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Graphene-Mediated Light-Matter Interaction
Get PDFAdvances in 2D materials have opened a wealth of possibilities for the control of emission and propagation of light on length scales much smaller than the wavelength of light. Graphene, with highly-confined electrostatically tunable plasmons, provides a strong platform for explore a number of avenues.
We show that graphene that can increase the luminescence of erbium by 80%, can induce population inversion in a three-level system, speed up the response time by over an order of magnitude, and has modulation depth of up to 14 dB for luminescence.
We experimentally demonstrated a tunable epsilon-near-zero metamaterial with a elliptic-to-hyperbolic transition. The device had been theorized for many years and we provide the first experimental realization.
We explore the properties of an isotropic tunable 2D heterostructure composed of black phosphorus, hexagonal boron nitride, and graphene. These symmetry-breaking materials create an effective permittivity that is biaxially anistropic and tunable. This material supports tunable beam steering based on propagation of energy along the hyperbolic dispersion lines.</p
Physical Processes Leading to Export of Fixed Carbon Out of the Surface Ocean
Get PDFThe ocean sequesters carbon on long time scales by depositing it deep in the ocean, where it is no longer in contact with the atmosphere. This sequestration is also termed "carbon export", and is accomplished via a vertical flux of carbon into the interior of the ocean. Marine photosynthesis by phytoplankton, which consume carbon dioxide dissolved in the surface ocean and are transported to depth to be eventually remineralized or form sediments at the ocean surface, is a key component of this flux (the biological pump). This mechanism is primarily thought to occur via sinking of particulates. However, research over the past few decades has highlighted the role of instabilities at the "submesoscale", or 0.1--20 km, to induce large, O(100 m day-1) vertical velocities in the ocean. These vertical velocities can potentially subduct carbon from the surface ocean into the interior, where it would contribute to export. Observations of the ocean are, however, rarely made at scales which would detect these submesoscale instabilities. In this thesis, I use in situ observations from autonomous underwater vehicles, Seagliders, which make measurements in the upper 1000 m of the water column at horizontal scales of 1-3 km, to understand when and where submesoscale instabilities are present, and the extent to which they act to transport biologically fixed carbon out of the surface ocean. Three different types of instabilities are active in the surface mixed layer: baroclinic, gravitational, and symmetric. Each of these has potential to subduct material below the mixed layer; however, these instabilities are generally strongest during the winter, when biological production is at its minimum. An interesting exception is in southern Drake Passage, where interactions between the intense frontal system and the continental shelf result in subduction of water masses off the continental shelf during summer, when phytoplankton are photosynthesizing. In general, however, carbon export via submesoscale instabilities is expected to be largest during spring, when phytoplankton become more productive but conditions can still be ripe for submesoscale subduction. Scaling up these observations to the global ocean system is difficult because in situ observations at submesoscales are sparse. This thesis explores the ability of surface flux measurements, from reanalysis products and remote sensing measurements, to accurately depict carbon export via subduction processes by modeling the water profile in a one-dimensional model following Lagrangian floats in the ocean. This approach holds promise to advance the ultimate goal of determining the global effect of submesoscale-driven carbon export
Next Generation Fast Shutter System for LIGO
Get PDFThe proposal for the next generation Fast Shutter system intended to replace the current electromagnetic system used to block an optical pulse that is part of the LIGO observational setup is presented and discussed. The current system setup, problems solved with the new setup, and experiments guiding the new proposal design are detailed along with future work remaining to verify the operation of the next generation Fast Shutter system before it is used in LIGO's observatories.
This system is an electro-mechanical device consisting of two coils of electrically conducting wires and a payload made up of two magnets of opposite polarity configured to move between the upper and lower parts of the device. The mirror is attached to the magnet payload and is used to reflect optical pulses. When a current is applied to the coils, the magnets and attached mirror can move vertically relative to the coils due to the Lorentz force from the current in the coils acting on the magnets. The system can be operated in air or in a vacuum environment over a wide range of temperature and cleanliness requirements.
This system offers a novel and robust solution compared to LIGO's previously patented electro-magnetic ultra-fast shutter due to the design of a moving payload consisting of magnets and a mirror with no wires attached, compared to the prior design consisting of a moving coil requiring electrical attachment. As a result, this system avoids the failure mode associated with wire fatigue caused by repeated flexure of the coil attachment wires. With stationary coils, this system uses permanent magnets for the upward propulsion of the payload. Eddy current damping provided by copper interacting with the payload magnets is included to damp the oscillatory transient response of the payload. This newly designed system achieves the same critical performance specifications as the previously patented ultra-fast mechanical shutter, while being physically smaller, cheaper to build, and vastly more reliable.</p
Coupled Cluster Green's Functions for Periodic Systems: Ab-Inito Computation and Applications
Get PDFThe Coupled Cluster Green’s function method is expanded to periodic systems and preliminary results of the spectral function for diamond and graphene are shown. Future improvements and potential applications are discussed.</p
The Spectre of Race
No full textAfter the election of Donald Trump as President of the United States, many are asking themselves why far-right political movements are gaining ground when it seemed in recent decades that the country was becoming more united and more tolerant
Testing Alternative Theories of Quantum Mechanics with Optomechanics, and Effective Modes for Gaussian Linear Optomechanics
Get PDFOptomechanics has made great strides in theory and experiments over the past decade, which culminated in the first direct detection of gravitational waves in 2015 by LIGO. This thesis explores how optomechanics can be used to test fundamental physics other than the theory of general relativity. Our emphasis will be on falsifiable theories (ultimately, only experiments can decide whether a theory is correct) that address two outstanding issues in quantum mechanics: the measurement problem, and reconciling quantum mechanics with the theory of general relativity. In particular, we show that the space experiment LISA pathfinder places aggressive bounds on two objective collapse models, which are non-linear stochastic modifications of the Schroedinger equation that can resolve the measurement problem. Moreover, we show that state-of-the-art torsion pendulum experiments can test the Schroedinger-Newton theory, which is the non-relativistic limit of a non-linear theory combining quantum mechanics with a fundamentally classical spacetime.
Along the way, we propose how to resolve two major difficulties with determining the predictions of non-linear quantum mechanics in an actual experiment. First, we cannot use the density matrix formalism in non-linear quantum mechanics and so we have to suggest and justify a particular ensemble for the thermal bath. Separating out quantum and classical fluctuations helped us propose a reasonable ensemble. Second, most researchers believe that deterministic non-linear quantum mechanics must violate the no-signaling condition. We show this isn't necessarily the case because different interpretations of quantum mechanics make different predictions in non-linear quantum mechanics. We propose an interpretation, the causal-conditional prescription, that doesn't violate causality by noticing that once we fix an initial state, the evolution of a system under many non-linear theories is equivalent to evolution under a linear Hamiltonian with feedback. The mapping allows us to leverage the tools of quantum control, and it tells us that if the non-linear parameters of a non-linear Hamiltonian respond causally (i.e. with an appropriate delay) to measurement results, then the theory can be made causal.
We also contribute to the theory of quantum optomechanics. We introduce two new bases that one can view environment modes with. In linear optomechanics a system interacts with an infinite number of bath modes. We show that the interaction can be reduced to one with finite degrees of freedom. Moreover, at any particular time, the system is correlated with only a finite number of bath modes. We show that if we make the assumption that we can measure any commuting environment modes, then this basis allows us to understand the one-shot quantum Cramer-Rao bound in a simple way, and allows us to sweep large parameter regimes and so find promising optomechanics topologies for quantum state preparation tasks that we can then analyze without the assumption of being able to measure any observable of the environment. We also use this basis to show that when we are interested in the conditional dynamics of a test mass, we can only adiabatically eliminate a lossy cavity when we measure the optomechanical system at a slow enough rate. Finally, we develop an analytic filter for obtaining the state of a generic optomechanical system that interacts linearly with its environment and is driven by Gaussian states, and where the outgoing light is measured with a non-linear photon-counting measurement. We hope that our work will help researchers explore optomechanics topologies that make use of photon counters.</p
Electromechanical Properties of 3D Multifunctional Nano-Architected Materials
Get PDFIn this thesis, we explore the fabrication and characterization of 3D architected multifunctional materials in three different categories: varied density for tailored mechanical response, stiff ultra low-k dielectric materials, and direct laser writing of piezoelectric structures at the micron scale. The density of an architected material plays a large role in determining its effective Young’s modulus, strength, and deformation behavior. The first section of this work explores the effect of incorporating two density regions into hollow nanolattices, which results in two distinct mechanical response regions for horizontal interfaces and a combined varying response for a diagonal interface. The second section of this work describes low dielectric constant (low-k) materials, which have gained increasing popularity because of their critical role in developing faster, smaller, and higher performance devices. We report the fabrication of 3D nanoarchitected hollow-beam alumina dielectrics with a k value of 1.06 - 1.10 at 1 MHz that is stable over the voltage range of -20 to 20 V and a frequency range of 100 kHz to 10 MHz, with an effective Young’s modulus of 30 MPa, a strength of 1.07 MPa, a nearly full shape recoverability to its original size after >50% compressions, and outstanding thermal stability with a thermal coefficient of dielectric constant (TCK) of 2.43 x 10-5K-1 up to 800° C. Finally, we report the fabrication of monolithic piezoelectric ZnO structures of arbitrary shape via a polymer complex route. We have confirmed the microstructure using XRD, TEM, and SAED, and have observed its electromechanical response using a novel in-situ experiment.</p
Searching for Gravitational Waves from Compact Binary Coalescences in Advanced LIGO Data
Get PDFAdvanced LIGO's first observing run marked the birth of gravitational-wave astronomy through the first detection of gravitational waves from coalescing black holes-GW150914. Advanced LIGO's second and Advanced Virgo's first observing run marked the birth of multimessenger astronomy with first joint observations of gravitational and electromagnetic radiation associated with coalescing neutron stars-GW170817. The electromagnetic observations included detection of a burst of gamma rays produced by the merger, and a kilonova powered by the radioactive decay of r-process nuclei synthesized in the neutron star coalescence ejecta. Gravitational waves from compact binary coalescences carry fingerprints of the sources that generated them. Studying them allows us to test Einstein’s general relativity in the strongest regimes, where it has never been tested before, and study matter at densities beyond reach of the most powerful laboratories on our planet. Moreover, we can gain insight about the evolution of stars, galaxies and even the Universe as a whole by studying the merger rate of compact objects. Joint electromagnetic and gravitational-wave observations help develop our understanding of the physical processes that occur in such systems, and provide a new method of probing cosmological parameters.
GW170817 was detected by the GstLAL pipeline in low-latency making the extensive electromagnetic followup possible. The GstLAL pipeline is a matched filtering pipeline that uses compact binary coalescence waveform models to filter the data from gravitational-wave detectors in the time-domain. It can detect gravitational waves from coalescing compact binaries in near real time and provide point estimates for binary parameters. This thesis describes the methods, developments, and the results from the GstLAL pipeline over the course of the first two observing runs of Advanced LIGO, focusing on the contributions made by the author. We also present a study about the prospects of observing a cosmological stochastic background which is expected to be buried under the astrophysical background from the population of coalesceing compact binaries with third-generation gravitational-wave detectors.</p
Dielectric Metasurfaces from Fundamentals to Applications
Get PDFIn the past few decades, the advancements in nanotechnology have significantly altered many fields of science and technology, especially electronics and integrated photonics. Free-space optics, on the other hand, has remained mostly unaffected, and even today "optics" reminds us of carefully shaped and polished pieces of various types of glasses and crystals lumped into lenses and beam shapers. Several of these devices are then combined into more complicated optical systems like microscopes and pulse shapers that are expensive, bulky, sensitive to various environmental factors, and require several alignment steps. This thesis contains my work on designing and utilizing structures engineered at the nano-scale, which are called metasurfaces, to implement compact optical elements and systems with capabilities beyond those of conventional refractive and diffractive optics. My contributions to this field are two-fold: I have developed and contributed to the development of new concepts that take metasurfaces beyond conventional difractive optics in various aspects, in addition to paradigm changing platforms for optical element and system design. Here, I first give an overview and a brief history about optical metasurfaces. Next I discuss the unprecedented capabilities of metasurfaces in controlling light based on its degrees of freedom like illumination angle and polarization. Then, I will focus on various novel metasurface platforms of conformal and tunable metasurfaces, 3D metasurface beam shapers, and integrated metasurfaces. I conclude with an outlook on future potentials and challenges that need to be overcome for realizing their wide-spread applications
Spatially Resolved Star Formation in Cosmological Zoom-in Simulations: Understanding the Role of Feedback in Scaling Relations
Get PDFTo understand the night sky is to understand how galaxies form their stars. Cosmological zoom-in simulations, which self-consistently evolve a small number of galaxies at very high resolution by embedding them within a fully cosmological box, have evolved over the last 25 years to a level of realism where they can begin to tackle questions of spatially resolved star formation within galaxies. Whereas a decade ago simulations faced difficulty in matching even global properties of observed galaxies (e.g., the ratio of stellar mass to total halo mass), the state of the art is now able to meaningfully recover resolved quantities in galaxies that were not put into the simulations by hand (e.g., the Kennicutt-Schmidt star formation scaling relation).
The research presented in this thesis seeks to understand how the physics of star formation and stellar feedback from massive stars shape and regulate the interstellar medium (ISM) within galaxies. Particularly, the focus lies on the scale of the largest coherent structures in galaxies -- the disk scale height. To explore these physics, the cosmological zoom-in simulations of the Feedback in Realistic Environments (FIRE) project (Hopkins et al. 2014, 2018) are used.
The chapters of this thesis explore various topics in spatially resolved star formation, including: the Kennicutt-Schmidt relation (Schmidt 1959, Kennicutt 1998), an empirical relation between gas surface density and star formation rates, in the FIRE-1 simulations (Orr et al. 2018), including an examination what set the extent of the star-forming disks in the simulations (i.e., what causes star formation to fire up in the outskirts); an examination of the observational method of analyzing stacks of galaxy observations, finding that temporal variations in spatially resolved star formation rates within individual galaxies were more than enough to bias stacking analysis of star formation rate profiles; a semi-analytic model of non-equilibrium star formation rates, relating to the competition between the feedback timescale associated with star formation and local dynamical times (Orr et al. 2019), which explores this as a source of scatter in the Kennicutt-Schmidt relation; and finally, investigating how gas velocity dispersions and star formation rates relate in FIRE-2 Milky Way-mass disk galaxies, exploring whether or not feedback is primarily driving the velocity dispersions in galaxies, and how quickly local patches can self-regulate with star formation (Orr et al. in prep.).</p