Physics
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Item Casimir-Lifshitz forces and torques(2018) Somers, David Andrew Templeton; Munday, Jeremy N; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum electromagnetic field fluctuations result in the well-documented Casimir-Lifshitz force between macroscopic objects. If the objects are anisotropic, theory predicts a corresponding Casimir-Lifshitz torque that causes the objects to rotate and align. In this work, we report the first measurements of the Casimir-Lifshitz torque, which confirm the predictions first made decades ago. The experimental design uses a nematic liquid crystal separated from a birefringent crystal by an isotropic Al2O3 layer with a thicknessItem Design of a nonlinear quasi-integrable lattice for resonance suppression at the University of Maryland Electron Ring(2018) Ruisard, Kiersten; Koeth, Timothy; Thomas, Antonsen; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Conventional particle accelerators use linear focusing forces for transverse confinement. As a consequence of linearity, accelerating rings are sensitive to myriad resonances and instabilities. At high beam intensity, uncontrolled resonance-driven losses can deteriorate beam quality and cause damage or radio-activation in beam line components and surrounding areas. This is currently a major limitation of achievable current densities in state-of-the-art accelerators. Incorporating nonlinear focusing forces into machine design should provide immunity to resonances through nonlinear detuning of particle orbits from driving terms. A theory of nonlinear integrable beam optics is currently being investigated for use in accelerator rings. Such a system has potential to overcome the limits on achievable beam intensity. This dissertation presents a plan for implementing a proof-of-principle quasi-integrable octupole lattice at the University of Maryland Electron Ring (UMER). UMER is an accelerator platform that supports the study of high-intensity beam dynamics. In this dissertation, two designs are presented that differ in both complexity and strength of predicted effects. A configuration with a single, relatively long octupole magnet is expected to be more stabilizing than an arrangement of many short, distributed octupoles. Preparation for this experiment required the development and characterization of a low-intensity regime previously not operated at UMER. Additionally, required tolerances for the control of first and second order beam moments in the proposed experiments have been determined on the basis of simulated beam dynamics. In order to achieve these tolerances, a new method for improved orbit correction is developed. Finally, a study of resonance-driven losses in the linear UMER lattice is discussed.Item 3D Magnetic Imaging using SQUIDs and Spin-valve Sensors(2016) Jeffers, Alex; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa2Cu3O7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from this inverse technique that demonstrate the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin-valve sensors I typically applied currents of 1 mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1 mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.Item Interaction of intense laser pulses with gas for two-color THz generation and remote magnetometry(2014) Johnson, Luke Alan; Antonsen Jr., Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interaction of intense laser pulses with atmospheric gases is studied in two contexts: (i) the generation of broadband terahertz radiation via two-color photoionization currents in nitrogen, and (ii) the generation of an electromagnetic wakefield by the induced magnetization currents of oxygen. (i) A laser pulse propagation simulation code was developed to investigate the radiation patterns from two-color THz generation in nitrogen. Understanding the mechanism for conical, two-color THz furthers the development of broadband THz sources. Two-color photoionization produces a cycle-averaged current driving broadband, conically emitted THz radiation. The THz emission angle is found to be determined by an optical Cherenkov effect, occurring when the front velocity of the ionization induced current source is greater than the THz phase velocity. (ii) A laser pulse propagating in the atmosphere is capable of exciting a magnetic dipole transition in molecular oxygen. The resulting transient current creates a co-propagating electromagnetic field behind the laser pulse, i.e. the wakefield, which has a rotated polarization that depends on the background magnetic field. This effect is analyzed to determine it's suitability for remote atmospheric magnetometry for the detection of underwater and underground objects. In the proposed approach, Kerr self-focusing is used to bring a polarized, high-intensity, laser pulse to focus at a remote detection site where the laser pulse induces a ringing in the oxygen magnetization. The detection signature for underwater and underground objects is the change in the wakefield polarization between different measurement locations. The magnetic dipole transition line that is considered is the b-X transition band of oxygen near 762 nm.Item Broadband In-plane Relative Permittivity Characterization of Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1) Thin Films(2010) Orloff, Nathan Daniel; Takeuchi, Ichiro; Booth, James C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a broadband on-wafer measurement technique for the characterization of the in-plane complex relative permittivity of a thin-film test wafer and a companion substrate test wafer from 100 Hz to 40 GHz, and potentially to 110 GHz. From 100 Hz to 300 MHz, the approach uses an ensemble of interdigitated capacitors with different interdigitated active lengths l = (0.100 mm, 0.325 mm, 0.875 mm, 1.835 mm, 2.9 mm) fabricated on both test wafers. Within this regime, from 100 Hz to 1 MHz, the measurements were performed with an inductance-capacitance-resistance meter. From 1 MHz to 300 MHz, the scattering parameters of the set of interdigitated capacitors were measured with a radio frequency vector network analyzer. In the high frequency regime, 300 MHz to 40 GHz, we measure scattering parameters of a set of coplanar waveguides of active lengths l = (0.420 mm, 1.270 mm, 2.155 mm, 3.22 mm, 3.993 mm, 5.933 mm) fabricated on the test wafers. We extracted the capacitance and conductance of the interdigitated capacitors and coplanar waveguides on the test wafers for the appropriate frequency regimes. We then obtained a mapping function from 2D finite element simulations that relates the change in capacitance of the thin-film test wafer relative to the companion substrate test wafer to the real part of the in-plane relative permittivity. The imaginary part of the in-plane relative permittivity was obtained from the real part of the in-plane relative permittivity and the in-plane loss tangent. We applied this broadband dielectric spectroscopy technique to explore the frequency-dependent relative permittivity of unstrained Ruddlesden-Popper series Srn+1TinO3n+1(n=1, 2, 3) thin films as a function of temperature and dc electric field. At room temperature, the in-plane relative permittivities (K11) obtained for Srn+1TinO3n+1(n=1, 2, 3) were 42 plus/minus 3, 54 plus/minus3, and 77 plus/minus2, respectively, and were independent of frequency. At low temperatures, K11 increased with a behavior consistent with an incipient ferroelectric, and paraelectric behavior developed in Sr4Ti3O10(n=3). In 2004, J. H. Haeni, et al. showed that SrTiO3 (n = infinity) on DyScO3 (110) undergoes a ferroelectric to paraelectric phase transition around room temperature. As a means to understand the origins of the loss and tunability in strained SrTiO3 (n = infinity), we performed our broadband dielectric spectroscopy technique on epitaxial thin-films of Ruddlesden-Popper series Srn+1TinO3n+1(n=2, 3, 4, 5, 6) on the rare-earth scandate substrates, DyScO3 (110) and GdScO3 (110). For these thin films, DyScO3 (110) and GdScO3 (110) corresponded to biaxial tensile strain of approximately 1% and 1.7%, respectively. The thin films were 50 nm thick on DyScO3 (110) and 25 nm thick on GdScO3 (110), which ensured uniform strain throughout the film. We report the dependence of the critical temperature, tunability, and loss tangent on series number and strain at 1 MHz. We also examined the broadband frequency dependent dielectric properties of these thin films as a function of temperature, electric field, series number and strain.