Physics

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HIGH PERFORMANCE NANOPHOTONIC CAVITIES AND INTERCONNECTS FOR OPTICAL PARAMETRIC OSCILLATORS AND QUANTUM EMITTERS
(2024) Perez, Edgar; Srinivasan, Kartik; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Integrated photonic devices like photonic crystals, microring resonators, and quantum emitters produce useful states of light, like solitons or single photons, through carefully engineered light-matter interactions. However, practical devices demand advanced integration techniques to meet the needs of cutting-edge technologies. High performance nanophotonic cavities and interconnects present opportunities to solve outstanding issues in the integration of nanophotonic devices. In this dissertation I develop three core tools required for the comprehensive integration of quantum emitters: wavelength-flexible excitation sources with sufficient pump power to drive down stream systems, photonic interconnects to spatially link the excitation sources to emitters, and cavities that can Purcell enhance quantum emitters without sacrificing other performance metrics. To create wavelength-flexible excitation sources, a high-performance χ(3)microring Optical Parametric Oscillator (OPO) is realized in silicon nitride. Microring OPOs are nonlinear frequency conversion devices that can extend the range of a high-quality on-chip (or off-chip) laser source to new wavelengths. However, parasitic effects normally limit the output power and conversion efficiency of χ(3)microring OPOs. This issue is resolved by using a microring geometry with strongly normal dispersion to suppress parasitic processes and multiple spatial mode families to satisfy the phase and frequency matching conditions. Our OPO achieves world-class performance with a conversion efficiency of up to 29% and an on-chip output power of over 18 mW. To create photonic interconnects, Direct Laser Writing (DLW) is used to fabricate 3-dimensional (3D) nanophotonic devices that can couple light into and out of photonic chips. In particular, polymer microlenses of 20 μm diameter are fabricated on the facet of photonic chips that increase the tolerance of the chips to misaligned input fibers by a factor of approximately 4. To do so, we develop the on-axis DLW method for photonic chips, which avoids the so-called "shadowing" effect and uses barcodes for automated alignment with machine vision. DLW is also used to fabricate Polymer Nanowires (PNWs) with diameters smaller than 1 μm that can directly couple photons from quantum emitters into Gaussian-like optical modes. Comparing the same quantum emitter system before and after the fabrication of a PNW, a (3 ± 0.7)× increase in the fiber-coupled collection efficiency is measured in the system with the PNW. To refine the design of quantum emitter cavities, a toy model is used to understand the underlying mechanisms that shape the emission profiles of Circular Bragg Gratings (CBGs). Insights from the toy model are used to guide the Bayesian optimization of high-performance CBG cavities suitable for coupling to single-mode fibers. I also demonstrate cavity designs with quality factors (Q) greater than 100000, which can be used in future experiments in cavity quantum electrodynamics or nonlinear optics. Finally, I show that these cavities can be optimized for extraction to a cladded PNW while producing a Purcell enhancement factor of 100 with efficient extraction into the fundamental PNW mode. The tools developed in this dissertation can be used to integrate individual quantum emitter systems or to build more complex systems, like quantum networks, that require the integration of multiple quantum emitters with multiple photonic devices.
Ultra-high impedance superconducting circuits
(2023) Mencia, Raymond; Manucharyan, Vladimir E; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Chains of Josephson junctions are known to produce some of the largest kinetic per unit-length inductance, which can exceed the conventional geometric one by about 104. However, the maximum total inductance is still limited by the stray capacitance of the chain, which results in parasitic self-resonances. This stray capacitance is unnecessarily large in most circuits due to the high dielectric constant of silicon or sapphire substrates used. Here, we explore a regime of ultra-high impedance superconducting circuits by introducing the technique of releasing the Josephson chain off the substrate. The ultra-high impedance regime (Z > 4xRQ ~ 25.8 kOhms) is realized by combining a maximal per-unit-length inductance with a minimal stray capacitance and demonstrating the highest impedance electromagnetic structures available today. We begin with suspended “telegraph” transmission lines, composed of 30,000+ junctions, and show that the wave impedance can exceed 5 x RQ (33 kOhms) while the line still maintains a negligible DC resistance. To quantify the effects of parasitic chain modes in ultra-high impedance circuits, we use high-inductance fluxonium qubits. We show that chain modes are ultra-strongly coupled to the qubit but can be moved to a higher frequency with the Josephson chain releasing technique. Finally, we create a superconducting quasicharge qubit (blochnium), dual of transmon, whose impedance reaches over 30 x RQ (200 kOhms) with no evidence of parasitic modes below 10 GHz. This qubit completes the periodic table of superconducting atoms and demonstrates the dual nature of a small Josephson junction in ultra-high impedance circuits, which we probe in a DC experiment in the final chapter.
QUANTUM CONTROL AND MEASUREMENT ON FLUXONIUMS
(2022) Xiong, Haonan; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Superconducting circuit is a promising platform for quantum computing and quantum simulation. A number of efforts have been made to explore the physics in transmon systems and optimize the qubit performance. Compared to transmon, fluxonium is a relatively new type of qubit and attracts more attention recently due to its high coherence time and large anharmonicity. In this thesis, we summarize recent progress toward high fidelity two-qubit gate and readout for fluxonium qubits. We report improved fluxonium coherence either in cavity or cavityless environment. In the former case, we demonstrate single-shot joint readout for two fluxonium qubits and explore various two-qubit gate schemes such as controlled-Z(CZ) gate, controlled-phase(CP) gate, bSWAP gate and cross-resonance(CR) gate. The CZ gate realized by near-resonantly driving the high transitions exhibits 99.2% fidelity from randomized benchmarking. A continuous CP gate set can be implemented by off-resonantly driving the high transitions and shows an average 99.2% fidelity from the cross-entropy benchmarking technique. Other gates involving only computational states are also explored to further improve the gate fidelity, which can take advantage of the high coherence of the fluxonium lower levels. In the cavityless environment, we demonstrate fluorescence shelving readout with 1.7 MHz radiative decay rate for the readout transition while maintaining 52 us coherence time for the qubit transition. Our research explores the basic elements for fluxonium-based quantum processors. The results suggest that fluxonium can be an excellent candidate for not only universal quantum computation but also quantum network and quantum optics studies.
ATOMICALLY PRECISE FABRICATION AND CHARACTERIZATION OF DONOR-BASED QUANTUM DEVICES IN SILICON
(2019) Wang, Xiqiao; Silver, Richard M; Appelbaum, Ian; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Atomically precise donor-based quantum devices in silicon are a promising candidate for scalable solid-state quantum computing and analog quantum simulation. This thesis demonstrates success in fabricating state-of-the-art silicon-phosphorus (Si:P) quantum devices with atomic precision. We present critical advances towards fabricating high-fidelity qubit circuitry for scalable quantum information processing that demands unprecedented precision and reproducibility to control and characterize precisely placed donors, electrodes, and the quantum interactions between them. We present an optimized atomically precise fabrication scheme with improved process control strategies to encapsulate scanning tunneling microscope (STM)-patterned devices and technological advancements in device registration and electrical contact formation that drastically increase the yield of atomic-precision fabrication. We present an atomic-scale characterization of monolayer step edges on Si (100) surfaces using spatially resolved scanning tunneling spectroscopy and quantitatively determine the impact of step edge density of states on the local electrostatic environment. Utilizing local band bending corrections, we report a significant band gap narrowing behavior along rebonded SB step edges on a degenerately boron-doped Si substrate. We quantify and control atomic-scale dopant movement and electrical activation in silicon phosphorus (Si:P) monolayers using room-temperature grown locking layers (LL), sputter profiling simulation, and magnetotransport measurements. We explore the impact of LL growth conditions on dopant confinement and show that the dopant segregation length can be suppressed below one Si lattice constant while maintaining good epitaxy. We demonstrate weak-localization measurement as a high-resolution, high-throughput, and non-destructive method in determining the conducting layer thickness in the sub-nanometer thickness regime. Finally, we present atomic-scale control of tunnel coupling using STM-patterned Si:P single electron transistors (SET). We demonstrate the exponential scaling of tunnel coupling down to the atomic limit by utilizing the Si (100) 2×1 surface reconstruction lattice as a natural ruler with atomic-accuracy and varying the number of lattices counts in the tunnel gaps. We analyze resonant tunneling spectroscopy through atomically precise tunnel gaps as we scale the SET islands down to the few-donor quantum dot regime. Finally, by combining single/few-donor quantum dots with atomically defined single electron transistors as charge sensors, we demonstrate single electron charge sensing in few-donor quantum dots and characterize the tunnel coupling between few-donor quantum dots and precision-aligned single electron charge sensors.
The effects of geometry and patch potentials on Casimir force measurements
(2017) Garrett, Joseph L.; Munday, Jeremy N.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Electromagnetic fluctuations of the quantum vacuum cause an attractive force between surfaces, called the Casimir force. In this dissertation, the first Casimir force measurements between two gold-coated spheres are presented. The proximity force approximation (PFA) is typically used to compare experiment to theory, but it is known to deviate from the exact calculation far from the surface. Bounds are put on the size of possible deviations from the PFA by combining several sphere-sphere and sphere-plate measurements. Electrostatic patch potentials have been postulated as a possible source of error since the first Casimir force measurements sixty years ago. Over the past decade, several theoretical models have been developed to characterize how the patch potentials contribute an additional force to the measurements. In this dissertation, Kelvin probe force microscopy (KPFM) is used to determine the effect of patch potentials on both the sphere and the plate. Patch potentials are indeed present on both surfaces, but the force calculated from the patch potentials is found to be much less than the measured force. In order to better understand how KPFM resolves patch potentials, the artifacts and sensitivities of several different KPFM implementations are tested and characterized. In addition, we introduce a new technique, called tunable spatial resolution (TSR) KPFM, to control resolution by altering the power-law separation dependence of the KPFM signal.
Surface Studies of Graphene and Graphene Substrates
(2013) Burson, Kristen M.; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Graphene has attracted a great deal of attention for its exceptional electronic and mechanical properties. As graphene, a two-dimensional lattice of carbon atoms, is an `all surface' material, its interactions with the underlying substrate play a crucial role in determining graphene device behavior. In order to tailor graphene device properties, the interaction between graphene and the underlying substrate must be clearly understood. This thesis addresses the question of the relationship between graphene and graphene substrates by considering both the substrate topography and the impact of charged impurities in the substrate. Utilizing scanning tunneling microscopy and high-resolution atomic force microscopy, we measure the topography of silicon dioxide (SiO₂) supported graphene and the underlying SiO₂(300nm)/Si substrates. We conclude that the graphene adheres conformally to the substrate with 99% fidelity and resolve finer substrate features by atomic force microscopy than previously reported. To quantify the density of charged impurities, simultaneous atomic force microscopy (AFM) and Kelvin probe microscopy are used to measure the potential and topographic landscape of graphene substrates, SiO₂ and hexagonal boron nitride (h-BN). We find that the surface potential of SiO₂ is well described by a random two-dimensional surface charge distribution with charge densities of ~1011 cm-2, while BN exhibits charge fluctuations that are an order of magnitude lower than this. Charged impurities in the substrate present a scattering source for transport through graphene transistors, and the difference in magnitude in measured substrate charged impurities densities for SiO₂ and BN is consistent with the observed improvement in charged carrier mobility in graphene devices on h-BN over graphene devices on SiO₂. Finally, this thesis presents a theoretical model elucidating the challenges of imaging corrugated substrates by non-contact AFM and an experimental work using Kelvin probe microscopy to characterize the electrostatic potential steps at interfaces of small-molecule organic heterojunctions.
TAILORING PROPERTIES AND FUNCTIONALITIES OF NANOSTRUCTURES THROUGH COMPOSITIONS, COMPONENTS AND MORPHOLOGIES
(2013) WENG, LIN; Ouyang, Min; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The field of nanoscience and nanotechnology has made significant progresses over the last thirty years. Sophisticated nanostructures with tunable properties for novel physics and applications have been successfully fabricated, characterized and underwent practical test. In this thesis, I will focus on our recent efforts to develop new strategies to manipulate the properties of nanostructures. Particularly, three questions have been answered from our perspective, based on the nanomaterials synthesized: (1) How does the composition affect a novel nanostructure? We started from single-molecule precursors to reach nanostructures whose bulk counterparts only exist under extreme conditions. Fe₃S and Fe₃S₂ are used as examples to demonstrate this synthetic strategy. Their potential magnetic properties have been measured, which may lead to interesting findings in astronomy and materials science. (2) How to achieve modularity control at nanoscale by a general bottom-up approach? Starting with reviewing the current status of this field, our recent experimental progresses towards delicate modularity control are presented by abundant novel heteronanostructures. An interesting catalytic mechanism of these nanostructures has also been verified, which involves the interaction between phonons, photons, plasmons, and excitons. (3) What can the morphology difference tell us about the inside of nanostructures? By comparing a series of data from three types of CdSe/CdS core-shell structures, a conclusion has been reached on the CdS growth mechanism on CdSe under different conditions, which also may lead to a solution to the asymmetry problem in the synthesis of CdSe/CdS nanorods. Finally this thesis is concluded by a summary and future outlook.
QUANTIFYING PARTICLE PROPERTIES FROM ION-MOBILITY MEASUREMENTS
(2012) Li, Mingdong; Zachariah, Michael R.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Nanoparticles have received considerable interest due to the wide variety of potential applications in biomedical, optical, and electronic fields. However, our capabilities for quantitatively charactering these materials, for example in number concentration or shape are limited. The objective of this work is to develop experimentally verified theories to quantify particle properties from aerosol based ion-mobility measurement. The use of aerosol tools is predicated on the idea that these methods offer the best chance for quantification, due to a better understanding of the physics of ion transport in the gas phase. Nevertheless this does not preclude us from using these techniques to characterize particles in liquids as will be show in the first part of this work which resolves problems associated with generating an aerosol from colloidal suspensions. In this dissertation I resolve the problem of artificial "droplet induced aggregation" during electrospray which can corrupt the eventual determination of particle size. I develop an experimentally verified statistical based model, to determine and correct this undesired artifact. Furthermore, I have found that this nominally undesired artifact can be used in a beneficial way that allows one to determine the absolute number concentration of nanoparticles in solution, without the need for calibration particles. Mobility is one of the most important and fundamental properties of a particle. However most particle characterization approaches interpret the results of mobility measurement in the context of spherical particle transport. I have undertaken to systematically explore the mobility properties of non-spherical particles. In this dissertation I develop a theory to quantify the effect of orientation on the mobility and the dynamic shape factor of charged axially symmetric particles in an electric field. The experimental results of well-defined doublets of NIST traceable size standard 127nm, 150nm, 200nm and 240nm PSL spheres are shown to be in excellent agreement with the expected values based on my theory. More general new theories of the mobility of nonspherical particles are also proposed and compared with current theories. I also propose a new instrument, a pulsed differential mobility analyzer (PDMA), to obtain shape information by measuring the electrical mobility under different electric fields.
Nanofabrication on engineered silicon (100) surfaces using scanning probe microscopy
(2011) Li, Kai; Silver, Richard M; Einstein, Theodore L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Fabricating and measuring sub-5 nanometer features brings to light several pressing issues in future semiconductor industry manufacturing and dimensional metrology. This dissertation presents a feasible process to create nanostructures using scanning probes with applications in dimensional metrology and nanomanufacturing processes. Using the lattice spacing of a crystal as the fundamental "ruler" or scale, sub 5 nm critical dimension reference standards can be created with atomic scale dimensional control. This technique relies on atomically sharp tips to provide robust imaging and patterning (nanolithography) capabilities. We have developed a comprehensive process to routinely produce high quality scanning tunneling microscope (STM) tips. The quality of STM tips are a critical factor in achieving reproducible patterning. A modified electrochemical etching method has been used to create sharp tips with preferred apex geometry. By using a field ion microscope (FIM), tip surfaces have been cleaned by field evaporation. Finally, a thermal ultra-high-vacuum (UHV) process is implemented to stabilize the atoms on the tip apex for improved performance. The process is also found to be capable of restructuring the apex to regain atomic resolution when tips fail during imaging or patterning. Silicon (100) samples with pre-patterned micrometer-size fiducial marks are used as templates in this technique. The fiducial marks are used as 2D references to relocate the tip scanned area and the lithographic patterns. Large atomically-flat reconstructed (100) surfaces are obtained after a wet chemical cleaning process and a high temperature annealing process. After the high temperature annealing process, we observed reproducible step-terrace patterns formed on surfaces due to the fiducial marks. A kinetic Monte-Carlo simulation was used to study quantitatively the evolution of surface morphology under the influence of fiducial marks. Some of the key aspects, such as the electromigration effect and step permeability have been extensively studied. Hydrogen-passivated silicon (100) reconstructed surfaces are used to create nanopatterns by selective depassivation lithography. Optimized depassivation procedures enable us to fabricate patterns from the microscale to the atomic scale consistently using an UHV STM. To preserve and later enhance the nanopatterns, SiO2 hard etch mask marks are formed by oxidizing the patterns using ambient humidity or gaseous oxygen. A reactive ion etching (RIE) process is used to further enhance the aspect ratio of oxidized nanopatterns so that they can be served as 3D nanostructures on silicon surfaces.
High Frequency Generation from Carbon Nanotube Field Effect Transistors Used as Passive Mixers
(2012) Tunnell, Andrew Jacob; Williams, Ellen; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The high mobilities, low capacitances (due to small sizes), and high current densities of carbon nanotube field-effect transistors (CNT FETs) make them valid candidates for high frequency applications. The high cost of high frequency measurement equipment has been the largest hurdle to observing CNT transistor behavior at frequencies above 50 GHz. One economic solution to this barrier is to use an external harmonic mixer to convert high frequency signals to lower frequencies where they can be detected by a standard spectrum analyzer. By using this detection method, a new regime of high frequency CNT FET behavior is available for study. In this dissertation, we describe the design and fabrication of CNT FETs on quartz substrates using aligned arrays of CNTs as the device channel. The nonlinear input voltage to output drain current behavior of the devices is explained and approximated to the first order by using a Taylor expansion. For the high frequency mixing experiments, two input voltages of different frequencies are sourced on the gate of the devices without any device biasing. The input frequencies are limited to 100 kHz to 40 GHz by the signal generators used. The nonlinearities of the fabricated CNT FETs cause the input frequencies to be mixed together, even in the absence of a source-drain bias (passive mixing). The device output is the drain current, which contains sum and difference products of the input frequencies. By using an external harmonic mixer in combination with a spectrum analyzer to measure the drain current, output frequencies from 75 to 110 GHz can be observed. Up to 11th order mixing products are detected, due to the low noise floor of the spectrum analyzer. Control devices are also measured in the same experimental setup to ensure that the measured output signals are generated by the CNTs. The cutoff frequencies from previous passive mixing experiments predict that our devices should stop operating near 13 GHz, however our measurement setup extends and overcomes these cutoffs, and the generation of high frequency output signals is directly observed up to 110 GHz. This is the highest output frequency observed in CNT devices to date.

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