Physics

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    Characterization of Quantum Efficiency and Robustness of Cesium-Based Photocathodes
    (2010) Montgomery, Eric J.; O'Shea, Patrick G.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    High quantum efficiency, robust photocathodes produce picosecond-pulsed, high-current electron beams for photoinjection applications like free electron lasers. In photoinjectors, a pulsed drive laser incident on the photocathode causes photoemission of short, dense bunches of electrons, which are then accelerated into a relativistic, high quality beam. Future free electron lasers demand reliable photocathodes with long-lived quantum efficiency at suitable drive laser wavelengths to maintain high current density. But faced with contamination, heating, and ion back-bombardment, the highest efficiency photocathodes find their delicate cesium-based coatings inexorably lost. In answer, the work herein presents careful, focused studies on cesium-based photocathodes, particularly motivated by the cesium dispenser photocathode. This is a novel device comprised of an efficiently photoemissive, cesium-based coating deposited onto a porous sintered tungsten substrate, beneath which is a reservoir of elemental cesium. Under controlled heating cesium diffuses from the reservoir through the porous substrate and across the surface to replace cesium lost to harsh conditions -- recently shown to significantly extend the lifetime of cesium-coated metal cathodes. This work first reports experiments on coated metals to validate and refine an advanced theory of photoemission already finding application in beam simulation codes. Second, it describes a new theory of photoemission from much higher quantum efficiency cesium-based semiconductors and verifies its predictions with independent experiment. Third, it investigates causes of cesium loss from both coated metal and semiconductor photocathodes and reports remarkable rejuvenation of full quantum efficiency for contaminated cesium-coated surfaces, affirming the dispenser prescription of cesium resupply. And fourth, it details continued advances in cesium dispenser design with much-improved operating characteristics: lower temperature and cleaner operation. Motivated by dispenser integration with semiconductor coatings, initial fabrication of those coatings are reported on dispenser-type substrates with measurement of quantum efficiency and analysis of thermal stability. Detailed investigations are performed on dispenser substrate preparation by ion beam cleaning and on dispenser pore structure by electron microscopy and focused ion beam milling. The dissertation concludes by discussing implications of all results for the demonstration and optimization of the future high quantum efficiency cesium dispenser photocathode.
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    New Technologies for Broadband Quantum Key Distribution: Sources, Detectors, and Systems
    (2008-11-12) Rogers, Daniel; Goldhar, Julius; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis I describe three independent projects that advance the development of broadband quantum cryptography. While each project pertains to a different part of the QKD chain, together they provide key developments in implementing QKD at bit rates that are practical for use in the modern telecommunications infrastructure. The first project comprises the bulk of the thesis and involves developing a novel source of correlated photon pairs for use in free-space QKD. This source is based on a birefringent semiconductor optical waveguide as a Kerr medium. We demonstrate the feasibility of using birefringent phase-matched four-wave mixing to generate correlated photon pairs. We further propose that, by reversing the process and pumping with conjugate wavelengths, one can use the same effect to produce entangled photon pairs with the same device. These pairs can then be used for QKD to realize the most secure and efficient quantum cryptographic data links. The second project examines the implications of operating a BB84 QKD protocol at clock rates that are faster than the recovery time of the constituent detectors. We show that operating such systems under conventional protocols results in a security violation that allows an eavesdropper to learn significant information about the key and present a modification to the BB84 protocol that maintains key security at fast transmission rates. This modification to the protocol will become vital to QKD viability as links become faster and clock rates go into the tens of gigahertz. We also demonstrate, rather counterintuitively, that there exists an optimal transmission rate for a QKD system that exceeds the inverse of an individual detector's dead time. The final project describes a new design for a free-space QKD link that centers around faster silicon detectors. These detectors have a peak quantum efficiency in the visible range, requiring that the system operate at a wavelength that is more susceptible to solar interference. To mitigate this effect, the link is designed around a Fraunhofer line in the solar spectrum where the background solar light levels are reduced by up to 90%. By implementing this system, we expect at least a two-fold increase in the secret key rate, coming ever closer to the goal of a 10 Mb/s QKD system compatible with first-generation ethernet technology.
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    Interaction of Lasers with Atomic Clusters and Structured Plasmas
    (2007-11-09) Palastro, John Patrick; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We examine the interaction of intense, short laser pulses with atomic clusters and structured plasmas, namely preformed plasma channels. In examining the laser pulse interaction with atomic clusters we focus on the optical response of an individual cluster when irradiated by a laser. Our analysis of the laser pulse interaction with plasma channels focuses on the mode structure of a laser pulse propagating within the channel. We then present a novel application of these channels: quasi-phased match acceleration of electrons. The optical properties of a gas of laser pulse exploded clusters are determined by the time-evolving polarizabilities of individual clusters. In turn, the polarizability of an individual cluster is determined by the time evolution of individual electrons within the cluster's electrostatic potential. We calculate the linear cluster polarizability using the Vlasov equation. A quasi-static equilibrium is calculated from a bi-maxwellian distribution that models both the hot and cold electrons, using inputs from a particle-in-cell simulation [Taguchi, T. et al., Phys. Rev. Lett., 2004. 92(20)]. We then perturb the system to first order in field and integrate the response of individual electrons to the self consistent field following unperturbed orbits. The dipole spectrum depicts strong absorption at frequencies much smaller than omega_p/√2. This enhanced absorption results from a beating of the laser field with electron orbital motion. The properties of pulse propagation within plasma are determined by the structure of the plasma. The preformed plasma channel provides a guiding structure for laser pulses unbound by the intensity thresholds of standard wave guides. In particular, the corrugated plasma channel [Layer et al. Phys. Rev. Lett. (2007)] allows for the guiding of subluminal spatial harmonics. These spatial harmonics can be phase matched to high energy electrons, making the corrugated plasma channel ideal for the acceleration of electrons. We present a simple analytic model of pulse propagation in a corrugated plasma channel and examine the laser-electron beam interaction. Simulations show accelerating gradients of several hundred MeV/cm for laser powers much lower than required by standard laser wakefield schemes.
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    Carbon Nanotube Devices: Growth, Imaging, and Electronic Properties
    (2006-01-11) Brintlinger, Todd Harold; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation focuses on growth, fabrication, and electronic characterization of carbon nanotube (CNT) devices. A technique for imaging CNTs on insulating substrates with the scanning electron microscope (SEM) will be described. This technique relies on differential charging of the CNT relative to the surrounding insulator. In addition, it is not only quicker than using scanning probe microscopy (SPM), but is also useful for identifying conducting pathways within an assortment of CNTs and metallic contacts. CNT field effect transitors (FETs) fabricated on strontium titanate gate dielectric show transconductances normalized by channel width of 8900 S/m, greatly exceeding that in Si FETs. Intriguingly, the transconductance cannot be explained within the conventional FET or Schottky-barrier models. To explain this, it is proposed that there is Schottky-barrier lowering due to high electric fields at the nanotube/contact interface. Exploring novel CNT-FET lithography, I demonstrate focused electron beam induced deposition (FEBID) of pure gold for CNT device electrodes. In examination of the CNT/electrode interface, equivalence between FEBID leads and leads deposited using conventional electron beam lithography is found with the majority device resistance in the CNT. Lastly, CNTs are suspended across wide trenches (>100 microns). These trenches are formed without lithography or etching and have metallic leads on either side of the trench for electrical transport measurements. Using a mechanical probe as a mobile gate, electrical transport can be performed on these suspended CNT devices, which show minimal hysteresis consistent with the absence of charge trapping.
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    Nonlinear Pulse Propagation Through An Optical Fiber: Theory and Experiment
    (2004-03-19) Khubchandani, Bhaskar; Roy, Rajarshi; Physics
    Pulse propagation through optical fibers is studied for two different phenomena, (i) the evolution of four-wave-mixing and (ii) the interplay between self- and cross-phase modulation for ultra-short pulses in a polarization maintaining fiber. For the four-wave-mixing case, we present the results of a study of the dynamical evolution of multiple four-wave-mixing processes in a single mode optical fiber with spatially and temporally delta-correlated phase noise. A nonlinear Schrodinger equation (NLSE) with stochastic phase fluctuations along the length of the fiber is solved using the Split-Step Fourier method. Good agreement is obtained with previous experimental and computational results based on a truncated-ODE model in which stochasticity was seen to play a key role in determining the nature of the dynamics. The full NLSE allows for simulations with high frequency resolution (60MHz) and frequency span (16THz) compared to the truncated ODE model (300GHz and 2.8THz respectively), thus enabling a more detailed comparison with observations. Fluctuations in the refractive index of the fiber core are found to be a possible source for this phase noise. It is found that index fluctuations as small as 1 part per billion are sufficient to explain observed features of the evolution of the four-wave-mixing sidebands. These measurements and numerical models thus may provide a technique for estimating these refractive index fluctuations which are otherwise difficult to measure. For the case of self- and cross-phase modulation, the evolution of orthogonal polarizations of asymmetric femtosecond pulses (810nm) propagating through a birefringent single-mode optical fiber (6.9cm) is studied both experimentally (using GRENOUILLE) and numerically (using a set of coupled NLSEs). A linear optical spectrogram representation is derived from the electric field of the pulses and juxtaposed with the optical spectrum and optical time-trace. The simulations are in good qualitative agreement with the experiments. Input temporal pulse asymmetry is found to be the dominant cause of output spectral asymmetry. The results indicate that it is possible to modulate short pulses both temporally and spectrally by passage through polarization maintaining optical fibers with specified orientation and length.