Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item EFFICIENT SIMULATION OF ELECTRON TRAPPING IN LASER AND PLASMA WAKEFIELD ACCELERATION(2009) Morshed, Sepehr; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma based laser Wakefield accelerators (LWFA) have been a subject of interest in the plasma community for many years. In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in the quasistatic approximation. The 2D, cylindrically symmetric, quasistatic simulation code, WAKE is an efficient tool for the modeling of short-pulse laser propagation in under dense plasmas [P. Mora & T.M. Antonsen Phys. Plasmas 4, 1997]. The quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons, through the pulse, cannot, however, treat electron trapping and beam loading. Here we modify WAKE to include the effects of electron trapping and beam loading by introducing a population of beam electrons. Background plasma electrons that are beginning to start their oscillation around the radial axis and have energy above some threshold are removed from the background plasma and promoted to "beam" electrons. The population of beam electrons which are no longer subject to the quasistatic approximation, are treated without approximation and provide their own electromagnetic field that acts upon the background plasma. The algorithm is benchmarked to OSIRIS (a standard particle in cell code) simulations which makes no quasistatic approximation. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBNL. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.Item An Optical MEMS Sensor for On-chip Catechol Detection(2008-12-08) Dykstra, Peter Hume; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis reports the successful design, fabrication and testing of an optical MEMS sensor for the detection of the toxic phenol, catechol. Catechol's presence in food and drinking water posses a health concern due to its harmful effects on cell respiration. By-products of catechol oxidation have demonstrated increased absorbance changes in a chitosan film in the UV and near UV range. Our reported sensor utilizes patterned SU-8 waveguides and a microfluidic channel to deliver catechol samples to an electrodeposited chitosan film for absorbance measurements at 472 nm. Concentrations as low as 1 mM catechol are detected while control experiments including ascorbic acid display no measurable response. By using optical detection methods, our device does not suffer from many of the problems which plague conventional electrochemical based sensors.Item Analysis and Experimental Demonstration of Conformal Adaptive Phase-Locked Fiber Array for Laser Communications and Beam Projection Applications(2008-10-07) Liu, Ling; Vorontsov, Mikhail A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The primary goal of this research is the analysis, development, and experimental demonstration of an adaptive phase-locked fiber array system for free-space optical communications and laser beam projection applications. To our knowledge, the developed adaptive phase-locked system composed of three fiber collimators (subapertures) with tip-tilt wavefront phase control at each subaperture represents the first reported fiber array system that implements both phase-locking control and adaptive wavefront tip-tilt control capabilities. This research has also resulted in the following innovations: (a) The first experimental demonstration of a phase-locked fiber array with tip-tilt wavefront aberration compensation at each fiber collimator; (b) Development and demonstration of the fastest currently reported stochastic parallel gradient descent (SPGD) system capable of operation at 180,000 iterations per second; (c) The first experimental demonstration of a laser communication link based on a phase-locked fiber array; (d) The first successful experimental demonstration of turbulence and jitter-induced phase distortion compensation in a phase-locked fiber array optical system; (e) The first demonstration of laser beam projection onto an extended target with a randomly rough surface using a conformal adaptive fiber array system. Fiber array optical systems, the subject of this study, can overcome some of the drawbacks of conventional monolithic large-aperture transmitter/receiver optical systems that are usually heavy, bulky, and expensive. The primary experimental challenges in the development of the adaptive phased-locked fiber-array included precise (<5>microrad) alignment of the fiber collimators and development of fast (100kHz-class) phase-locking and wavefront tip-tilt control systems. The precise alignment of the fiber collimator array is achieved through a specially developed initial coarse alignment tool based on high precision piezoelectric picomotors and a dynamic fine alignment mechanism implemented with specially designed and manufactured piezoelectric fiber positioners. Phase-locking of the fiber collimators is performed by controlling the phases of the output beams (beamlets) using integrated polarization-maintaining (PM) fiber-coupled lithium niobate phase shifters. The developed phase-locking controllers are based on either the SPGD algorithm or the multi-dithering technique. Subaperture wavefront phase tip-tilt control is realized using piezoelectric fiber positioners that are controlled using a computer-based SPGD controller. Both coherent (phase-locked) and incoherent beam combining in the fiber array system are analyzed theoretically and experimentally. Two special fiber-based beam-combining testbeds have been built to demonstrate the technical feasibility of phase-locking compensation prior to free-space operation. In addition, the reciprocity of counter-propagating beams in a phase-locked fiber array system has been investigated. Coherent beam combining in a phase-locking system with wavefront phase tip-tilt compensation at each subaperture is successfully demonstrated when laboratory-simulated turbulence and wavefront jitters are present in the propagation path of the beamlets. In addition, coherent beam combining with a non-cooperative extended target in the control loop is successfully demonstrated.Item STUDIES OF THE OPTICAL PROPERTIES OF PLASMONIC NANOSTRUCTURES(2007-11-28) Hung, Yu-Ju; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Various properties of Surface Plasmon Polaritons (SPPs) at the interface between a layer of PMMA (polymethyl methacrylate) gratings and a 50 nm thick gold film have been studied. Gold has a negative dielectric constant at visible wavelength range which results in negative refraction phenomenon without medium of both permittivity () and permeability () constants negative. A direct observation of negative refraction has been demonstrated. It verifies our assumption that in the 1-D stripe PMMA gratings on top of a gold film, SPPs experience negative group velocity and positive phase velocity. With this criterion, negative refraction is the natural choice in Snell's Law. Correspondingly, it was previously claimed that with a highly anisotropic layered structure (metal/dielectric stack), the high spatial frequency k vectors scattered from an object can be preserved in an imaging system and the conventional diffraction limit is defeated. In this thesis, this kind of layered structure, a so-called "hyperlens" or "superlens", has been experimentally demonstrated and the results verify theoretical predictions. A proof of concept on corner resonators has also been demonstrated. Four squares with PMMA/Au and Air/Au are arranged so that SPPs are trapped in the corner. It shows the possibility of making a tiny resonator with zero phase paths in the cavity. An experiment utilizing the field enhancement of SPPs is designed. A surface field is excited on R6G(Rhodamine 6G, fluorophore)/PMMA gratings/Au substrate. The enhanced pumping light pushes up the emission intensity 10-fold or higher compared to a sample with a R6G/PMMA gratings/Glass platform, a transparent substrate. This device with a R6G/PMMA gratings/Au platform has the advantage that the emission light is converted to the normal direction; the collection efficiency is high and the directivity makes the examination easy under a commercial fluorescence optical microscope. This device shows the potential of R6G/PMMA/Au platforms in gene chip industry.Item Indium Phosphide MEMS Cantilever Waveguides with Integrated Readout for Chemical Sensing(2007-11-26) Siwak, Nathan Paul; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the development towards an integrated, monolithic, micro-electro-mechanical system (MEMS) cantilever waveguide resonator chemical sensor using the III-V semiconductor indium phosphide (InP). Waveguide cantilevers with resonant frequencies as high as 5.78 MHz, a quality factor of 340, and a sensitivity of 4.4x10^16 Hz/g are shown for the first time in this system. The first demonstration of vapor detection using the sensor platform is performed utilizing an organic semiconductor Pentacene absorbing layer. Vapors are measured from mass shifts of 6.56x10^-14 and 7.28x10^-14 g exhibiting a mass detection threshold of 5.09x10^-15 g. The design, fabrication, and testing of an integrated waveguide PIN photodetector with an In0.53Ga0.47As absorbing layer is reported. Dark currents as low as 8.7 nA are measured for these devices. The first demonstration of a resonating cantilever waveguide measurement is also performed using the monolithically integrated waveguide photodiodes with uncertainty of less than ± 35 Hz. Finally, a future outlook is presented for this monolithic InP sensor system.Item Generation of a CW Local Oscillator Signal Using a Stabilized Injection Locked Semiconductor Laser(2007-04-25) Pezeshki, Jonah Massih; Goldhar, Julius; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In high speed-communications, it is desirable to be able to detect small signals while maintaining a low bit-error rate. Conventional receivers for high-speed fiber optic networks are Amplified Direct Detectors (ADDs) that use erbium-doped fiber amplifiers (EDFAs) before the detector to achieve a suitable sensitivity. In principle, a better method for obtaining the maximum possible signal to noise ratio is through the use of homodyne detection. The major difficulty in implementing a homodyne detection system is the generation of a suitable local oscillator signal. This local oscillator signal must be at the same frequency as the received data signal, as well as be phase coherent with it. To accomplish this, a variety of synchronization techniques have been explored, including Optical Phase-Lock Loops (OPLL), Optical Injection Locking (OIL) with both Fabry-Perot and DFB lasers, and an Optical Injection Phase-Lock Loop (OIPLL). For this project I have implemented a method for regenerating a local oscillator from a portion of the received optical signal. This regenerated local oscillator is at the same frequency, and is phase coherent with, the received optical signal. In addition, we show that the injection locking process can be electronically stabilized by using the modulation transfer ratio of the slave laser as a monitor, given either a DFB or Fabry-Perot slave laser. We show that this stabilization technique maintains injection lock (given a locking range of ~1GHz) for laser drift much greater than what is expected in a typical transmission system. In addition, we explore the quality of the output of the slave laser, and analyze its suitability as a local oscillator signal for a homodyne receiver.Item DISTRIBUTED TWO-DIMENSIONAL FOURIER TRANSFORMS ON DSPs: WITH AN APPLICATIONS FOR PHASE RETRIEVAL(2006-12-08) Smith, Jeffrey Scott; Jacob, Bruce L; Yeung, Donald; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many applications of two-dimensional Fourier Transforms require fixed timing as defined by system specifications. One example is image-based wavefront sensing. The image-based approach has many benefits, yet it is a computational intensive solution for adaptive optic correction, where optical adjustments are made in real-time to correct for external (atmospheric turbulence) and internal (stability) aberrations, which cause image degradation. For phase retrieval, a type of image-based wavefront sensing, numerous two-dimensional Fast Fourier Transforms (FFTs) are used. To meet the required real time specifications, a distributed system is needed, and thus, the 2-D FFT necessitates an all-to-all communication among the computational nodes. The 1-D floating point FFT is very efficient on a digital signal processor (DSP). For this study, several architectures and analysis of such are presented which address the all-to-all communication with DSPs. Emphasis of this research is on a 64-node cluster of Analog Devices TigerSharc TS-101 DSPs.Item Novel Organic Polymeric and Molecular Thin-Film Devices for Photonic Applications(2006-12-08) Kim, Younggu; Lee, Chi H.; Herman, Warren N.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The primary objective of this thesis is to explore the functionalities of new classes of novel organic materials and investigate their technological feasibilities for becoming novel photonic components. First, we discuss the unique polarization properties of optical chiral waveguides. Through a detailed experimental polarization analysis on planar waveguides, we show that eigenmodes in planar chiral-core waveguides are indeed elliptically polarized and demonstrate waveguides having modes with polarization eccentricity of 0.25, which agrees very well with recent theory. This is, to the best of our knowledge, the first experimental demonstration of the mode ellipticities of the chiral-core optical waveguides. In addition, we also examine organic magneto-optic materials. Verdet constants are measured using balanced homodyne detection, and we demonstrate organic materials with Verdet constants of 10.4 and 4.2 rad/T · m at 1300 nm and 1550 nm, respectively. Second, we present low-loss waveguides and microring resonators fabricated from perfluorocyclobutyl copolymer. Design, fabrication and characterization of these devices are addressed. We demonstrate straight waveguides with propagation losses of 0.3 dB/cm and 1.1 dB/cm for a buried channel and pedestal structures, respectively, and a microring resonator with a maximum extinction ratio of 4.87 dB, quality factor Q = 8554, and finesse F = 55. In addition, from a microring-loaded Mach-Zehnder interferometer, we demonstrate a modulation response width of 30 ps and a maximum modulation depth of 3.8 dB from an optical pump with a pulse duration of 100 fs and a pulse energy of 500 pJ when the signal wavelength is initially tuned close to one of the ring resonances. Finally, we investigate a highly efficient organic bulk heterojunction photodetector fabricated from a blend of P3HT and C60. The effect of multilayer thin film interference on the external quantum efficiency is discussed based on numerical modeling. We experimentally demonstrate an external quantum efficiency ηEQE=87±2% under an applied bias voltage V = −10 V, leading to an internal quantum efficiency ηIQE≈97%. These results show that the charge collection efficiency across the intervening energy barriers can indeed reach near 100% under a strong electric field.Item Polarization-Insensitive Techniques for Optical Signal Processing(2006-09-01) Salem, Reza; Murphy, Thomas E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates polarization-insensitive methods for optical signal processing. Two signal processing techniques are studied: clock recovery based on two-photon absorption in silicon and demultiplexing based on cross-phase modulation in highly nonlinear fiber. The clock recovery system is tested at an 80 Gb/s data rate for both back-to-back and transmission experiments. The demultiplexer is tested at a 160 Gb/s data rate in a back-to- back experiment. We experimentally demonstrate methods for eliminating polarization dependence in both systems. Our experimental results are confirmed by theoretical and numerical analysis.Item Interaction of Intense Short Laser Pulses with Gases of Nanoscale Atomic and Molecular Clusters(2006-08-09) Gupta, Ayush; Antonsen, Thomas M.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the interaction of intense laser pulses with gases of van der Waals bound atomic aggregates called clusters in the range of laser-cluster parameters such that kinetic as well as hydrodynamic effects are active. The clustered gas absorbs the laser pulse energy efficiently producing x-rays, extreme ultraviolet radiation, energetic particles and fusion neutrons. First, we investigate the effect of pulse duration on the heating of a single cluster in a strong laser field using a 2-D electrostatic particle-in-cell (PIC) code. Heating is dominated by a collision-less resonant absorption process that involves energetic electrons transiting through the cluster. A size-dependent intensity threshold defines the onset of this resonance [Taguchi et al., Phys. Rev. Lett., v90(20), (2004)]. It is seen that increasing the laser pulse width lowers this intensity threshold and the energetic electrons take multiple laser periods to transit the cluster instead of one laser period as previously recorded [Taguchi et al., Phys. Rev. Lett.,v90(20), (2004)]. Results of our numerical simulations showing the effect of pulse duration on the heating rate and the evolution of the electron phase space are presented in this dissertation. Our simulations show that strong electron heating is accompanied by the generation of a quasi-monoenergetic high-energy peak in the ion kinetic energy distribution function. The energy at which the peak occurs is pulse duration dependent. Calculations of fusion neutron yield from exploding deuterium clusters using the PIC model with periodic boundary conditions are also presented. We also investigate the propagation of the laser pulse through a gas of clusters that is described by an effective dielectric constant determined by the single cluster polarizability. For computational advantage, we adopt a uniform density description of the exploding clusters, modified to yield experimentally consistent single cluster polarizability, and couple it to a Gaussian description of the laser pulse. This model is then used to study self-focusing, absorption, and spectral broadening of the laser pulse. The model is further extended to allow for a fraction of the gas to be present as unclustered monomers and to include the effect of unbound electrons produced in the laser-cluster interaction.