Department of Physics
http://hdl.handle.net/1721.1/7864
2016-10-25T08:34:50Z21 cm cosmology with optimized instrumentation and algorithms
http://hdl.handle.net/1721.1/104536
21 cm cosmology with optimized instrumentation and algorithms
Zheng, Haoxuan, Ph. D. Massachusetts Institute of Technology
Precision cosmology has made tremendous progress in the past two decades thanks to a large amount of high quality data from the Cosmic Microwave Background (CMB), galaxy surveys and other cosmological probes. However, most of our universe's volume, corresponding to the period between the CMB and when the first stars formed, remains unexplored. Since there were no luminous objects during that period, it is called the cosmic "dark ages". 21 cm cosmology is the study of the high redshift universe using the hyperfine transition of neutral hydrogen, and it has the potential to probe that unchartered volume of our universe and the ensuing cosmic dawn, placing unprecedented constraints on our cosmic history as well as on fundamental physics. My Ph.D. thesis work tackles the most pressing observational challenges we face in the field of 21 cm cosmology: precision calibration and foreground characterization. I lead the design, deployment and data analysis of the MIT Epoch of Reionization (MITEoR) radio telescope, an interferometric array of 64-dual polarization antennas whose goal was to test technology and algorithms for incorporation into the Hydrogen Epoch of Reionization Array (HERA). In four papers, I develop, test and improve many algorithms in low frequency radio interferometry that are optimized for 21 cm cosmology. These include a set of calibration algorithms forming redundant calibration pipeline which I created and demonstrated to be the most precise and robust calibration method currently available. By applying this redundant calibration to high quality data collected by the Precision Array for Probing the Epoch of Reionization (PAPER), we have produced the tightest upper bound of the redshifted 21 cm signals to date. I have also created new imaging algorithms specifically tailored to the latest generation of radio interferometers, allowing them to make Galactic foreground maps that are not accessible through traditional radio interferometry. Lastly, I have improved on the algorithm that synthesizes foreground maps into the Global Sky Model (GSM), and used it to create an improved model of diffuse sky emission from 10 MHz through 5 THz.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 213-236).
2016-01-01T00:00:00ZNovel angular and frequency manipulation of light in nano-scaled dielectric photonic systems
http://hdl.handle.net/1721.1/104534
Novel angular and frequency manipulation of light in nano-scaled dielectric photonic systems
Shen, Yichen, Ph. D. Massachusetts Institute of Technology
Humankind has long endeavored to control light. In modern society, with the rapid development of nanotechnology, the control of light is moving toward devices at micrometer and even nanometer scales. At such scales, traditional devices based on geometrical optics reach their fundamental diffraction limits and cease to work. Nano-photonics, on the other hand, has attracted wide attention from researchers, especially in the last decade, due to its ability to manipulate light at the nanoscale. In this thesis, we explore novel control of light created by nanophotonic structures, with a common theme on light interference in nanoscaled dielectric photonic systems. The first part of the thesis focuses on broadband angular selective nanophotonic systems. We survey the literatures and the current state of the art focused on enabling optical broadband angular selectivity. We also present a novel way of achieving broadband angular selectivity using Brewster mode in nanophotonic systems. We propose two categories of potential applications for broadband angularly selective systems. The first category aims at enhancing the efficiency of solar energy harvesting, through photovoltaic process or solar thermal process. The second category aims at enhancing light extracting efficiency and detection sensitivity. Finally, we discuss the most prominent challenges in broadband angular selectivity and some prospects on how to solve these challenges. The second part of the thesis focuses on spectrum control of light using all-dielectric surface resonator. We proposes a new structural color generation mechanism that produces colors by the Fano resonance effect on thin photonic crystal slab. We experimentally realize the proposed idea by fabricating the samples that show resonance-induced colors with weak dependence on the viewing angle. We also show that the colors can be dynamically tuned by stretching the photonic crystal slab fabricated on an elastic substrate. In a follow up work, we address how to overcome the challenge of mode leaking on dielectric substrate. We present a class of low-index zigzag surface structure that supports resonance modes even without index contrast with the substrate. In the third part, we investigate neuromorphic computation using the interference of light in on-chip dielectric photonic waveguide network. We first mathematically prove that conventional neural networks architecture can be equivalently represented by nanoscaled optical systems. We then experimentally demonstrate that our optical neural networks are able to give equivalent accuracy on a standard training datasets. In the last part, we show that in principle optical neural nets are at least 3 orders of magnitude faster and power efficient in forward propagation than conventional neural nets.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 93-114).
2016-01-01T00:00:00ZSemiclassical studies of decoherence produced by scattering
http://hdl.handle.net/1721.1/104533
Semiclassical studies of decoherence produced by scattering
Schram, Matthew Christopher
The conventional notion of coherent atom-surface scattering originates from the existence of Bragg peaks in elastic scattering. The helium atom acts as a quantum mechanical matter wave that is coherent with itself; the well-defined phase relationship of the particle beam at the different spatial positions at surface impact implies the possibility of different non-specular outgoing beams thanks to the constructive interference of the emitted waves from each surface atom. Moreover, we still observe diffraction peaks when scattering off a lattice at finite temperature, although the peaks are here diminished by the Debye-Waller factor. However, in the case of inelastic scattering, the surface particles are displaced by the scattering atom itself and may then emit or absorb one or more phonons to the scatterer. Acoustic phonons produced by this process are gapless excitations; hence, extremely long-wavelength phonons will contribute vanishingly small shifts in energy and momentum. The difficulty in observing this is exacerbated due to the roughly 1eV resolution of high energy helium scattering experiments. So through phonon excitation the surface has "measured" the particle's presence which acts to destroy quantum coherence, though we still observe diffraction spots which imply coherent scattering. How do we reconcile these disparate viewpoints? We propose a new way of looking at the question of coherence in atom-surface scattering. Instead of considering a single beam of helium particles, we instead use semiclassical techniques to simulate an initially coherent superposition of helium particles with equal probabilities of interacting with the surface or not interacting with the surface. We then evolve the classical mechanical trajectories, and recombine the atoms after scattering to observe the resulting interference pattern. The degree to which phonons are excited in the lattice by the scattering process dictates the fringe contrast of the interference pattern of the resulting beams. We show that for a wide range of conditions, despite the massive change in the momentum perpendicular to the surface, we can still expect to have coherent (in the superposition sense) scattering.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 146-152).
2016-01-01T00:00:00ZEffects of resonances and spin-curvature coupling in extreme mass ratio inspirals
http://hdl.handle.net/1721.1/104532
Effects of resonances and spin-curvature coupling in extreme mass ratio inspirals
Ruangsri, Uchupol
Since Einstein proposed the theory of general relativity (GR) as a theory of gravity, it has passed all experimental checks and tests. Until recently, all of these tests have been done in the weak gravity limit. The first test of strong-field GR came just a few months ago, when the LIGO collaboration directly detected gravitational waves for the first time. Using gravitational waves as a tool to test the validity of GR requires us to know the waveforms that GR predicts from various sources. The ultimate goal of the research described in this thesis is to compute the waveform generated by a stellar mass Kerr black hole as it inspirals into a much more massive black hole (SMBH). To compute this waveform, we must first compute the inspiral trajectory of the stellar mass black hole. The trajectory of the smaller black hole differs from the geodesic structure taught in GR textbooks due to the influence of this body's mass and spin. In this thesis, I examine these two effects separately. Later work will need to consider the two effects simultaneously, but the separate impact of these effects provides insight which helps us to understand how to model these sources. The small body's mass perturbs the spacetime and pushes its trajectory away from textbook geodesic motion. I show how to compute the dissipative part of this "self force," whose average impact is equivalent to the loss of energy and angular momentum due to gravitational wave emission. I study in particular how the self force's averaged behavior changes near orbital resonances, quantifying the impact that such resonances will have on the small body's inspiral. The small body's spin couples to spacetime curvature. This coupling leads to a force which also pushes the small body's trajectory away from the geodesic. This force is comparable in magnitude to the self force associated with the small body's mass, indicating that future work will need to assess the impact of these effects together in a self consistent way in order to make accurate inspiral waveforms.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references.
2016-01-01T00:00:00Z