The phenomena that occur when an intense laser beam interacts with matter have been of interest for some time, mainly due to the possibility of constructing a laser-driven fusion reactor. In particular a number of models describing the effect of a laser striking a plane solid target, and the behaviour of the resulting plasma, have been proposed. In 1965-66 the self-regulating model was developed independently by Caruso et al and Krokhin et al. The validity of this model has been confirmed by experiment and shown to be applicable in the case of relatively low-powered lasers. To accommodate higher intensity lasers Fauquignon and Floux,and Bobin developed the deflagration-wave model in 1970-71. The fact that these models are really two regimes of the same basic model has been demonstrated by Puell. In 1973 Pert showed that at higher intensities the effects of thermal conduction modify these models to such an extent that it is necessary to introduce two new models, these being termed thick self-regulating and thick deflagration-wave. An alternative method of attacking the problem is to solve the hydrodynamic equations governing the plasma motion by means of a computer code. It was found that the Lagrangian form of the equations of motion was the most suitable for this treatment, and a number of one-dimensional codes of this type have been written. Notable among these are, in chronological order: Fader (l), Kidder (2), Shearer and Barnes (3), Mulser (4), Nuckolls et al (LASNEX) (5), Goldman (6) and Clarke et al (7). The production of a two-dimensional code was delayed by the fact that Lagrangian, coordinates become extremely difficult to work with in more than one dimension, and simple Eulerian techniques cannot handle the strong shocks produced in the problem. However with the advent of more sophisticated numerical methods the production of a working two-dimensional Eulerian code became feasible. There is now a code of this type due to Winsor at the Naval Research Laboratory Washington, and in this country there is the CASTOR code due to Christiansen, LASERB of Craxton, and 2DEL of Pert. There is also now a 2-D Lagrangian version of LASNEX at Livermore. All of these codes include, among other sophistications, the effects due to magnetic fields generated within the plasma. The object of the first part of this thesis is to compare the results of a specially written two-dimensional code with steady-state versions of the theoretical models, and in particular to examine the 'thick' models of Pert and to determine the extent to which these models are lost due to flux limitation effects. To ensure that a steady-state is attained, relatively long runs of the computer code are required; therefore the code was reduced to the simplest form that could still model the physical situation adequately. Hence sophistications such as magnetic field effects were neglected. These omissions are justified since it is basically hydrodynamic effects that are of interest. In Chapter 2 the plasma model is described, the equations of motion are presented, and a description is given of the computer code. Chapter 3 examines the various analytic steady-state models and Chapter 4 presents the results of the computer code and compares them with the models of the previous chapter. The second part of this thesis deals with the development of a ray-tracing code to examine the effects of refraction in a laser-plasma interaction. The code is described in Chapter 5 and the results obtained presented in Chapter 6.

This work examines the application of selected estimation methods to path integrated direct detection CO2 lidar data, with the objective of improving the precision in the estimates of the log power, and log power ratios. Particular emphasis is given to the optimal estimation techniques of Kalman filtering theory, and to the consequent requirements for system and measurement model identification. A dual wavelength system was designed and constructed, employing two hybridised TEA lasers, a co-axial transceiver, and direct detection. Over a period of several months, a database of differential absorption measurements was accumulated, each consisting of 10,000 dual wavelength lidar returns. Various wavelength pairs were used, including those recommended for the monitoring of H2O, CO2, NH3 and C2H4. A subset of this database is used to evaluate the above mentioned estimation methods. The results are compared with simulated data files in which it was possible to control precisely process models which are believed to form an approximation to the real processes latent in the actual lidar data.

abstract: Phononic crystals are artificially engineered materials that can forbid phonon propagation in a specific frequency range that is referred to as a “phononic band gap.” Phononic crystals that have band gaps in the GHz to THz range can potentially enable sophisticated control over thermal transport with “phononic devices”. Calculations of the phononic band diagram are the standard method of determining if a given phononic crystal structure has a band gap. However, calculating the phononic band diagram is a computationally expensive and time-consuming process that can require sophisticated modeling and coding. In addition to this computational burden, the inverse process of designing a phononic crystal with a specific band gap center frequency and width is a challenging problem that requires extensive trial-and-error work. In this dissertation, I first present colloidal nanocrystal superlattices as a new class of three-dimensional phononic crystals with periodicity in the sub-20 nm size regime using the plane wave expansion method. These calculations show that colloidal nanocrystal superlattices possess phononic band gaps with center frequencies in the 102 GHz range and widths in the 101 GHz range. Varying the colloidal nanocrystal size and composition provides additional opportunities to fine-tune the phononic band gap. This suggests that colloidal nanocrystal superlattices are a promising platform for the creation of high frequency phononic crystals. For the next topic, I explore opportunities to use supervised machine learning for expedited discovery of phononic band gap presence, center frequency and width for over 14,000 two-dimensional phononic crystal structures. The best trained model predicts band gap formation, center frequencies and band gap widths, with 94% accuracy and coefficients of determination (R2) values of 0.66 and 0.83, respectively. Lastly, I expand the above machine learning approach to use machine learning to design a phononic crystal for a given set of phononic band gap properties. The best model could predict elastic modulus of host and inclusion, density of host and inclusion, and diameter-to-lattice constant ratio for target center and width frequencies with coefficients of determinations of 0.94, 0.98, 0.94, 0.71, and 0.94 respectively. The high values coefficients of determination represents great opportunity for phononic crystal design. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2020

The recent emergence of a new class of two dimensional layered materials (2DLMs) have not only opened up the potential for exciting new technological opportunities but also established a new platform to explore exciting new fundamental physics and chemistry at the limit of atomic thickness. Among several of these newly rediscovered 2DLMs, transition metal dichalcogenides (TMDCs) as well as other elemental combinations of Group III and Group VI represent a large family of 2D layered materials, which can be isolated into few atomic layers. These materials show remarkable promise for future electronic and opto-electronics applications. The scope of this dissertation, thus, broadly covers the electronic and opto-electronic properties of such few layered 2D materials. Extensive investigation of electronic and opto-electronic transport phenomena of charge carriers in few layer MoS2 synthesized using a variety of methods such as Chemical Vapor Deposition (CVD), liquid phase exfoliation and mechanical exfoliation as well as CVT grown mechanically exfoliated WSe2 and ternary alloy of CuIn7Se11 is reported. Specifically, it is shown that in case of MoS2, the ac conductance (σ(ω); measured in the range of 10mHz < ω < 0.1 MHz) of atomically thin 2D layers of chemical vapor deposited (CVD) Molybdenum Disulphide (MoS2) as well as thin films of exfoliated flakes of MoS2, show "universal" power law behavior (with σ(ω) ~ ωs). The temperature dependence of 's' indicate that the mechanism of ac transport in CVD MoS2 is due to electron hopping by quantum mechanical tunneling (QMT) process whereas the ac transport in exfoliated MoS2 films is due to correlated barrier hoping (CBH) mechanism. The ac conductivity also show scaling behavior, manifested by collapse of the ac conductivity data for both the samples at various temperatures to one single master curve. The T-γ dependence of the d.c conductance suggests that in case of the CVD – grown and mechanically exfoliated MoS2, γ=1/3 which corresponds to the Mott’s variable range hopping (VRH) transport where as in case of liquid phase exfoliated MoS2, γ=1 which relates to thermally activated Arrhenius type transport mechanism. Opto-electronic measurements were also performed in a variety of 2DLM samples. From the field effect transport measurements on the mechanically exfoliated samples of few layers of MoS2, WSe2 and CuIn7Se11, we found at room temperature the charge carrier mobility is ~ 47 cm2/V.s, 80 cm2/V.s and 37 cm2/V.s for MoS2, WSe2 and CuIn7Se11 respectively. The photoconductivity measurements performed on these samples show that it is possible to achieve photo-responsivities values~50 μA/W, 0.2 A/W, 1 A/W and 51 A/W at room temperature for liquid exfoliated MoS2, mechanically exfoliated MoS2, WSe2 and CuIn7Se11 based devices respectively. Mechanisms of photoconduction in these materials were explained on the basis of intensity dependent photo-current measurements. From the intensity dependent photo-current along with the low temperature photoconduction measurements we found that in case of liquid phase exfoliated MoS2 thin film devices the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. On the other hand for CVT grown mechanically exfoliated WSe2 based devices bimolecular recombination mechanism is the most dominant process for photoconduction. The result obtained are discussed and compared with the available literature on the subject.

The impact of liquid drops on solid surfaces is a ubiquitous phenomenon in our everyday experience; nevertheless, a general understanding of the dynamics governing droplet impact remains elusive. The impact event is understood within a commonly accepted hydrodynamic picture: impact initiates with a rapid shock and a subsequent ejection of a sheet leading to beautiful splashing patterns. However, this picture ignores the essential role of the air that is trapped between the impacting drop and the surface. We describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. We directly visualize the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. We show that the approach of the spreading liquid toward the surface is unstable, and lift-off of the spreading front away from the surface occurs. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. We show that when a drop impacts an atomically smooth mica surface, a strikingly stable nanometer thin layer of air remains trapped between the liquid and the solid. This layer occludes the formation of contact, and ultimately causes the complete rebound of the drop. / Engineering and Applied Sciences

<p> We provide estimates of both cross section and rate for the stimulated attachment of a second positron into the (1<i>s</i>² ¹<i>S^e</i>) state of the <i>H&macr; </i>⁺ ion using Ohmura and Ohmura&rsquo;s (1960 Phys. Rev. 118 154) effective range theory, Reiss&rsquo;s strong field approximation (1980 Phys. Rev. A 22, 1786), and the principle of detailed balancing. Our motivation for producing <i>H&macr;</i>⁺ ion include its potential to be used as an intermediate state in bringing antihydrogen to ultra-cold (sub-mK) temperatures required for a variety of studies, which include both spectroscopy and the probing of the gravitational interaction of the anti-atom. We show that both cross section and rate are increased with the use of a resonant laser field.</p><p>

<p> The Langmuir-Blodgett deposition process is investigated for creating polystyrene nanosphere monolayers on a hydrophilic silicon substrate. The monolayers are fabricated over areas ~1 cm² and sputter coated with 100&Aring; of permalloy. The quality of the monolayers is analyzed with optical microscope image processing, and 2D Fourier transforms of electron microscope images. The magnetic switching behavior of the sputtered samples is measured using an alternating gradient magnetometer, and compared to completely flat permalloy. The magnetic hysteresis measurements are done at different angle between the easy and hard axis of the flat permalloy films. The measurements show different hysteresis shapes for nanosphere patterned permalloy and flat permalloy, with the difference becoming greater nearer the hard axis of the flat permalloy samples. The ambiguity of an easy or hard axis on a curved surface is likely to contribute to the difference in magnetic switching behavior between the two sample types.</p><p>

The fundamental physics of spin-exchange optical pumping (SEOP) has been explained in detail by many brilliant scientists since its discovery in the 50’s and 60’s. Although some interactions remain only tenuously understood, mathematical relationships have been mapped across many trajectories with meticulous care. Despite these foundational descriptions, many of the larger scale dynamics remain capricious in practice, especially as SEOP strives to take advantage of rapidly developing laser technologies. This presents a difficulty for implementing the large-scale production of hyperpolarized gases that is required for clinical and some specific experimental applications. This research, performed over the past four years, was designed to shed light on some of the practical effects that become critical for scaling up production while keeping polarizations high, particularly in a “stopped-flow” polarizer environment. This dissertation is divided into eight main chapters. The first chapter is written to provide a historical context for the SEOP field and summarize the evolutionary stages that have led to current methodologies. The second chapter provides a brief summary of SEOP theory and mathematically outlines the transfer of quantum order from photon polarization, to electron polarization via optical pumping, and finally to long-lived nuclear polarizations via spin-exchange. Chapter 3 discusses the practical implementation of SEOP, and the specific designs and techniques used throughout this project to create and monitor polarization. Chapter 4 presents data with some unexpected trends collected by Dr. Nicholas Whiting and Dr. Peter Nikolaou using high densities of xenon and high resonant laser powers. This data inspired a set of simulations designed to locate the cause of these trends, and map the expected trajectory for further studies. Chapter 5 features a clinical-scale polarizer with 170 W of highly resonant cw laser power, capable of producing >0.8 L of hyperpolarized gas per SEOP cycle with 129Xe polarization values of ~30-90% (depending on the xenon density). Multidimensional data maps were created over various temperatures, gas mixes, and laser powers; the results are used to guide optimal performance and describe the conditions that cause SEOP to fail. Chapter 6 reintroduces helium into stopped-flow gas mixes to help mitigate the central difficulties found in Chapter 5 with thermal regulation, and discusses the improvements and difficulties observed as a result. Chapter 7 contrasts the tactics for high 129Xe polarization with the strategies that lead to high 131Xe polarization. Specifically this study is designed to assess whether 131Xe is capable of becoming polarized via SEOP to sufficient levels—and in sufficient amounts—to be used for some specific fundamental physics studies and other biomedical applications. Finally Chapter 8 presents a short proof of concept for the use of an aluminum optical pumping cell instead of the glass optical pumping cells predominantly used for SEOP.

<p> Thin films of chlorine (Cl) and copper (Cu) doped zinc selenide (Cl:ZnSe and Cu:ZnSe) were fabricated by pulsed laser deposition (PLD) with the goal of enabling a multilayered semiconductor structure for a mid-infrared (mid-IR) electrically excited laser. Doping of ZnSe is achieved by varying the mass ratio of zinc chloride (ZnCl₂) or copper selenide (Cu₂Se) to ZnSe precursors in starting pressed powder targets. Appropriate adjustment of the fraction of dopant precursor in the mixtures allows for the control of the dopant concentration, N<i>_D</i>&ndash;N<i>_A</i> for N<i>_D</i> >> N<i>_A</i> (or N<i>_A</i>-N<i>_D</i> for N<i>_A</i> >> N<i>_D</i>) in the thin films, where N<i>_D</i> is the donor concentration and N<i>_A</i> is the acceptor concentration. PLD is used to ablate the Cl:ZnSe or Cu:ZnSe targets, to produce thin films on gallium arsenide (GaAs) substrates. Impedance spectroscopy allows current-voltage and capacitance-voltage (C-V) characterization. Specifically Mott-Schottky measurements determine N<i>_D</i>-N<i>_A</i> (or N<i>_A</i>-N<i>_D</i>) of the fabricated thin film samples with comparisons to the nominal dopant concentration of the targets. The Mott-Schottky, 1/C² vs. V, measurements for determining N<i>_D</i>-N<i>_A</i> were calibrated against well-characterized silicon wafers with known values of N<i>_D</i>. The goal of this project was to demonstrate a reliable method for controlling the dopant concentration in PLD-deposited Cl:ZnSe and Cu:ZnSe thin films. The results obtained allows for the fabrication of Cl:ZnSe and Cu:ZnSe thin films with known N<i>_D</i>-N<i>_A</i> for use in a mid-IR electrically-excited laser devices under development in our research group.</p><p>

Gallium nitride (GaN) based high-electron-mobility transistors (HEMTs) are used in a wide variety of areas, such as 5G, automotive, aeronautics/astronautics and sensing elds ranging from chemical, mechanical, biological to optical applications. Owing superior material properties, the GaN based HEMTs are especially useful in harsh operation environments e.g. in the combustion engine, exhaust, space, and medical instruments where the reliability and resilience are highly demanded. In this thesis the e ect of proton irradiation on the GaN HEMTs as well as the possible incorporation of them in biomedicine and diagnostics are investigated. The thesis includes mainly two parts: one is on theoretic background of GaN HEMTs, and another presents the experiment/simulation details of the devices before and after proton radiation. In the background section, the HEMTs function, manufacture technique and defect formation mechanism in the device under di erent proton radiation conditions are introduced. Then, the characterizations of the HEMT devices and related test structures before and after the proton radiation with dose range from 1011 to 1015 protons=cm2 are emphasized, as well as the comparison with simulation results obtained using SRIM/TRIM program. In addition, the biocompatibility of GaN devices and their biomedicine applications in proton radiation scenarios are also described and discussed in this thesis. / Gallium Nitrid (GaN) baserade high electron mobility transistors (HEMTs) används inom många olika områden, såsom 5G, bil-industrin, yg/rymd och i sensorer fö kemiska, mekaniska, biologiska och optiska applikationer. Tack vare dess goda materialegenskaper GaN baserade HEMTs särskilt användbara i harda miljöer, som till exempel i förbränningsmotorer, avgaser, i rymden, samt till medicinska instrument där pålitlighet och tålighet är eftersträvat. I det här examensarbetet sa undersöks e ekten av protonbestrålning pa GaN HEMTs samt möjligheten till användning av dem inom biomedicin och diagnostik. Arbetet är uppdelat i två delar: den ena behandlar den teoretiska bakgrunden av GaN HEMTs och den andra presenterar de experiment/simuleringar som utförts för att se efekterna på komponenterna före och efter protonbestrålning. I bakgrunds-sektionen så beskrivs hur HEMTs fungerar, tillverkningstekniker och mekanismerna för hur defekter uppkommer under olika former av protonbestrålning. Därefter sa karaktäriseras HEMT komponenterna och relaterade teststrukturer före och efter protonbestralning, med ett fokus på doser mellan 1011 to 1015 protoner=cm2, samt en jämförelse med resultat som fatts fran simuleringar med SRIM/TRIM-program. Utöver detta sa beskrivs och diskuteras även biokompatibiliteten och applikationer inom biomedicin av GaN komponenter vid protonbestralnings-scenarion i arbetet.