One consequence of a quantum theory in which resonances are mathematically unified with decaying states is an asymmetry in time evolution, even for closed quantum mechanical systems. This time asymmetry is different from the environmentally induced, dynamical irreversibility that is experienced only by open quantum systems and that is quantified by an increase in entropy. To investigate the connection between time asymmetry and dynamical irreversibility, we study open systems within a time asymmetric theory. We find that, when using a time asymmetric theory for open systems, one must relax the assumption that measurements are perfectly repeatable. To treat this problem, we develop a framework in which one can incorporate the interference from multiple environmental systems affecting a single experiment. We also study a kinematic effect of indistinguishability that affects only open systems, and we show how it leads to a monotonic increase in entropy without requiring an active measurement. Finally, within our framework we develop two models that reproduce for open systems the expected and observed phenomena. One is a model of photons scattering inefficiently from a beam splitter. The other is a model of systems undergoing Rabi oscillations and suffering environmental interference. We find that the kinematic effect of indistinguishability can explain for such systems the generally measured Excitation Induced Dephasing, which has previously been treated dynamically. / text

<p>A detailed analysis of coupled cavity semiconductor lasers with asymmetric multiple quantum well (AMQW) active regions is presented in this thesis. The analysis involved design, fabrication, characterization, and simulation of these devices. Although the coupled cavity devices can be multi sectioned, the devices discussed in this thesis are two sectioned.</p><p> A below threshold model for an AMQW coupled cavity device is developed. Non-linear fits of the below threshold spectral data to that obtained from the model were used to extract optimized device parameters. These fits helped to create an understanding of the operation of the devices and paved the way for improved device performance. Optimized device parameters obtained from the below threshold model were later used as input parameters in the development of an above threshold model. This model verified the wavelength selection mechanism employed by coupled cavity diode lasers and predicted the longitudinal modes for sets of injection currents.</p><p> Optical coherence tomography (OCT) is an application where much interest has recently been drawn. The coupled cavity devices fabricated in this work applied with proper modulation of the injection currents and followed by subsequent time averaging have demonstrated short coherence length (-15 μm) and can be an excellent source for synthesized OCT. Rapid wavelength switching (-70 ns, the measurement was limited by detector response time) over the whole range has also been experimentally shown. Because of the high speed (relative to mechanical) wavelength switching ability, AMQW coupled cavity devices have the potential for applications requiring real time measurements including real time synthesized OCT.</p> / Thesis / Doctor of Philosophy (PhD)

This thesis is about excitation, detection, properties and applications of multiple quantum coherences applied to different dipolar coupled spin systems. Major focus of the work is on spectral simplification, measurement of residual dipolar couplings and discrimination of enantiomers in chiral aligning media. The first chapter gives a brief account on the fundamentals of nuclear magnetic resonance spectroscopy and multiple quantum coherences. This includes a description of product operator and polarization operator formalisms of pulses and evolution of magnetization. Subsequently a detailed account of two dimensional multiple quantum – single quantum (MQ-SQ) correlation experiments is given. Demonstration of the homonuclear MQ-SQ pulse sequence on a weakly coupled spin system and analysis of the spectrum obtained are also discussed. Homo-nuclear multiple quantum studies carried out to obtain relative the signs of the couplings have been reported in the initial part of the second chapter. The technique has been applied on doubly labeled acetonitrile (13CH313C15N) aligned in a liquid crystalline medium. Special situations like ambiguity in the determination of relative signs of the couplings from the appearance of two dimensional MQ-SQ spectra and the explanation for the same are also discussed. Homo-nuclear MQ experiments on indistinguishable spins, like protons in a methyl group of 13CH313C15N oriented in liquid crystal, and distinguishable spins, like the two carbons in the same molecule, have been carried out. Different directions of approach in which these results need to be analyzed have been discussed. Subsequent part of the chapter is about the correlation of connected MQ-SQ coherences. These experiments are significant in reducing the cross-peaks further from the MQ-SQ spectra. This concept is extended for the discrimination of optical enantiomers dissolved in chiral aligning medium made of poly-Γ-benzyl-L-glutamate (PBLG) and CDCl3. In molecules of Chemical and biological interest one encounters several nuclei such as, 1H, 13C, 15N and 19F. It will be of general interest to determine magnitudes and relative signs of the couplings among these coupled nuclei by NMR experiments. Utilization of hetero-nuclear MQ Experiments in solving such problems is discussed in the third Chapter. Hetero-nuclear MQ experiments were carried out on dipolar coupled 13CH313C15N, with the aim of obtaining the values and signs of various hetero-nuclear couplings in the molecule. The splitting of transitions in the spectra of oriented molecules is always influenced by the sum of dipolar and scalar couplings. Hence precise determination of dipolar couplings requires the knowledge of scalar couplings. To determine the J couplings, experiments were carried out on the same molecule in isotropic medium. When many coupled nuclei are involved one has to carry out several experiments to derive all the spectral parameters. In circumventing this problem heteronuclear multiple quantum experiments involving more than two nuclei as active spins are advantageous. This reduces the number of experiments and thereby reducing the total experimental time. Second part of this chapter demonstrates how a triple resonance triple quantum experiment can provide majority of the couplings from a given coupled system. The feasibility of the experiment is demonstrated even for molecules containing natural abundant isotopes. Application of multiple quantum j-resolved technique for chiral discrimination and obtaining complete one dimensional spectrum of each enantiomer from their racemic mixture is discussed in the fourth chapter. The two dimensional experiment consists of a selective double quantum excitation period followed by selective refocusing during indirect time domain, isotropic mixing and nonselective detection of SQ transitions. Hence this pulse sequence is named as DQSERF-COSY (Double Quantum Selective Refocused Correlation Spectroscopy). The experiment exploits the existence of different intra-methyl couplings between the enantiomers dissolved in chiral liquid crystal medium to separate the one dimensional spectra of each enantiomer in different cross sections. This is possible due to the fact that all the nuclei in any one of the enantiomers are coupled among themselves and there is no inter molecular interaction between the two enantiomers. Also one can extract all the couplings between protons in each enantiomer, which can subsequently be utilized for determination of the residual dipolar couplings, structure and orientation parameters.

<p>This dissertation focuses on a very special topic in the field of Nuclear Magnetic Resonance (NMR) in solution: Intermolecular Multiple Quantum Coherences, or iMQCs, which can only be created by intermolecular dipolar couplings. Since the very beginnings of NMR, it has been known that dipolar couplings dominate the solid-state linewidth for spin-1/2 nuclei, but the effects are still not fully understood. The angular dependency (1-3cos2&#952;ij) and distant dependency (rij-3) of dipolar coupling led to an oversimplified conclusion that it can be ignored in an isotropic liquid. Thus, it was surprising when COSY Revamped by Asymmetric Z-gradient Echo Detection (CRAZED) was first introduced in the early `90s and showed strong iMQC signals. Since then, CRAZED has inspired a wide range of applications for iMQCs and led to two different but equivalent mathematical frameworks to describes these effects, which we call the conventional DDF theory.</p><p>However, several disagreements between the conventional DDF theory and experiments have grasped our attention recently. This dissertation will: first, demonstrate how conventional picture fails by two examples, Multi-axis CRAZED (MAXCRAZED) and Gradient-embedded COSY Experiment (GRACE); second, provide a corrected DDF theory; and, third, discuss what impact this correction will bring.</p><p>Intermolecular double quantum coherences (iDQCs) are very sensitive to the local anisotropy (10&#956;m - 1mm) and can be used to create positive contrast highlighting superparamagnetic iron oxide nanoparticles (SPIONs). This dissertation will show the design and optimization of iDQC anisotropy by a series of phantom experiments. A set of numerical simulations will then be provided for a sub-voxel level explanation. We will also demonstrate how the newly corrected DDF theory can be quickly adapted to improve the iDQC anisotropy.</p><p>Finally, as a side product of this research, the mechanism of diacetyl hydration/dehydration as solved by NMR will be provided.</p> / Dissertation

In this work, we have reported the design, MBE-growth and fabrication of strain-balanced n-type modulation-doped (MD) InGaAlAs/InGaAs multiple quantum wells laser/SOAs on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Two kinds of samples having similar structure but different fundamental transition wavelength of 1.55 £gm and 1.48 £gm are separately prepared for investigating their characteristics in optical amplification under forward bias and electro-absorption under reversed bias. Also, the technique of growing high-quality InGaAlAs with solid-source molecular beam epitaxy has been established and the resulting InGaAlAs bulk and QWs samples are extensively characterized by double-crystal X-ray diffraction, transmission electron microscopy, electroluminescence, and photoluminescence measurements. For £f = 1.55 £gm samples, ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated by a new developed multi-step wet-etching process. When injection current density > 20A/cm^2, electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at £f = 1460 nm than the e1-hh1 transition at £f = 1550 nm. The FP laser shows a lasing peak of £f = 1514 nm at threshold. Additional lasing wavelength at £f =1528 nm and 1545 nm were observed sequentially as the injection current increased. However, for the TEF laser, only the emission at £f = 1511 nm was observed. These TE-polarized lasing wavelengths are consistent with the £_-like absorption peaks in photocurrent spectra. The lasing performance is possible attributed to optical transitions within quantum dots/wires which are formed by the strain-field profile and alloy segregation/migration. For £f = 1.48 £gm samples, the differential absorption spectroscopy, which measures the change of transmission (£GT/T) in the presence of electric field, is used to study the electro-absorption modulation behavior of MD-SOA¡¦s. A sample with n-type modulation-doping amounting to a sheet density of 3.5 ¡Ñ 10^11 cm^-2 per QW and combining with a hole-stopping barrier represents the largest chirp parameter (£Gn/£Gk) under reversed bias, which offers an excellent platform to realize electro-refractive devices with larger refractive index changes (£Gn) but lower differential absorption (£G£\) near £f = 1.55 £gm, which is also our interested region of operation. In addition, we have succeeded in reducing the length of conventional constant-width multimode interference (MMI) coupler of K = 0.15 and 0.28 more than 32% by a novel stepped-width design concept. By extending the stepped-with idea, we show that it is possible to obtain 2x2 waveguide couplers with new power splitting ratios of 7%, 64%, 80% and 93% for cross coupling by cascading two short MMI sections. We further realize freely chosen power splitting ratio by interconnecting a pair of unequal-width waveguides as the phase-tuning section into the middle of two short MMI sections. These compact and low loss MMI-based devices use only rectangular geometry without any bent, curved, and tapered waveguides. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.

This PhD thesis presents a study of structure-composition correlations of bioactive glasses (BGs) by employing solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. Silicate-based Na2O−CaO−SiO2−P2O5 BGs are utilized clinically and are extensively investigated for bone regeneration purposes. Once implanted in the human body, they facilitate bone regeneration by partially dissolving in the body fluids, followed by the formation of a biomimetic surface-layer of calcium hydroxy-carbonate apatite (HCA). Eventually, the implanted BG totally integrates with the bone. The bioactivity of melt-prepared BGs depends on their composition and structure, primarily on the phosphorus content and the average silicate-network connectivity (NC). We explored these composition-structure relationships for a set of BGs for which the NC and phosphorus contents were varied independently. The short-range structural features of the glasses were explored using 29Si and 31P magic-angle-spinning (MAS) NMR spectroscopy. 31P MAS NMR revealed that the orthophosphate content is directly proportional to the total P content of the glass, with a linear correlation observed between the orthophosphate content and the silicate network connectivity. The bearings of the results for future BG design are discussed. By using multiple-quantum coherence-based 31P NMR experiments, the spatial distribution of orthophosphate groups was probed in the melt prepared BGs, as well as in two mesoporous bioactive glasses prepared by an evaporation-induced self-assembly technique. The results evidence randomly distributed orthophosphate groups in the melt-prepared BGs, whereas the pore-walls of the mesoporous bioactive glasses constitute nanometer-sized clusters of calcium phosphate. The distribution of Na+ ions among the phosphate/silicate groups were studied by heteronuclear dipolar-based 23Na−31P NMR experiments, verifying that sodium is dispersed nearly randomly in the glasses. The phosphorus and proton environments in biomimetically grown HCA were investigated by using 1H and 31P MAS NMR experiments. Our studies revealed that the biomimetic HCA shared many local structural features with synthetic and well-ordered hydroxy-apatite. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Accepted.</p>

The main objective of this research is to fabricate and characterize an optically integrated interferometric sensor on standard silicon and silicon CMOS circuitry. An optical sensor system of this nature would provide the high sensitivity and immunity to electromagnetic interference found in interferometric based sensors in a lightweight, compact package capable of being deployed in a multitude of situations inappropriate for standard sensor configurations. There are several challenges involved in implementing this system. These include the development of a suitable optical emitter for the sensor system, the interface between the various optically embedded components, and the compatibility of the Si CMOS with heterogeneous integration techniques. The research reported outlines a process for integrating an integrated sensor on Si CMOS circuitry using CMOS compatible materials, integration techniques, and emitter components.

Since Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki won the 2014 Nobel prize in Physics owing to theircontributions on the invention of efficient blue GaN light emitting diodes, GaN became an even more appealingmaterial system in the research field of optoelectronics. On the other hand, quantum structures or low-dimensionalstructures with properties derived from quantum physics demonstrate superior and unique electrical and opticalproperties, providing a significant potential on novel optoelectronic applications based on the employment of quantumconfinement.   In 2012, our research team at Linköping University utilized pyramid templates, which is an established approach toform quantum structures, to successfully grow GaN pyramids with InGaN hybrid quantum structures, includingquantum wells, quantum wires, and quantum dots. This growth enabled site-controlled pyramids based on selectivearea growth (SAG). After numerous studies on the photoluminescence properties, the mature and controlled growthtechnique was proposed to be adapted for fabrication of AlGaN pyramids on which GaN hybrid quantum structurescan be hosted.   This thesis is dedicated to the subsequent problems of the growth of AlGaN pyramids. It was found that there wasan undesired deposition of a considerable thickness on top the desired AlGaN pyramid with GaN multiple quantumwells. In this thesis, two different directions are explored to find the key solution with a potential of furtheroptimization. On one hand, the growth parameters such as precursors cut-off, carrier gas during cooling, temperatureholding, cooling pressure, III/V ratio, and the possible effect of GaN surfaces are investigated. However, due to theactual inherent properties of the metal-organic chemical vapor deposition reactor used, no promising parameter tuningcan been identified. On the other hand, from post-growth point of view, a KOH aqueous etching solution exhibits apositive result toward removing the undesired deposition. This etching process is suggested to be further optimized toachieve the final goal of eliminating the undesired deposition.

In this thesis experimental samples of multiple quantum wells in the InGaN/GaN structures will be compared using methods of laser spectroscopy. In particular, the optical properties of the samples will be investigated. The samples were prepared under different conditions; therefore one of the aims is to compare them. The knowledge of the influence of preparation enables utilization not only for fundamental research, but also for the construction of radiation sources or scintillation detectors. Measurements of absorption and photoluminescence will be carried out and their dynamic properties will be measured as well. There will be examined the effect of different excitation power and different excitation wavelength on the intensity of photoluminescence. From dynamic properties there will be examined the effect of different excitation wavelength on the lifetime of the absorption and how does temperature influence the lifetime of the photoluminescence. Individual quantities will be compared amongst samples and their suitability for further applications will be discussed.

<p>  </p> <p>Research that seeks to understand and develop the growth of III-nitride materials by molecular beam epitaxy (MBE) is beneficial to a broad range of the device community. MBE and the III-nitrides have been used to develop transistors, diodes, electroacoustic devices, solar cells, LEDs, LDs, intersubband devices, and quantum-cascade lasers. In this work we focus on the growth of III-nitride materials specifically for applications in near-infrared intersubband (NIR ISB) optical devices, however all this work is broadly applicable. </p> <p><br></p> <p>We begin by investigating the reduced indium incorporation in non-polar m-plane InGaN films. We find that InGaN grown on m-plane GaN has an effective activation energy for thermal decomposition of 1 eV, nearly half that reported for similar c-plane films. We produce high quality m-plane In0.16Ga0.84N and utilize it in AlGaN/InGaN devices designed for near-infrared ISB absorption measurements. We continue this work by exploring the growth of low-temperature AlGaN, necessary for these devices. We find that the utilization of an indium surfactant during low-temperature AlGaN growth enhances adatom diffusion, resulting in smoother surface morphologies, sharper interfaces, and reduced defects within the material. This growth method also prevents the anomalous suppression of the AlGaN growth rate, which we link to a reduction in the formation of high-aluminum containing defects. These investigations result in the demonstration of an Al0.24Ga0.76N/In0.16Ga0.84N heterostructure with a conduction band offset large enough to enable NIR ISB transitions.</p> <p><br></p> <p>Lastly, we explore the novel material ScAlN. This material’s large bandgap, large spontaneous polarization, ferroelectricity, and ability to be lattice matched to GaN at ~18% scandium composition make it an ideal candidate for a variety of devices, including NIR ISB devices. We investigate the reported temperature dependence of ScAlN’s <em>c</em>-lattice constant and confirm this dependence is present for high growth-temperature ScxAl1-xN with 0.11 < x < 0.23. We find that this temperature dependence is no longer present below a certain composition-dependent growth temperature. This finding, coupled with observations that samples grown at lower temperatures exhibit lower defect densities, smoother surfaces, and homogeneous chemical compositions suggest that high growth temperatures lead to defect generation that may cause the observed change in lattice parameters. We demonstrate lattice-matched, 50 repeat Sc0.18Al1-xN/GaN heterostructures with ISB absorption in excess of 500 meV with FWHM as little as 45 meV. </p>