Matching Items (13)

1D LASING AT THE NANOSCALE: METHOD AND BEHAVIOR

Description

The importance of lasing cannot be overstated – from improving medicine through surgery uses and industry through laser cutting and micro-wielding (just to name a few), to the development of

The importance of lasing cannot be overstated – from improving medicine through surgery uses and industry through laser cutting and micro-wielding (just to name a few), to the development of laser cooling to isolate the first Bose-Einstein condensate in 1995. Not only do the technological benefits encourage research but, as could probably be deduced, lasers are expensive devices. From Ruby crystals to Rubidium gasses, the materials required to construct lasers can be rare and highly specialized. Since the advancement of computer technology, computational physics has proved exceedingly useful. As a combination of both theoretical and experimental physcis, computational physics proves itself invaluable for allowing the testing of various theories and running of experiments in a time efficient and far less expensive way. For the purpose of this paper, having a clear understanding of the computational lasing system allows for simulations that are incredibly expensive or might not even be possible yet, to be conducted and the groundwork to be laid for future theory, experiment, or product.
The response of a molecular sheet with varying densities of simple, two-level system without lasing due to an ultra-short, wideband pulse centered at 2 eV is first investigated. The Fabry-Pérot modes rising from interference are observed, as well as the expected redshift in the transmission and reflection frequencies in the thin molecular sheet regime. Cautions regarding numerical instability and implementation of the Fast Fourier Transform are discussed. Upon activating the lasing levels of the molecules (creating a four-level system), the transmission and refection responses are measured for four combinations of molecular density and molecular sheet thickness. Lasing threshold and saturation phenomenon are observed and a clear lasing region is seen in the Power input/output analysis.
Population inversion is driving force that triggers lasing through stimulated emission. To investigate this, the populations of each of the four molecular energies levels are tracked for the same combinations of parameters in the previous tests. The population inversions and the threshold/saturation phenomena do not correspond to within reasonable limits. Inspection of the population data reveals a highly varied distribution within the molecular, suggesting that the system does not reach steady-state, and therefore and alternate method of analysis will need to be developed.
Having experimented with the simulations above, both the development of appropriate population analysis framework and the investigation of higher order dimensions (2-D and 3-D) will be pursed.

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  • 2016-05

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Spectral Analysis of Four Times Ionized Iron and Nickel in the Vacuum Ultraviolet with Applications in Astrophysics

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Observations of four times ionized iron and nickel (Fe V & Ni V) in the G191-B2B white dwarf spectrum have been used to test for variations in the fine structure

Observations of four times ionized iron and nickel (Fe V & Ni V) in the G191-B2B white dwarf spectrum have been used to test for variations in the fine structure constant, α, in the presence of strong gravitational fields. The laboratory wavelengths for these ions were thought to be the cause of inconsistent conclusions regarding the
variation of α as observed through the white dwarf spectrum. This thesis presents 129 revised Fe V wavelengths (1200 Å to 1600 Å) and 161 revised Ni V wavelengths (1200 Å to 1400 Å) with uncertainties of approximately 3 mÅ. A systematic calibration error
is identified in the previous Ni V wavelengths and is corrected in this work. The evaluation of the fine structure variation is significantly improved with the results
found in this thesis.

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Date Created
  • 2016-05

Thermally enhanced photoinduced electron emission from nitrogen-doped diamond films on silicon substrates

Description

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on p-type silicon substrates. It has been shown that photon-enhanced thermionic emission

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on p-type silicon substrates. It has been shown that photon-enhanced thermionic emission (PETE) can substantially enhance thermionic emission intensity from a p-type semiconductor. An n-type diamond/p-type silicon structure was illuminated with 400–450 nm light, and the spectra of the emitted electrons showed a work function less than 2 eV and nearly an order of magnitude increase in emission intensity as the temperature was increased from ambient to ∼400 °C. Thermionic emission was negligible in this temperature range. The results are modeled in terms of contributions from PETE and direct photoelectron emission, and the large increase is consistent with a PETE component. The results indicate possible application in combined solar/thermal energy conversion devices.

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  • 2014-09-15

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System Size Dependence in Fermionic Superfluids at Unitarity

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While the use of super-cooled gasses as a tool for the study of macroscopic quantum effects has only become experimentally viable in recent years, theories involing such gasses have existed

While the use of super-cooled gasses as a tool for the study of macroscopic quantum effects has only become experimentally viable in recent years, theories involing such gasses have existed almost as long as quantum theory itself. Albert Einstein first proposed the concept of what is known today as a Bose-Einstein condensate; the driving principle behind his theory was a deliberate exploitation of the symmetric property of multiparticle bosonic wavefunctions. Specifically, since the Bose-Einstein statistics of bosons dic- tate that any arbitrary number of particles can occupy the same state, it is possible in an extremely low energy environment for particles on the order of Avagadro's number to all condense into the ground state. This state of matter is now called a Bose-Einstein condensate (hereafter referred to as a BEC). This state of matter is interesting because having such a large number of particles in the same state allows for the observation of macroscopic quantum effects.

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  • 2012-12

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Prediction of heat transport in multiple tokamak devices with neural networks

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The OMFIT (One Modeling Framework for Integrated Tasks) modeling environment and the BRAINFUSE module have been deployed on the PPPL (Princeton Plasma Physics Laboratory) computing cluster with modifications that have

The OMFIT (One Modeling Framework for Integrated Tasks) modeling environment and the BRAINFUSE module have been deployed on the PPPL (Princeton Plasma Physics Laboratory) computing cluster with modifications that have rendered the application of artificial neural networks (NNs) to the TRANSP databases for the JET (Joint European Torus), TFTR (Tokamak Fusion Test Reactor), and NSTX (National Spherical Torus Experiment) devices possible through their use. This development has facilitated the investigation of NNs for predicting heat transport profiles in JET, TFTR, and NSTX, and has promoted additional investigations to discover how else NNs may be of use to scientists at PPPL. In applying NNs to the aforementioned devices for predicting heat transport, the primary goal of this endeavor is to reproduce the success shown in Meneghini et al. in using NNs for heat transport prediction in DIII-D. Being able to reproduce the results from is important because this in turn would provide scientists at PPPL with a quick and efficient toolset for reliably predicting heat transport profiles much faster than any existing computational methods allow; the progress towards this goal is outlined in this report, and potential additional applications of the NN framework are presented.

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  • 2015-05

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Optimal Coherent Control of Spin State Transfer in Silicon

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In this paper, optimal control routines are applied to an existing problem of electron state transfer to determine if spin information can successfully be moved across a chain of donor

In this paper, optimal control routines are applied to an existing problem of electron state transfer to determine if spin information can successfully be moved across a chain of donor atoms in silicon. The additional spin degrees of freedom are introduced into the formulation of the problem as well as the control optimization algorithm. We find a timescale of transfer for spin quantum information across the chain fitting with a t > π/A and t > 2π/A transfer pulse time corresponding with rotation of states on the electron Bloch sphere where A is the electron-nuclear coupling constant. Introduction of a magnetic field weakens transfer
efficiencies at high field strengths and prohibits anti-aligned nuclear states from transferring. We also develop a rudimentary theoretical model based on simulated results and partially validate the characteristic transfer times for spin states. This model also establishes a framework for future work including the introduction of a magnetic field.

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  • 2015-05

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Optical characterization of III nitride semiconductors using cathodoluminescence techniques

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Group III-nitride semiconductors have attracted much attention for applications on high brightness light-emitting diodes (LEDs) and laser diodes (LDs) operating in the visible and ultra-violet spectral range using indium gallium

Group III-nitride semiconductors have attracted much attention for applications on high brightness light-emitting diodes (LEDs) and laser diodes (LDs) operating in the visible and ultra-violet spectral range using indium gallium nitride in the active layer. However, the device efficiency in the green to red range is limited by quantum-confined Stark effects resulting from the lattice mismatch between GaN and InGaN. In this dissertation, the optical and micro-structural properties of GaN-based light emitting structures have been analyzed and correlated by utilizing cathodoluminescence and transmission electron microscopy techniques. In the first section, optimization of the design of GaN-based lasers diode structures is presented. The thermal strain present in the GaN underlayer grown on sapphire substrates causes a strain-induced wavelength shift. The insertion of an InGaN waveguide mitigates the mismatch strain at the interface between the InGaN quantum well and the GaN quantum barrier. The second section of the thesis presents a study of the characteristics of thick non-polar m-plane InGaN films and of LED structures containing InGaN quantum wells, which minimize polarization-related electric fields. It is found that in some cases the in-plane piezoelectric fields can still occur due to the existence of misfit dislocations which break the continuity of the film. In the final section, the optical and structural properties of InGaAlN quaternary alloys are analyzed and correlated. The composition of the components of the film is accurately determined by Rutherford backscattering spectroscopy.

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Date Created
  • 2011

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Nanoscale heterogeneities in visible light absorbing photocatalysts: connecting structure to functionality through electron microscopy and spectroscopy

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Photocatalytic water splitting over suspended nanoparticles represents a potential solution for achieving CO2-neutral energy generation and storage. To design efficient photocatalysts, a fundamental understanding of the material’s structure, electronic properties,

Photocatalytic water splitting over suspended nanoparticles represents a potential solution for achieving CO2-neutral energy generation and storage. To design efficient photocatalysts, a fundamental understanding of the material’s structure, electronic properties, defects, and how these are controlled via synthesis is essential. Both bulk and nanoscale materials characterization, in addition to various performance metrics, can be combined to elucidate functionality at multiple length scales. In this work, two promising visible light harvesting systems are studied in detail: Pt-functionalized graphitic carbon nitrides (g-CNxHys) and TiO2-supported CeO2-x composites.

Electron energy-loss spectroscopy (EELS) is used to sense variations in the local concentration of amine moieties (defects believed to facilitate interfacial charge transfer) at the surface of a g-CNxHy flake. Using an aloof-beam configuration, spatial resolution is maximized while minimizing damage thus providing nanoscale vibrational fingerprints similar to infrared absorption spectra. Structural disorder in g-CNxHys is further studied using transmission electron microscopy at low electron fluence rates. In-plane structural fluctuations revealed variations in the local azimuthal orientation of the heptazine building blocks, allowing planar domain sizes to be related to the average polymer chain length. Furthermore, competing factors regulating photocatalytic performance in a series of Pt/g-CNxHys is elucidated. Increased polymer condensation in the g-CNxHy support enhances the rate of charge transfer to reactants owing to higher electronic mobility. However, active site densities are over 3x lower on the most condensed g-CNxHy which ultimately limits its H2 evolution rate (HER). Based on these findings, strategies to improve the cocatalyst configuration on intrinsically active supports are given.

In TiO2/CeO2-x photocatalysts, the effect of the support particle size on the bulk
anoscale properties and photocatalytic performance is investigated. Small anatase supports facilitate highly dispersed CeO2-x species, leading to increased visible light absorption and HERs resulting from a higher density of mixed metal oxide (MMO) interfaces with Ce3+ species. Using monochromated EELS, bandgap states associated with MMO interfaces are detected, revealing electronic transitions from 0.5 eV up to the bulk bandgap onset of anatase. Overall, the electron microscopy/spectroscopy techniques developed and applied herein sheds light onto the relevant defects and limiting processes operating within these photocatalyst systems thus suggesting rational design strategies.

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Date Created
  • 2019

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A GPU accelerated discontinuous Galerkin conservative level set method for simulating atomization

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This dissertation describes a process for interface capturing via an arbitrary-order, nearly quadrature free, discontinuous Galerkin (DG) scheme for the conservative level set method (Olsson et al., 2005, 2008). The

This dissertation describes a process for interface capturing via an arbitrary-order, nearly quadrature free, discontinuous Galerkin (DG) scheme for the conservative level set method (Olsson et al., 2005, 2008). The DG numerical method is utilized to solve both advection and reinitialization, and executed on a refined level set grid (Herrmann, 2008) for effective use of processing power. Computation is executed in parallel utilizing both CPU and GPU architectures to make the method feasible at high order. Finally, a sparse data structure is implemented to take full advantage of parallelism on the GPU, where performance relies on well-managed memory operations.

With solution variables projected into a kth order polynomial basis, a k+1 order convergence rate is found for both advection and reinitialization tests using the method of manufactured solutions. Other standard test cases, such as Zalesak's disk and deformation of columns and spheres in periodic vortices are also performed, showing several orders of magnitude improvement over traditional WENO level set methods. These tests also show the impact of reinitialization, which often increases shape and volume errors as a result of level set scalar trapping by normal vectors calculated from the local level set field.

Accelerating advection via GPU hardware is found to provide a 30x speedup factor comparing a 2.0GHz Intel Xeon E5-2620 CPU in serial vs. a Nvidia Tesla K20 GPU, with speedup factors increasing with polynomial degree until shared memory is filled. A similar algorithm is implemented for reinitialization, which relies on heavier use of shared and global memory and as a result fills them more quickly and produces smaller speedups of 18x.

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  • 2015

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Combined photo- and thermionic electron emission from low work function diamond films

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In this dissertation, combined photo-induced and thermionic electron emission from low work function diamond films is studied through low energy electron spectroscopy analysis and other associated techniques. Nitrogen-doped, hydrogen-terminated diamond

In this dissertation, combined photo-induced and thermionic electron emission from low work function diamond films is studied through low energy electron spectroscopy analysis and other associated techniques. Nitrogen-doped, hydrogen-terminated diamond films prepared by the microwave plasma chemical vapor deposition method have been the most focused material. The theme of this research is represented by four interrelated issues. (1) An in-depth study describes combined photo-induced and thermionic emission from nitrogen-doped diamond films on molybdenum substrates, which were illuminated with visible light photons, and the electron emission spectra were recorded as a function of temperature. The diamond films displayed significant emissivity with a low work function of ~ 1.5 eV. The results indicate that these diamond emitters can be applied in combined solar and thermal energy conversion. (2) The nitrogen-doped diamond was further investigated to understand the physical mechanism and material-related properties that enable the combined electron emission. Through analysis of the spectroscopy, optical absorbance and photoelectron microscopy results from sample sets prepared with different configurations, it was deduced that the photo-induced electron generation involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. (3) Based on results from the first two studies, possible photon-enhanced thermionic emission was examined from nitrogen-doped diamond films deposited on silicon substrates, which could provide the basis for a novel approach for concentrated solar energy conversion. A significant increase of emission intensity was observed at elevated temperatures, which was analyzed using computer-based modeling and a combination of different emission mechanisms. (4) In addition, the electronic structure of vanadium-oxide-terminated diamond surfaces was studied through in-situ photoemission spectroscopy. Thin layers of vanadium were deposited on oxygen-terminated diamond surfaces which led to oxide formation. After thermal annealing, a negative electron affinity was found on boron-doped diamond, while a positive electron affinity was found on nitrogen-doped diamond. A model based on the barrier at the diamond-oxide interface was employed to analyze the results. Based on results of this dissertation, applications of diamond-based energy conversion devices for combined solar- and thermal energy conversion are proposed.

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Date Created
  • 2013