes to interrogate length-scale effects. The measured steady-state creep deformation is characterized with three different modeling approaches, that rely on: (i) conventional deviatoric creep potential; (ii) pressure-sensitive Drucker-Prager creep potential; and (iii) length-scale dependent deviatoric creep potential. The creep flow rule for all three cases is Norton’s power-law creep. The materials in this study are from a family of sintered silver materials used for interconnects and die-attach in high-temperature electronics and for conductor traces in printed electronics. The dissertation focuses on identifying and quantifying the length-scale dependence presented by sintered materials due to their non-homogeneous morphology. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: 25°C, 75°C, and 125°C. The indentation results were first analyzed using two different post-processing methods: an empirical approach with closed-form models (CFM) and a computational FEA approach based on classical continuum mechanics. Differences found between the CFM and numerical (FEA) analyses, while significant at room temperature, reduce with temperature. Both models reveal that indenters of different radii cause significantly different viscoplastic behavior. This dependence on tip radius increases with temperature The research was extended to examine two second-order influences of the metallic agglomerated phase and the discontinuous compliant phase of the microstructure of sintered silver on its viscoplastic behavior: (i) dependence on hydrostatic stress; (ii) dependence on microstructural length-scale. The aim of incorporating the pressure-sensitive modeling was to investigate what effect the intrinsic compressive hydrostatic stress in indentation tests might have on the measured viscoplastic properties. Results from using the Drucker-Prager creep model further confirmed the increasing dependence on length-scale with temperature. The length-scale dependence seen in all the above results is investigated and quantified with the help of a simplified strain-gradient viscoplastic model. This modeling approach is motivated by the conventional mechanism-based strain gradient (CMSG) model that is widely used in plasticity theory to quantify length-scale effects. The characteristic length-scale metric in this problem is presented by the agglomerate size distribution in the sintered material and is quantified in this study with ‘watershed analysis’ of cross-sectional features observed via electron microscopy. This discrete length scale is believed to cause the variations in the observed creep response when queried with indenters of different radii, because of the different strain gradients produced by the two different indenters. The length-scale dependence is incorporated in a strain-gradient viscoplastic constitutive model suitable for finite element modeling of deformation fields containing strong strain-gradients (e.g. in the die attach layer in microelectronics chip assembly). Finally, a procedure is proposed, to incorporate the scale-dependence in the empirical closed-form approach, currently available in the literature, for extracting viscoplastic properties from indentation tests. This approach provides corrected model constants for the strain-gradient viscoplastic model, using simple closed-form equations instead of expensive finite element modeling.Item INVESTIGATION AND ENGINEERING OF HfZrO2 INTERFACES FOR FERROELECTRIC BASED NEUROMORPHIC DEVICES(2024) Pearson, Justin Seth; Takeuchi, Ichiro; Najmaei, Sina; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes the study of ferroelectric hafnium zirconium oxide (HZO) and its integration into ferroelectric field effect transistors (FeFET). Ferroelectric HZO is uniquely situated for energy efficient, non-volatile memory applications such as FeFETs due to its CMOS compatibility and ferroelectricity at scaled thicknesses less than 10 nm[1]. This work covers material growth of HZO via atomic layer deposition (ALD), as well as electrode metallization (W and Pt) via sputtering and electron beam physical vapor deposition, to optimize ferroelectricity in capacitive structures. Preliminary results show Pt-based devices were sufficient in producing ferroelectric HZO, but had issues in electrode degradation at high thermal processing > 450 °C. In contrast, HZO capacitors in W devices showed drastic improvement in the ferroelectric response reaching remnant polarization values > 40 μC/cm2. To integrate into a FET structure, gate dielectrics (Al2O3 and HfO2) and the 2D semiconductor tungsten diselenide (WSe2) are introduced to the HZO stack. Material and electrical characterization was performed and gave indication of challenges such as: low remnant polarization (<10 μC/cm2), surface roughness (> 20 nm), and high trap characteristics in FeFET modulation. Electrical characterization was performed via variable pulsing, high frequency cycling, current vs voltage, capacitance vs voltage, and polarization vs voltage testing. Challenges such as low remnant polarization, leaky dielectrics, and surface roughness are identified through transmission electron microscopy, atomic force microscopy, and electrical characterization. These challenges were addressed by altering the growth conditions, scaling the thickness of each material, and thermally processing within the bounds of material stability. Upon integration of these various materials into FETs, the challenges of reliability, stochasticity, and consistency were evaluated on through various means of electrically testing such as, variable pulsing, high frequency cycling, current vs voltage, capacitance vs voltage, and polarization vs voltage. A greater depth of understanding of fundamental aspects of these device architectures is required to untangle the complex electrical characteristics of the fabricated devices. Characterization of material properties is performed by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Throughout the studies performed in this dissertation, the phase landscape of HZO was investigated on inert Ti/Pt electrodes. While the ferroelectric nature of the HZO was sufficiently explored at CMOS compatible temperatures, yielding remanent polarization values of 20 μC/cm2 and demonstrating multi state memory within Ferroelectric field effect devices (3.5 order of magnitude conductivity change), due to the phase landscape evolution under thermal processing. Higher temperatures were found to be incompatible with the electrode choice as the interdiffusion and breakdown resulted in poor device performance. W electrode HZO capacitors were then used to study the higher temperature ferroelectric devices as well as incorporate scaling of the ferroelectric films to better match the needs of modern device architectures. The optimal ferroelectric films were found to have remanent polarization values > 40 μC/cm2 and when implemented in a FEFET were able to demonstrate a memory window of 6.3 volts, allowing for a large range of modulation for neuromorphic devices.Item Insulating Materials for an Extreme Environment in a Supersonically Rotating Fusion Plasma(2024) Schwartz, Nick Raoul; Koeth, Timothy W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fusion energy has long been sought as the “holy grail” of energy sources. One of the most critical remaining challenges in fusion is that of plasma-facing materials, even denoted by the National Academies of Science. The materials challenge is particularly acute for centrifugal mirrors, an alternative concept to the industry-standard tokamak that may offer a more efficient scheme with a faster path to development. The centrifugal mirror incorporates supersonic rotation into a conventional magnetic mirror scheme, providing three primary benefits: (1) increased confinement, (2) suppression of instabilities, and (3) plasma heating through shear flow. However, this rotation, which is driven by an axial magnetic field and a radial electric field, requires the magnetic field lines to terminate on electrically insulating surfaces to avoid “shorting” the plasma. This unique requirement presents a novel materials challenge: the insulator must not only resist irradiation and thermal damage, but also be an excellent electrical insulator and thermal conductor that can be actively cooled. To address this materials challenge, the Centrifugal Mirror Fusion Experiment (CMFX) was developed at the University of Maryland. CMFX serves as a test bed for electrically insulating materials in a fusion environment, as well as a proof-of-concept for the centrifugal mirror scheme. To guide the design of future power plants and better understand the neutronand ion flux on the insulators, a zero-dimensional (0-D) scoping tool, called MCTrans++, was developed. This software, discussed in Chapter 2, demonstrates the ability to rapidly model experimental parameter sets in CMFX and predict the scaling to larger devices, informing material selection and design. Assuming the engineering challenges have been met, the centrifugal mirror has been demonstrated as a promising scheme for electricity production via fusion energy. One of the key aspects to the operation of CMFX is the high voltage system. This system, discussed in Chapter 3, was developed in incremental stages, beginning with a 20 kV, then 50 kV pulsed power configuration, and finally culminating in a 100 kV direct current power supply to drive rotation at much higher voltages, creating an extreme environment for materials testing. This work identified hexagonal boron nitride (hBN) as a promising insulator material. Computational modeling (Chapter 4) demonstrated hBN’s superior resistance to ion-irradiation damage compared to other plasma-facing materials. Additionally, fusion neutrons are crucial for assessing both material damage and power output. Chapter 5 details the neutronics for CMFX, including 3He proportional counters, which have been installed on CMFX to measure neutron production. In parallel, Monte Carlo computational methods were used to predict neutron transport and material damage in the experiment. Ultimately, a materials test stand was installed on CMFX to expose electrically insulating materials to high energy fusion plasmas (Chapter 6). Comparative analysis of hBN and silicon carbide after exposure revealed superior performance of hBN as a plasma-facing material. Two primary erosion mechanisms were identified by surface morphology and roughness measurements: grain ejection and sputtering, both more pronounced in silicon carbide. This work advances our understanding of insulating material behavior in fusion environments and paves the way for the development of the next-generation centrifugal mirror fusion reactors. Chapter 7 discusses conclusions and proposes future work. In particular this section suggests some changes that may allow CMFX to operate at much higher voltages, unlocking higher plasma density and temperature regimes for further material testing.Item Phonon Transport and Nonequilibrium Kinetics with Stimulation Modeling in Molecular Crystals(2024) Liu, Zhiyu; Chung, Peter W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An important family of materials known as molecular crystals has been used extensively in fields such as organic semiconductors, energy, optoelectronics, and batteries. Due to their periodic crystal structure, phonons are the predominant heat and energy carriers. Phonons and their transport behaviors are crucial to the performance of semiconductors, the figure of merit of thermoelectrics, shock-induced properties of molecular crystals, and light-matter interactions of materials. Recent decades have seen significant advancements in the understanding of the phonon transport behaviors in inorganic crystals. However, a comprehensive understanding of phonon properties in molecular crystals is still lacking. While various theoretical models and computational simulations have been developed to study vibrational energy transfer in molecular crystals and to correlate vibrational structure with the stability of materials, these approaches often suffer from limitations. Many of these studies either neglect anharmonic scattering entirely or rely on simplified representations of phonon scattering. In this dissertation, we focus on investigating the phonon transport and nonequilibrium kinetics in molecular crystals. In the first work, we study the harmonic phonon properties of cellulose Iβ using tapered reactive force fields (ReaxFF). While geometry optimization with the original ReaxFF potential often results in structures with negative eigenvalues, indicating structural instability, the modified potential with a tapering function yields structures with no associated negative eigenvalues. Three ReaxFF parameterizations are evaluated by comparing lattice properties, elastic constants, phonon dispersion, temperature-dependent entropy, and heat capacity with experimental results from the literature. In the second study, we study the phonon transport behavior of Si, Cs2PbI2Cl2, cellulose Iβ, and α-RDX by calculating the thermal conductivity using different thermal transport models including the Phonon gas model, Cahill-Watson-Pohl, and the Allen-Feldman model and the Wigner formulation. By comparing the calculated thermal conductivity with experimental values, we highlight the significant contributions of wave-like heat carriers in cellulose Iβ and α-RDX. We show how different phonon properties influence particle-like and wave-like behavior in various materials and reveal unusual mechanisms present in molecular crystals. Lastly, we investigate nonequilibrium phonon kinetics resulting from direct vibrational excitations by employing the phonon Boltzmann transport equations. The results of our mid-IR pump-probe spectroscopy simulations align closely with experimental data from the literature. Additionally, by exciting different phonon modes at varying frequencies, we uncover distinct stages and pathways of vibrational energy transfer. To gain insights into the decomposition mechanism of RDX under excitation, we further calculate the bond activities of the N-N and N-O bonds, identifying possible stimuli that could trigger bond cleavage.Item Analyzing Inverse Design Problems from a Topological Perspective(2024) Chen, Qiuyi; Fuge, Mark; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inverse design (ID) problems are inverse problems that aim to rapidly retrieve the subset of valid designs having the desired performances and properties under the given conditions. In practice, this can be solved by training generative models to approximate and sample the posterior distributions of designs. However, little has been done to understand their mechanisms and limitations from a theoretical perspective. This dissertation leverages theoretical tools from general and differential topology to answer these three questions of inverse design: what does a set of valid designs look like? How helpful are the data-driven generative models for retrieving the desired designs from this set? What topological properties affect the subset of desired designs? The dissertation proceeds by dismantling inverse (design) problems into two major subjects: that is, the representing and probing of a given set of valid designs (or data), and the retrieval of the desired designs (or data) from this given set. It draws inspiration from topology and geometry to investigate them and makes the main contributions below: 1. Chapter 3 details a novel representation learning method called Least Volume, which has properties similar to nonlinear PCA for representing datasets. It can minimize the representation's dimension automatically and, as shown in Chapter 4, conducts contrastive learning when applied to labeled datasets. 2. Two conditional generative models are developed to generate performant 2-D airfoils and 3-D heat sinks in Chapter 5 and 6 respectively. They can produce realistic designs to warm-start further optimization, with the relevant chapters detailing their acceleration effects. 3. Lastly, Chapter 7 describes how to use Least volume to solve high-dimensional inverse problems efficiently. Specifically, using examples from physic system identification, the chapter uncovers the correlation between the inverse problem's uncertainty and its intrinsic dimensions.Item INTEGRATION OF ATOMIC EMITTERS IN PHOTONIC PLATFORMS FOR CLASSICAL AND QUANTUM INFORMATION APPLICATIONS(2024) Zhao, Yuqi; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Integrated photonics provide a powerful toolbox for a wide range of classical and nonclassical applications. In addition to their scalability and significantly lower power consumption, integrated photonic structures enable new design knobs and functionalities that are inaccessible in their bulk counterparts.Solid-state atomic emitters, such as rare-earth ions (REIs) and quantum dots, serve as excellent sources for scalable quantum memories and exhibit strong nonlinear resonant absorption. Integrating atomic emitters with photonic devices enhances light-matter interactions, unlocking new opportunities for advanced optoelectronic systems in both classical and quantum regimes. This thesis tackles two main challenges utilizing the integration of photonic devices and atomic emitters: (1) developing scalable quantum network components, and (2) creating low-power nonlinear components for classical on-chip optical signal processing. Specifically, we focus on a platform of rare-earth ion doped thin-film lithium niobate (TFLN), leveraging the ions’ stable optical transitions with thin-film lithium niobate’s rich toolbox of high-performance photonics. We first demonstrate an integrated atomic frequency comb (AFC) memory in this platform, an essential component for quantum networks. This memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. As the first demonstrated integrated AFC memory, it features a significantly enhanced optical confinement compared to the previously demonstrated REI memories based on ion-diffused waveguides, leading to a three orders of magnitude reduction in optical power required for a coherent control. Next, we develop reconfigurable narrowband spectral filters using ring resonators in the REI:TFLN platform. These on-chip optical filters, with linewidths in the MHz and kHz range and extinction ratios of 13 dB – 20 dB, are crucial for reducing background noise in quantum frequency conversion. By spectral hole burning at 100 mK temperature in a critical-coupled resonance mode, we achieve bandpass filters with a linewidth of as narrow as 681 kHz. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental component for optical signal processing and optical memories on-a-chip. We also demonstrate picowatt-threshold power nonlinearity in TFLN, utilizing the strong resonant nonlinear absorption induced by three-level REIs and enhanced by TFLN ring resonators. This work presents three distinct nonlinear transmission functions by adjusting the ring’s coupling strength. The lifetime of the nonlinear transmission is measured to be ~3 ms, determined by the ion’s third-level lifetime. Finally, we propose a novel nonlinear device design based on a different material system and mechanism - an ultrathin optical limiter with low threshold intensity (0.45 kW/cm2), utilizing a 500 nm-thick GaAs zone plate embedded with InAs quantum dots. The optical limiting performance, enabled by the zone plate’s nonlinear focusing behavior, is investigated using FDTD simulations. We also explore the effects of the zone plate’s thickness and radius on its optical limiting performance.