Dissertations / Theses on the topic 'Cascade'

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.<br>Includes bibliographical references.<br>This thesis addresses issues of the design and modeling of the Bipolar Cascade Laser (BCL), a new type of quantum well laser. BCLs consist of multiple single stage lasers electrically coupled via tunnel junctions. The BCL ideally operates by having each injected electron participate in a recombination event in the topmost active region, then tunnel from the valence band of the first active region into the conduction band of the next active region, participate in another recombination event, and so on through each stage of the cascade. As each electron may produce more than one photon the quantum efficiency of the device can, in theory, exceed 100%. This work resulted in the first room temperature, continuous-wave operation of a BCL, with a record 99.3% differential slope efficiency. The device was fully characterized and modeled to include light output and voltage versus current bias, modulation response and thermal properties. A new singlemode bipolar cascade laser, the bipolar cascade antiresonant reflecting optical waveguide laser, was proposed and modeled.<br>by Steven G. Patterson.<br>Ph.D.

A few things that influenced this collection of poems: shifting memory, mirroring, opposites, river rapids, patterns that repeat and spread both in the natural world and in writing... rhizomatic root systems, veins, an aerial view of rivers...

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.<br>Includes bibliographical references (p. 297-310).<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>The development of the terahertz frequency range has long been impeded by the relative dearth of compact, coherent radiation sources of reasonable power. This thesis details the development of quantum cascade lasers (QCLs) that operate in the terahertz with photon energies below the semiconductor Reststrahlen band. Photons are emitted via electronic intersubband transitions that take place entirely within the conduction band, where the wavelength is chosen by engineering the well and barrier widths in multiple-quantum-well heterostructures. Fabrication of such long wavelength lasers has traditionally been challenging, since it is difficult to obtain a population inversion between such closely spaced energy levels, and because traditional dielectric waveguides become extremely lossy due to free carrier absorption. This thesis reports the development of terahertz QCLs in which the lower radiative state is depopulated via resonant longitudinal-optical phonon scattering. This mechanism is efficient and temperature insensitive, and provides protection from thermal backfilling due to the large energy separation between the lower radiative state and the injector. Both properties are important in allowing higher temperature operation at longer wavelengths. Lasers using a surface plasmon based waveguide grown on a semi-insulating (SI) GaAs substrate were demonstrated at 3.4 THz in pulsed mode up to 87 K, with peak collected powers of 14 mW at 5 K, and 4 mW at 77 K.<br>Additionally, the first terahertz QCLs have been demonstrated that use metalmetal waveguides, where the mode is confined between metal layers placed immediately above and below the active region. These devices have confinement factors close to unity, and are expected to be advantageous over SI-surface-plasmon waveguides, especially at long wavelengths. Such a waveguide was used to obtain lasing at 3.8 THz in pulsed mode up to a record high temperature of 137 K, whereas similar devices fabricated in SI-surface-plasmon waveguides had lower maximum lasing temperatures due to the higher losses and lower confinement factors. This thesis describes the theory, design, fabrication, and testing of terahertz quantum cascade laser devices. A summary of theory relevant to design is presented, including intersubband radiative transitions and gain, intersubband scattering, and coherent resonant tunneling transport using a tight-binding density matrix model. Analysis of the effects of the complex heterostructure phonon spectra on terahertz QCL design are considered. Calculations of the properties of various terahertz waveguides are presented and compared with experimental results. Various fabrication methods have been developed, including a robust metallic wafer bonding technique used to fabricate metal-metal waveguides. A wide variety of quantum cascade structures, both lasing and non-lasing, have been experimentally characterized, which yield valuable information about the transport and optical properties of terahertz devices. Finally, prospects for higher temperature operation of terahertz QCLs are considered.<br>by Benjamin S. Williams.<br>Ph.D.

Twenty-two-year-old Macy Oman narrates the book in retrospect from Cascade, Oregon, where she is visiting her mother. Macy's father moved with her to Portland shortly after the accidental death of her brother, Nick, seven years before the narration begins. Macy's mother stayed behind in Cascade. Thematically the work centers on the emotional repercussions of these losses. Macy's, and her older lover Jason's, involvement with Nick's death is unknown to everyone. Her guilt and her mother's perceived betrayal are disabling. Taking her longing for closeness to nature and to her reclusive friend Celia, Macy discovers folklore that inspires a vision quest to seek her own personal healer, a shaman inside. When Macy accepts and reveals her part in Nick's death, it opens the way to further revelations about the real root of her parents' separation, the divisive nature of assumptions, and the healing power of acceptance. This story attempts to loosely rewrite, subvert or reclaim the early life of the mythological Medea, who betrayed her father and her brother by aiding her lover, Jason the Argonaught, in his quest to obtain the golden fleece that hung on an oak tree, guarded by a dragon. In this story, the golden fleece is represented by an Apollo scarf that Macy's father Richard, an eccentric art-history buff, bought for her mother. Mythologically, the fleece was a powerful artifact, heavily guarded, worthy of war. In this rendition, the value of the scarf, rather than being material, is emotional. In comparison with the golden fleece, the Apollo scarf, an expensive item that is only appreciated for the relationships it represents, is meant to signal the superiority of the emotional over the material. Ultimately Macy does not betray her family; it is Mari who stays behind and Macy who instigates a reconciliation when she reveals the truth about her somewhat inadvertent participation in Nick's death. (352 pages)

Modeling of nonlinear systems based on special Hammerstein forms has been considered. In Hammerstein system modeling a static nonlinearity is connected to a dynamic linearity in cascade form. Fundamental contributions of this work are: 1) Introduction of Bezier curve nonlinearity representations<br>2) Introduction of B-Spline curve nonlinearity representations instead of polynomials in cascade modeling. As a result, local control in nonlinear system modeling is achieved. Thus, unexpected variations of the output can be modeled more closely. As an important demonstration case, a model is developed and named as Middle East Technical University Neural Networks and Cascade Model (METU-NN-C). Application examples are chosen by considering the Near-Earth space processes, which are important for navigation, telecommunication and many other technical applications. It is demonstrated that the models developed based on the contributions of this work are especially more accurate under disturbed conditions, which are quantified by considering Space Weather parameters. Examples include forecasting of Total Electron Content (TEC), and mapping<br>estimation of joint angle of simple forced pendulum<br>estimation of joint angles of spring loaded inverted double pendulum with forced table<br>identification of Van der Pol oscillator<br>and identification of speakers. The operation performance results of the International Reference Ionosphere (IRI-2001), METU Neural Networks (METU-NN) and METU-NN-C models are compared qualitatively and quantitatively. As a numerical example, in forecasting the TEC by using the METU-NN-C having Bezier curves in nonlinearity representation, the average absolute error is 1.11 TECu. The new cascade models are shown to be promising for system designers and operators.

This thesis is concerned with the design, fabrication and characterisation of monolithic tuneable quantum cascade lasers (QCLs), which are suitable for tuneable diode laser based absorption spectroscopy and polarisation dependent spectroscopy in the mid-infrared wavelength range. All investigations and device development work were carried out using the QCL structure based on strain-compensated Ga0.331In0.669As/Al0.659In0.341As grown on an InP substrate that emits light around 4500 nm wavelength. To make the QCLs electrically tuned, two laser designs were investigated: the double ring quantum cascade laser based on the Vernier-tuning effect, and the integrated tuneable birefringent waveguide utilising current controlled birefringence in quantum-wells. The key advantage of the Vernier tuning effect based the double ring laser design is that it can facilitate both a single mode and wide-tuning range operation. The Vernier tuning enhancement factor associated with the coupled waveguide is responsible for a wide-tuning range observed in double ring configuration. However, the tuning range is limited by the available gain bandwidth (i.e. FWHM of spontaneous spectra) in the material and the maximum obtainable index change of the tuner ring. Theoretically, the tuning range of 155 nm was estimated for the double ring quantum cascade laser (DRQCL) design employed in this thesis. However, experimentally, a single mode (~19 dB single sideband suppression ratio) and tuning range of 59 nm which covers almost half the bandwidth were observed. For the first time in the history of the QCL, a research into the design, fabrication and characterisation of integrated polarisation mode convertors (PMCs) has been carried out. The PMC design is based upon etching trenches, using the RIE lag effect, of sub-wavelength dimensions into one side of a waveguide in order to achieve an asymmetric cross-sectional profile, resulting in a waveplating effect. This thesis presents such PMCs integrated with QCLs that emit 69% TE light with the polarisation angle of 65 degree from one facet and a pure TM light emitted from the other facet using a 256 μm long PMC design (design D2). An integrated tunable birefringent waveguide (ITBW) consisting of two PMCs with a differential phase shift (DPS) section between them. To probe the birefringence operation, a sub-threshold electroluminescence was employed to investigate the single pass operation of the ITBW. A theory based on the electro-optic properties of birefringence in QCL waveguides was used combined with a Jones-matrix based description to gain an understanding of the electroluminescence results. With the QCL operating above threshold, polarisation and wavelength tuning of the signal output was demonstrated. By comparing the sub-threshold electroluminescence and active polarisation angle measurement result with the Jones matrix model, the material birefringence (no DPS current), 4n, was estimated to be around 0.005 for the QCL employed in this work. However, single mode emission was not observed and 24 nm discontinuous tuning was recorded. Despite this, using a QCL incorporating an ITBW device, active polarisation control over 45 degree was demonstrated, and currently, to the best of the authors knowledge there has been no other QCL device that is capable of electronically controlling the output polarisation.

A travelling-wave microwave fibre-optic hnk (TWMFL) is proposed consisting of two transmission line structures that are periodically loaded with laser diodes and photodiodes, connected to one another by a fibre array.

This Thesis is divided into three parts: Introduction (Chapter 1), Result and Discussion (Chapter 2) and Experimental (Chapter 3). The Introduction (Chapter 1) provides a review of the recent literature relating to the 1,3dipolar cycloaddition reactions. It· covers nitrones, azomethine ylides and azomethine imines. The Result and Discussion. section (Chapter 2) deals with the author's work which is . concerned with developing four novel multicomponent cascade reactions. These are (i) a 3- component cascade involving bifunctional aryl halide! Michael acceptors, allene and arriine or sulphonamide nucleophiles (ii) a 3-component cascade involving bifunctional arylhalide! Michael acceptors, carbon monoxide and primary ~nes (iii) a 4-component cascade involving a bifunctional aryliodide! aldehyde, hydrazines, allene and· a dipolarophile and (iv) a 3-component cascade involving 2-bromomethyl iodobenzene, carbon monoxide and / ' hydrazines. { Cascade (i) and (ii) employ a common bifunctional Michael acceptor! arylhalide and a series of these were prepared all of which had an a, .p- unsaturated amide as the Michael acceptor. In·niost caSe these contained a chiral amide function while enabled the author to study chiral induction and diastereomer product ratios. In series (i) product yields ranged from 54-75% but there was essentially no chiral induction (dr's 1:1-1:1.6). Series (ii) gave a range of interesting isoindolones in 61-78% yield but again there was no significant chiral induction (dr 1:1-1:1.7). In both series we concluded the chiral amide centre is too distant from the site of the newly created chiral centre to have a controlling influence. Series (iii), the 4-component cascade proceeded regia-and stereoselectively in 55-72% yield. The IH NMR spectra of the pyrazolidine products showed line broadening due to N-Ione pair inversion processes which was confirmed by variable temperature IH NMR studies. An approximate inversion barrier of 12-15 Kcals.morl wa~ estimated. The final series, series (iv), was only briefly studi~d with two examples completed in 61-64% yield. .The thesis concludes (Chapter 3) with full experimental details' of all new compounds together with their appropriate physical, spectroscopic and analytical data.

Quantum Cascade Lasers (QCLs) are unipolar lasers based on intersubband transitions that have emission wavelengths in the mid- to far-infrared. Mid-infrared QCLs have recently reached a high level of technological maturity with watt level continuous wave output powers being demonstrated at room temperature. However, the performance of these conventional QCLs falls away very rapidly below -4 urn where a number of important applications exist. This thesis details the design, fabrication and characterisation of high performance 'short wavelength' QCLs that operate in the 3-4 urn region. The high conduction band offset of the InGaAs/ AIAsSb material system and its compatibility with InP based waveguides and fabrication technology has made it the most attractive solution to achieving high performance QCLs in the 3-4 urn region. This thesis covers the development of InGaAs/AIAsSb based QCLs and covers the demonstration of a number of record high output powers and maximum operating temperatures. Watt level peak powers at room temperature and operation up to at least 400 K have been achieved across the -3.3-3.7 urn range. These high performance InGaAs/ AIAsSb lasers are well suited for the realization of single-mode devices that can be used to create compact, ultra-sensitive trace-gas sensors based on absorption spectroscopy. To this effect, single-mode distributed feedback (DFB) QCLs where developed at -3.35-3.45 urn with side mode suppression ratios of up to 30 dB. The lasers employed buried third order DFB gratings and operated at room temperature. Finally, the development of strain compensated InGaAs/AlInAs QCLs grown by metal- organic vapour phase epitaxy (MOVPE) is detailed. This growth technique presents significant advantages for commercial device production when compared to the more typically employed molecular beam epitaxy growth technique. MOVPE grown InGaAs/ AlInAs QCLs with 70% indium composition in the wells operating at -4 urn are reported.

Although terahertz quantum cascade lasers (QCLs) are promising devices for cheap, compact and coherent sources of terahertz radiation, the full potential of these lasers has not been realised. In this work we shall examine the active region design of THz QCLs and study the behaviour of heterogeneous active regions, where there are several quantum cascade designs combined in the same active region. We will present a study of the systematic design of THz QCLs and show that it is possible to systematically tune the frequency of these devices by simple modifications to the active region design. We then present some active region designs, together with results for some high-performance designs, operating around 2 THz. Two of these designs have shown high efficiency and high temperature performance at this frequency range. From this we move on to the subject of heterogeneous active regions, and present electrically switchable emission from heterogeneous THz QCLs; devices which emit at different, widely separated frequencies depending on the electrical conditions. This is the first reported electrically switchable emission from THz QCL devices without a magnetic field.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.<br>Includes bibliographical references (p. 57-58).<br>In this thesis, I implemented and extended a face detector, based on cascades of boosted features, for use in real time systems. The extensions are twofold. First, I designed a way of combining several cascades into a cascade tree, and showed how such a tree provides a powerful mechanism for combining detector efficiency and accuracy. When the training data has large variations, the cascade tree yields a faster detector, and when the data has only small variations, there is a distinct detection rate improvement. As a second extension, I designed a system for pose estimation based on an array of cascades. I performed an evaluation of this system and compared to normalized cross-correlation.<br>by Patrik Sundberg.<br>M.Eng.

Speech coding or speech compression is one of the important aspects of speech communications nowadays. By coding the speech, the speed needed to transmit the digitized speech, called the bit rate, can be reduced. This means that for a certain speech communications channel, the lower the bit rate of the speech coding, the more communicating parties can be carried on that channel. This research has as its main application the extraction of the parameters of human speech for speech coding purposes. We propose an RLS-based cascade adaptive filter structure that can significantly reduce the computational effort required by the RLS algorithm for inverse filtering types of applications. We named it the Cascade RLS with Subsection Adaptation (CRLS-SA) algorithm. The reduction in computational effort comes from the fact that, for inverse filtering applications, the gradients of each section in the cascade are almost uncorrelated with the gradients in other sections. Hence, the gradient autocorrelation matrix is assumed to be block diagonal. Since we use a second order filter for each section, the computation of the adaptation involves only the 2x2- gradient autocorrelation matrix for that section, while still being based on a global minimization criterion. The gradient signal of a section itself is defined as the derivative of the overall output error with respect to the coefficients of the particular section, which can be computed efficiently by passing the overall output of the cascade to a filter with coefficients that are derived from the coefficients of that section. The computational effort of the CRLS-SA algorithm is approximately 20*L*N/2, where L is the data record length and N is the order of the filter. We analyze the convergence rate of the CRLS-SA algorithm based on the convergence time constant concept, which is the ratio of the condition number and the sensitivity. The CRLS- SA structure is shown to satisfy the DeBrunner-Beex conjecture which says that a structure with a smaller convergence time constant converges faster than a structure with a larger convergence time constant. We show that CRLS-SA converges faster than the Direct Form RLS (DFRLS) algorithm and that its convergence time constant is lower than that of the direct form. The convergence behavior is verified by looking at how fast the estimated system approaches the true system. Here we use the Itakura distance as the measure of closeness between the estimated and the true system. We show that the Itakura distance associated with the CRLS-SA algorithm approaches zero faster than that associated with the direct form RLS algorithm. The CRLS-SA algorithm is applied in this dissertation to general linear prediction, to the direct adaptive computation of the LSF and their representation in quantized form using a split vector quantization (VQ) approach, and to the detection and tracking of the frequencies in signals consisting of multiple sinusoids in noise.<br>Ph. D.

Diese Arbeit untersucht den Einfluss verschiedener physikalischer Parameter auf das Verhalten von Frequenz-abstimmbaren External-Cavity Quantenkaskadenlasern (EC-QCLs) theoretisch und experimentell. Diese beinhalten unter anderen die Antireflexschicht, die Art der Optiken, die geometrischen und die mechanisch/strukturellen Eigenschaften. Dies wurde erreicht durch Aufbau dreier sehr unterschiedlicher EC-Konfigurationen, der Diskussion und dem Vergleich ihrer Leistungsmerkmale und ihrer Vor- und Nachteile für verschiedene Anwendungen unter hauptsächlicher Verwendung von QCLs desselben Wafers der Vergleichbarkeit wegen. Für den letzten Teil dieser Arbeit wurde ein neuer Typus EC-QCL mit vielversprechenden Eigenschaften entwickelt, sodass wir glauben er hat das Potential das Littrow Design langfristig abzulösen. Dieses selbststabilisierende Design verwendet einen Retroreflektor als externen Reflektor. Für die Demonstration dieses Konzepts war die Entwicklung eines Tuning-Elements in Form eines Winkel-verstimmbaren Mittinfrarot-Bandpass-Interferenzfilters mit sehr hohem Gütefaktor vonnöten. Für das Design des Filters wurden Materialien mit sehr strengen Toleranzen bezüglich ihrer physikalischen und optischen Eigenschaften auf Basis von theoretischen Überlegungen ausgewählt und eine Fabrikationsmethode mit hochoptimierten Prozessparametern entwickelt. Die ersten Filter auf Basis von Yttriumfluorid/Yttriumoxid/Germanium/Silizium haben eine Transmissionsbandbreite von 0.14% der Zentralwellenlänge und eine maximale Transmission von etwa 60%. Die EC Konfiguration resultierte in verminderter Empfindlichkeit gegenüber Mechanischen Störungen des Reflektors um zwei Größenordnungen. Das Design behebt die grundsätzliche Limitierung des Littrow Designs bezüglich Miniaturisierung, da kein großer Strahldurchmesser vonnöten ist um kleine Bandbreiten des Littrowgitters zu erreichen.<br>This thesis thoroughly investigates theoretically and experimentally the effects many physical parameters have on the performance of Tunable External-Cavity Quantum-Cascade Lasers (EC-QCLs). These include, among others, the anti-reflection coating, the type of optics, and the geometrical as well as mechanical and structural properties of the EC setup. This was done by assembling three very different EC setups and comparing and discussing their performance, as well as advantages and disadvantages for different purposes using mainly QCLs from the same original wafer for better comparability. For the last part of this thesis, a new type of EC-QCL configuration was developed with properties so promising that we believe it has the potential to replace the Littrow Cavity in the long term. This is an alignment-stabilized and interference filter-tuned design using a retroreflector as the external reflector. For the demonstration of this concept, development of the tuning element in the form of an angle-tunable high-Q mid-infrared bandpass filter was necessary. For the design of the filter, materials with very strict tolerances on the physical and optical properties were selected from theoretical considerations and a fabrication method with highly optimized process parameters was developed. The first filters on the basis of yttrium fluoride/yttrium oxide/germanium/silicon have a transmission bandwidth of 0.14% of the central wavelength and a peak transmission of approximately 60%. The EC configuration resulted in a sensitivity reduction to mechanical perturbations of the reflector by two orders of magnitude, with a calculated potential for three orders of magnitude using optimized optics. This design lifts the fundamental constraint on miniaturization imposed on the Littrow design that requires large beam diameters to ensure a small bandwidth of the Littrow grating.

Quantenkaskadenlaser (QCLs) wurden vor gerade zwanzig Jahren erfunden und haben seitdem stetig im weltweiten Markt der optoelektronischen Bauelemente für den Infrarot an Bedeutung gewonnen. Anwendungsbeispiele für aktuelle und potenzielle Einsatzgebiete von QCLs sind photoakustische Spektroskopie, Umweltüberwachung, Simulation von heißen Körpern, und optische Freiraumdatenübertragung. Rekord optische Leistungen von 14 W und Leistungseffizienzen zwischen 15-35 % wurden bei mittelinfraroten QCLs für Betriebstemperaturen zwischen 80-300 K erreicht. Die weitere Verbesserung dieser Eigenschaften hängt nicht nur von Aspekten wie Wärmemanagement und Chip-Packaging ab, sondern auch von Verbesserungen im Laserdesign zwecks der Reduzierung des Ladungsträgerleckstroms. Dennoch sind die verschiedenen Mechanismen und Komponenten des Leckstroms in Quantenkaskadenlasern leider noch nicht gründlich untersucht worden. Die vorliegende Arbeit liefert a realistische Beschreibung der Ladungsträgertransports in QCLs. Wir beschreiben u.a. Leckströme vom Quantentopf- in höhere Zustände und diskutieren elastische und inelastische Streumechanismen von Ladungsträgern bei mittelinfraroten Quantenkaskadenlasern. Wir illustrieren außerdem die Notwendigkeit zur Berücksichtigung der Elektronentemperatur für eine vollständigere Analyse der Ladungsträgertransporteigenschaften von Quantenkaskadenlasern. Methoden zur experimentellen Ermittlung des temperaturabhängigen Leckstroms in Quantenkaskadenlasern werden präsentiert. Unser Ansatz liefert eine Methode zur effektiven Analyse von der QCL-Leistung und Vereinfacht die Optimierung von QCL aktive Regionen.<br>Two decades after their invention in 1994, quantum-cascade lasers (QCLs) become increasingly important in the global infrared optoelectronics market. Photoacoustic spectroscopy, environment monitoring, hot object simulation, and free-space communication systems are selected examples of the current and potential applications of QCLs. Record optical powers as large as 14 W and power-conversion efficiencies ranging between 15-35 % have been reported for MIR QCLs for temperatures 80-300 K. Further improvement of these characteristics depends not only of aspects as heat management and chip-packaging, but also on improving the active-region design to reduce several leakage channels of charge carriers. However, mechanisms through which leakage of charge carriers affects QCLs performance have not been thoroughly researched. A better understanding of the several (non-radiative) scattering mechanisms involved in carrier transport in QCLs is needed to design new structures and optimize their performance. This work provides a realistic description of charge carriers transport in QCLs. We discuss in particular carrier leakage from QCL quantum-well confined states into higher and lower states. The two main mechanisms for non-radiative intersubband scattering in MIR QCLs are electron-longitudinal-optical-phonon scattering and interface roughness-induced scattering. We present methods for the experimental determination of the leakage current in QCLs at and above laser threshold, which allowed us to estimate the sheet distributions of conduction band states and better understand the impact of temperature activated leakage on QCLs characteristics. We found that even at temperatures low enough to neglect ELO scattering, carriers leakage due to IFR becomes significant for devices operating at high electron temperatures. Altogether, this approach offers a straightforward method to analyze and troubleshoot new QCL active region designs and optimize their performance.

Cancer is a serious family of disease that continues to cripple and claim those afflicted. For the last several decades, America has invested in a national program to alleviate cancer and cancer related suffering, ultimately seeking a cure. As part of this goal, the National Cancer Institute (NCI) has spent significant effort scouring the globe with the hope of finding naturally occurring compounds that can successfully combat cancer. Presently, this effort has uncovered many natural products with chemotherapeutic potential and many of the lead agents used in the fight against cancer are either natural products themselves or are compounds inspired by a natural product. This work describes one family of natural products uncovered by the NCI that is being explored for chemotherapeutic applications, namely the schweinfurthins. The schweinfurthins were isolated by the NCI; however the natural source, Macaranga schweinfurthii, did not provide these compounds in ample quantity to permit further study. The paucity of natural material indicated that a chemical synthesis of these compounds would be the most reliable method to provide meaningful amounts of schweinfurthins. The present work describes the chemical synthesis of four of the most potent schweinfurthins, describes the synthesis of numerous structural analogues, and details advances to the field of cascade cyclizations which makes their synthesis possible.

Different aspects of the bathyal ecosystem have been treated with the final aim to investigate trophic cascades driven by fishery (top-down control), a phenomenon that in bathyal systems has never been studied. This was exemplified by the in-depth study of the exploited soft-bottom continental slope off Catalonia (NW Mediterranean), where the fishery of the red shrimp Aristeus antennatus is carried out and the main structure of the trophic web is known. In Part I the infaunal assemblages inside the canyon and on the adjacent slope have been related to environmental variables. A seasonal variability has been detected along the year related to the primary production, river efflux and the occurrence of the Levantine Intermediate Waters (LIW). All variables, used as proxy for these three causes, explain the seasonal variability in both assemblages. The canyon was found to be more influenced by seasons and the terrigenous provision than the open slope, that conversely was more related to the primary production and showed less evidence of seasonality. The canyon showed high variability in biomasses, taxonomic composition and trophic types, whereas the adjacent slope was dominated by sub-surface deposit feeders and in February by the carnovorous caudofoveats eating on foraminifera. This study was mandatory in order to parametrise the models in Part III. Part II concerns a long term study of A. antennatus LPUE performed through both frequentist and Bayesian additive regression models. The variability explained by a total of six predictors captured the 43% of the total LPUE variability. The set of fishery-related variables (the daily trips performed by vessels, the gross registered tonnage and the factor vessel) was the most important source, with an explained deviance (ED) of 20.58%, followed by temporal (ED=13.12%) and finally economic variables (the ex-vessel shrimp price, ED=9.30%). We found that data derived from fishery, as well as independent data provided by scientific surveys, provide similar indices for the exploited species. The study also showed that it may be appropriate reduce the limit of the number of trips per month for a reasonable management. Finally has been shown that a mixed effect model permits to account for correlated data and if vessels hold random effects, there is no need to use them for standardization purposes. Part III covers the ecosystem approach of this thesis. A total of 40 carbon flows among 7 internal and 6 external compartments, were reconstructed using linear inverse modelling (LIM). The total carbon flux to the community was 2.62 mmol C m-2 d-1, entering as vertical (5.2E-03 mmol C m-2 d-1) and advective organic matter (2.60 mmol C m-2 d-1). The influx was then partitioned between the total organic matter (TOM) in sediment, 87.05% and suspension feeding (12.95%) mainly by the macrobenthos (95.74%). The fate of carbon deposited in sediments was its burial, its degradation or the ingestion by metazoan deposit feeders. The ingestion of C in sediments by the fauna was the 36.34%, the rest was used by the prokaryotes and nematods (69.28%) or trapped in the sediment (32.19%). The total respiration was 1.89 mmol C m-2 d-1, of which the 83.75% was represented by sediments, including prokaryotes and meiofauna. The dynamic simulation based on a system of ordinary differential equations predicts biomass trends during 5 years after perturbations induced by red shrimp fishery (top-down driver) and food supply (bottom-up driver). We only found very ephemeral indirect effects induced by fishery, persisting less than 10 days, that we considered not enough to demonstrate the occurrence of trophic cascade in the system. On the contrary we found effects driven by source limitation. We explain these results with the important role of detritus of allochthonous origin, that we hypothesised to be the component controlling the food web dynamics. A mechanism known as donor-control.<br>Diferentes aspectos del ecosistema batial han sido tratados con el objetivo final de investigar mecanismos de cascada trófica impulsados por la pesca, un fenómeno que en estos sistemas nunca ha sido estudiado. Esto fue ejemplificado con el estudio del talud continental de fondo suave frente Cataluña (Mediterráneo noroccidental), donde se lleva a cabo la pesquería de la gamba roja Aristeus antennatus. En la Parte I se ha estudiado la composición de la infauna dentro del cañón y en el talud adyacente, y se ha relacionado con variables ambientales, que se pueden reconducir a tres causas principales (la producción primaria, el flujo desde ríos y a la aparición de las Aguas Intermedias de Levante). El cañón fue más influenciado por las estaciones y la provisión terrígena, contrariamente, el talud abierto resultó ser más estable y relacionado con la producciónn primaria. El cañón mostró variabilidad en las biomasas, en la composición taxonómica y en los principales hábitos tróficos, mientras que el talud adyacente estaba dominado por detritívoros de detríto refractario y en febrero por carnivoros que se alimenta de foraminiferos. Este estudio fue obligatorio para desarrollar los modelos en la Parte III. La Parte II se refiere a un estudio de los desembarques por unidad de esfuerzo (LPUE) de A. antennatus realizado a través de análisis de regresión frecuentistas y bayesianas. La variabilidad explicada por un total de seis predictores capturó el 43% de la variabilidad total del LPUE. Las variables relacionadas con la pesca (los viajes diarios realizados por los barquos el tonelaje de registro bruto y los barcos mismos como factor) fueron la fuente más importante, con una devianza explicada (DE) de 20.58%, seguido de variables temporales (DE=13.12%) y económicas (el precio de la gamba, DE=9.30%). Los datos derivados de la pesca, así como lo datos independientes de ella, devuelven índices similares para esta especie. También encontramos que puede ser apropiado reducir el límite de la cantidad de viajes al mes a un umbral definido por el modelo para una gestión razonable del recurso. Por último, se demostró que los modelos de efectos mixtos permiten evitar dificultades en la estimación cuando los datos temporales estan correlados y que si los barcos son de efecto aleatorio, no son útiles para fines de normalización. La parte III cubre el enfoque ecosistémico de esta tesis. Un total de 40 flujos de carbono entre 7 compartimentos internos y 6 externos, fueron reconstruidos utilizando modelación lineal inversa (LIM). El flujo total de C a la comunidad fue 2.62 mmol C m-2 d-1, entrando como materia orgánica vertical (5.2E-03 mmol C m-2 d-1) o por trasporte advectivo (2.6 mmol C m-2 d-1). Después, ese flujo se repartió entre la deposición (87.05%) y el alimentación por suspención (12.95%), sobre todo por parte del macrobentos (95.74%). El destino del C depositado fue en parte la ingestión por los metazoos (36.34%). El resto fue utilizado por los procariotas (y nemátodos) (69.28%) o atrapado en el sedimento (32.19%). La respiración total fue de 1.89 mmol C m-2 d-1, por la mayoría debido a procariotas y meiofauna (83.75%). La simulación dinámica se basa en un sistema de ecuaciones diferenciales ordinarias para predecir las tendencias de biomasa durante 5 años después de las perturbaciones inducidas por la pesca de la gamba (proceso top-down) y por las limitaciones de alimento primario (proceso bottom-up). Sólo encontramos efectos muy efímeros inducidas por la pesca, que persisten menos de 10 días, y que no se consideraron suficientes para demostrar la ocurrencia de cascadas tróficas. Por el contrario hemos encontrado efectos impulsado por la fuente de energía. Explicamos estos resultados con la importante función del detrito de orígen allóctono, y hipotetizamos que el detrito es el componente que controla la dinámica de la red trófica batial, un mecanismo conocido como control de los donantes.

Arynes are reactive intermediates that have been an academic curiosity for over a century. A recent renaissance of interest in the chemistry of these intermediates can be traced back to the development of ortho-(silyl)aryl triflates as aryne precursors. The application of aryne chemistry outside academia has been precluded by the expense and laborious preparation of these precursors. Diphenyliodonium-2-carboxylate has been shown to be a stable and inexpensive benzyne precursor, however application has been limited due to the high temperature (>160 ºC) and long reaction times required to generate benzyne by this protocol. Described within is an investigation whereby diphenyliodonium-2-carboxylate is successfully decomposed using microwave irradiation to generate benzyne. This proof of concept investigation shows diphenyliodonium-2-carboxylate can be applied as an off-the-shelf benzyne precursor; by using microwave radiation, significantly reduced reaction times and lower b.p. solvents can facilitate a more universal application of this protocol than previously described. The investigation into the reactions of allylamino malonates with arynes is also described. Simple allylamino malonates are shown to perform a novel cascade aryne capture/ring-closure/[2,3]-rearrangement to generate indolin-3-one products. The influence of substitution of the indolin-3-one products on the photophysical properties is probed. Tetrahydropyridine derived aminomalonates result in a ring contraction by [2,3]-rearrangement to N-phenyl pyrrolidine products. Further investigations show N-allyl proline methyl esters also generate indolin-3-one products by this novel cascade mechanism. The photophysical properties of these products are also probed. N-diallylalanine methyl ester is shown to generate indolin-3-one with benzyne however N-allyl sarcosine ethyl ester generates the N-phenyl -allylated amino esters product by aryne capture/[2,3]-rearrangement.

The energy cascade process in turbulent flows is studied. Kolmogorov inertial range theories are critically reviewed and the multifractal characterization is discussed. Multiplicative cascade models are compared to the energy dissipation field (EDF) measured in the atmosphere. Landau's objection to the 1941 Kolmogorov theory is extended to the predictions of statistical fluid mechanics. The hypothesis $ rm Delta v( lambda L) { buildrel{d} over=} lambda sp{1/3} Delta v(L)$ is rejected with a statistical test. The moments $ rm langle( log varepsilon(L)) sp{p} rangle,$ where $ varepsilon$(L) denotes the EDF averaged over a volume of size L, are shown to be gaussian. For the EDF: Convergence tests showed that the exponents $ tau$(q) were not reliable for q $<$ 0; the correlations obey $ rm langle( mu sb{x}( delta)) sp{p}( mu sb{x+ delta}( delta)) sp{q} rangle propto delta sp{ gamma(p,q)}$ but $ gamma$ does not always equal the value obtained with a multinomial measure; a privileged scale ration r $ approx$ 1/2 is suggested by the prefactor oscillations of the correlation function. The implications of these results for the modelling of the EDF are discussed.

This thesis focuses on experiments involving coupled cavity THz QCLs. These take the form of either monolithic devices with etched gaps to form two cavities on the same device, or separate devices aligned under vacuum and at cryogenic temperatures using a specially built piezo-electric micropositioner system. After an introductory chapter briefly reviewing the current state of QCL research, processing techniques and other standard experimental techniques, the thesis begins with the presentation of some experiments dealing with the effects of optical feedback between the two cavities. This leads on to a theoretical chapter in which a mathematical model of a double section QCL is presented, along with some explanation of some unexpected results seen experimentally. The final chapter returns to experimental work. Findings from the previous chapters are used to attempt to develop methods for the measurement of gain and loss in QCLs using coupled cavities, something for which there is currently no reliable method at THz frequencies and which involves considerable experimental difficulties. The thesis concludes with a discussion of possible future work including an extension of the mathematical model, and other possibilities to improve the gain measurement method.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.<br>Includes bibliographical references (p. 321-340).<br>The terahertz or the far-infrared frequency range of the electromagnetic spectrum ( ... ) has historically been technologically underdeveloped despite having many potential applications, primarily due to lack of suitable sources of coherent radiation. Following on the remarkable development of mid-infrared ( ... ) quantum-cascade lasers (QCLs) in the past decade, this thesis describes the development of electrically-pumped terahertz quantum-cascade lasers in GaAs/AlsGal_.As heterostructures that span a spectral range of 1.59 - 5.0 THz ( ... ). A quantum-cascade laser (QCL) emits photons due to electronic intersubband transitions in the quantum-wells of a semiconductor heterostructure. The operation of terahertz QCLs at frequencies below the Reststrahlen band in the semiconductor ( ... ), is significantly more challenging as compared to that of the mid-infrared QCLs. Firstly, due to small energy separation between the laser levels various intersubband scattering mechanisms are activated, which make it difficult to selectively depopulate the lower laser level. Additionally, as electrons gain enough kinetic energy in the upper laser level thermally activated longitudinal-optical (LO) phonon scattering reduces the level lifetime and makes it difficult to sustain population inversion at higher temperatures.<br>(cont.) Secondly, waveguide design for terahertz mode confinement is also more challenging due to higher free-carrier losses in the semiconducting doped regions at the terahertz frequencies. For successful designs reported in this work, the lower radiative state depopulation is achieved by a combination of resonant-tunneling and fast LO phonon scattering, which allow robust operation even at relatively high temperatures. An equally important enabling mechanism for these lasers is the development of metal-metal waveguides, which provide low waveguides losses, and strong mode confinement due to subwavelength mode localization in the vertical dimension. With these techniques some record performances for terahertz QCLs are demonstrated including the highest pulsed operating temperature of 169 K, the highest continuous-wave (cw) operating temperature of 117 K, and the highest optical power output (248 mW in pulsed and 138 mW in cw at 5 K) for any terahertz QCL. Towards the bigger goal of realizing a 1-THz solid-state laser to ultimately bridge the gap between electronic and optical sources of electromagnetic radiation, QCLs with a unique one-well injection scheme, which minimizes intersubband absorption losses that occur at longer wavelengths, are developed.<br>(cont.) Based on this scheme a QCL operating at 1.59 THz (A - 189 ym) is realized, which is amongst the lowest frequency solid-state lasers that operate without the assistance of a magnetic field. This thesis also reports on the development of distributed-feedback lasers in metal-metal waveguides to obtain single-mode operation, with greater output power and better beam quality. The subwavelength vertical dimension in these waveguides leads to a strongly coupled DFB action and a large reflection from the end-facets, and thus conventional coupled-mode theory is not directly applicable to the DFB design. A design technique with precise control of phase of reflection at the end-facets is developed with the aid of finite-element analysis, and with some additional unique design and fabrication methods, robust DFB operation has been obtained. Single-mode surface-emitting terahertz QCLs operating up to - 150 K are demonstrated, with different grating devices spanning a range of approximately 0.35 THz around v - 3 THz using the same gain medium. A single-lobed far-field radiation pattern, higher output power due to surface-emission, and a relatively small degradation in temperature performance compared to the Fabry-Perot ridge lasers makes these DFB lasers well suited for practical applications that are being targeted by the terahertz quantum-cascade lasers.<br>by Sushil Kumar.<br>Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 113-118).<br>In this thesis, microelectromechanical systems (MEMS) assisted electrically tunable terahertz quantum cascade lasers (THz QCLs) are designed and demonstrated. Two MEMS tuner devices are proposed to achieve electrically tunable THz QCLs. One is the electrostatic comb drive actuated tuner design and the other one is a two-stage flexure design that is actuated by an external piezo nano-positioning actuator. The MEMS tuner devices are all fabricated using standard foundry process SOIMUMPs from MEMSCAP Inc. with some additional in-house post-processings. First order distributed-feedback (DFB) THz wire QCLs with robust mode selectors are designed and fabricated at the MIT Microsystems Technology Laboratories (MTL) using processes developed at our group. By integrating the MEMS tuner chips with the THz QCL chips, broadband electrically tunable THz QCLs are successfully demonstrated. This thesis work provides an important step towards realizing turn-key tunable THz coherent sources for a variety of applications such as THz spectroscopy and THz coherent tomography.<br>by Ningren Han.<br>S.M.

Thesis (DSc (Mathematical Sciences))--University of Stellenbosch, 2007.<br>ENGLISH ABSTRACT: (Math symbols have changed) Wavelet and subdivision techniques have developed, over the last two decades, into powerful mathematical tools, for example in signal analysis and geometric modelling. Both wavelet and subdivision analysis are based on the concept of a matrix–refinable function, i.e. a finitely supported matrix function which is self-replicating in the sense that it can be expressed as a linear combination of the integer shifts of its own dilation with factor 2: F = TAF = å k∈Z F(2 ・ −k)Ak. The coefficients Ak, k ∈ Z of d × d matrices, of this linear combination constitute the so-called matrix- mask sequence. Wavelets are in fact constructed as a specific linear combination of the integer shifts of the 2-dilation of a matrix- refinable function cf. [2; 9], whereas the convergence of the associated matrix- subdivision scheme c0 = c, cr+1 = SAcr, r ∈ Z+, SA : c = (ck : k ∈ Z) 7→ SAc = å ℓ∈Z Ak−2ℓ cℓ : k ∈ Z ! , subject to the necessary condition that rank := dim   \ ǫ∈{0,1} n y ∈ Rd : Qǫy = y o   > 0, Qǫ := å j∈Z Aǫ+2j, ǫ ∈ {0, 1}, ( cf. [26]) , implies the existence of a finitely supported matrix- function which is refinable with respect to the mask coefficients defining the refinement equation and the subdivision scheme. Throughout this thesis, we investigate in time–domain for a given matrix mask sequence, the related issues of the existence of a matrix–refinable function and the convergence of the corresponding matrix– cascade algorithm, and finally we apply some results to the particular research area of Hermite interpolatory subdivision schemes. The dissertation is organized as follows: In order to provide a certain flexibility or freedom over the project, we established in Chapter 1 the equivalence relation between the matrix cascade algorithm and the matrix subdivision scheme, subject to a well defined class of initial iterates. Despite the general noncommutativity of matrices, we make use in the full rank case Qǫ = I, ǫ ∈ {0, 1}, of a symbol factorization, to develop in Chapter 2 some useful tools, yielding a convergence result which comes as close to the scalar case as possible: we obtained a concrete sufficient condition on the mask sequence based on the matrix version of the generating function introduced in [3, page 22] for existence and convergence. Whilst the conjecture on nonnegative masks was confirmed in 2005 by Zhou [29], our result on scalar case provided a progress for general mask sequences. We then applied to obtain a new one-parameter family of refinable functions which includes the cardinal splines as a special case, as well as corresponding convergent subdivision schemes. With the view to broaden the class of convergent matrix-masks, we replaced in chapter 3 the full rank condition by the rank one condition Qǫu = u, ǫ ∈ {0, 1}, u := (1, . . . , 1)T, then improved the paper by Dubuc and Merrien [13] by using the theory of rank subdivision schemes by Micchelli and Sauer [25; 26], and end up this improvement with a generalization of [13, Theorem 13, p.8] in to the context of rank subdivision schemes. In Chapter 4, we translated the concrete convergence criteria of the general theory from Theorem 3.2, based on the r-norming factor introduced in [13, Definition 6, p.6], into the context of rank, factorization and spectral radius (cf. [26]), and presented a careful analysis of the relationship between the two concepts. We then proceed with generalizations and improvements: we classified the matrix cascade algorithms in term of rank = 1, 2, . . . , d, and provided a complete characterization of each class with the use of a more general r−norming factor namely τ(r)-norming factor. On the other hand, we presented numerical methods to determine, if possible, the convergence of each class of matrix cascade algorithms. In both the scalar and matrix cases above, we also obtained explicitly the geometric constant appearing in the estimate for the geometric convergence of thematrix-cascade algorithm iterates to the matrix- refinable function. This same geometric convergence rate therefore also holds true for the corresponding matrix–cascade algorithm. Finally, in Chapter 5, we apply the theory and algorithms developed in Chapter 4 to the particular research area of Hermite interpolatory subdivision schemes: we provided a new convergence criterium, and end up with new convergence ranges of the parameters’ values of the famous Hermite interpolatory subdivision scheme with two parameters, due to Merrien [23].<br>AFRIKAANSE OPSOMMING :(Wiskundige simbole het verander) Golfie en subdivisietegnieke het oor die afgelope twee dekades ontwikkel in kragtige wiskundige gereedskap, byvoorbeeld in seinanalise en geometriesemodellering. Beide golfie en subdivisie analise is gebaseer op die konsep van ’n matriks-verfynbare funksie; oftewel ’n eindig-ondersteunde matriksfunksie F wat selfreproduserend is in die sin dat dit uitgedruk kan word as ’n lineêre kombinasie van die heelgetalskuiwe van F se eie dilasie met faktor 2: F = Σ F(2 · −α)A(α), met A(α), α ∈ Z, wat aandui die sogenaamde matriks-masker ry. Golfies kan dan gekonstrueer word as ’n spesifieke lineêre kombinasie van die funksie ry {F(2 · −α) : α ∈ Z} (sien [2; 9]), terwyl die konvergensie van die ooreenstemmende matriks-subdivisie skema cº = c, cr+1 =(Σ β∈Z A(α − 2β) cr(β) : α ∈ Z ! , r ∈ Z+, onderhewig aan die nodige voorwaarde dat rank := dim   \ ǫ∈{0,1} n y ∈ Rd : Qǫy = y o   > 0, Qǫ := å α∈Z A(ǫ + 2α), ǫ ∈ {0, 1}, (sien [27]) die bestaan impliseer van ’n eindig-ondersteunde matriksfunksie F wat verfynbaar ismet betrekking tot diemaskerko¨effisi¨entewat die subdivisieskema definieer, en in terme waarvan die limietfunksie F van die subdivisieskema uitgedruk kan word as F = å α∈Z F(· − α)c(α). Ons hoofdoel hier is om , in die tydgebied, en vir ’n gegewematriks-masker ry, die verwante kwessies van die bestaan van ’nmatriks-verfynbare funksie en die konvergensie van die ooreenstemmende matriks-kaskade algoritme, en matriks-subdivisieskema, te ondersoek, en om uiteindelik sommige van ons resultate toe te pas op die spesifieke kwessie van die konvergensie van Hermite interpolerende subdivisieskemas. Summary v Eerstens, in Hoofstuk 1, ondersoek ons die verwantskap tussen matriks-kaskade algoritmes en matriks-subdivisie skemas, met verwysing na ’n goedgedefinieerde klas van begin-iterate. Vervolgens beskou ons die volle rang geval Qǫ = I, ǫ ∈ {0, 1}, om, in Hoofstuk 2, nuttige gereedskap te ontwikkel, en wat daarby ’n konvergensie resultaat met ’n sterk konneksie ten opsigte van die skalaar-geval oplewer. Met die doelstelling om ons klas van konvergente matriks-maskers te verbreed, vervang ons, in Hoofstuk 3, die volle rang voorwaarde met die rang een voorwaarde Qǫu = u, ǫ ∈ {0, 1}, u := (1, . . . , 1)T, en verkry ons dan ’n verbetering op ’n konvergensieresultaat in die artikel [14] deur Dubuc en Merrien, deur gebruik te maak van die teorie van rang subdivisieskemas van Micchelli en Sauer [26; 27], waarna ons die resultaat [14, Stelling 13, page 8] na die konteks van rang subdivisieskemas veralgemeen. InHoofstuk 4 herlei ons die konkrete konvergensie kriteria van Stelling 3.2, soos gebaseer op die r-normerende faktor gedefinieer in [14, Definisie 6, page 6] , na die konteks van rang, faktorisering en spektraalradius (sien [27]), en gee ons ’n streng analise van die verwantskap tussen die twee konsepte. Verder stel ons dan bekend ’n nuwe klassifikasie van matriks-kaskade algoritmes ten opsigte van rang, en verskaf ons ’n volledige karakterisering van elke klasmet behulp van ’nmeer algemene r-normerende faktor, nl. die τ(r)-normerende faktor. Daarby gee ons doeltreffende numeriesemetodes vir die implementering van ons teoretiese resultate. Ons verkry ook eksplisiet die geometriese konstante wat voorkom in die afskatting van die geometriese konvergensie van die matriks-kaskade algoritme iterate na die matriks-verfynbare funksie. Ten slotte, in Hoofstuk 5, pas ons die teorie en algoritmes ontwikkel in Hoofstuk 4 toe om die konvergensie van Hermite-interpolerende subdivisieskemas te analiseer. Spesifiek lei ons ’n nuwe konvergensie kriterium af, wat ons dan toepas om nuwe konvergensie gebiede vir die parameter waardes te verkry vir die beroemde Hermite interpolerende subdivisieskema met twee parameters, soos toegeskryf aan Merrien [24].

The aim of this project was to develop a cascade approach towards perhydropyrrolo-[1,2-a]-quinolines and to apply this to the asymmetric synthesis of the gephyrotoxin alkoids. Chapters Two and Three outline the development of a synthetic route towards a range of cascade precursors, whilst Chapter Four outlines investigations into the enamine-Michael cascade. Central to understanding the cascade process was the discovery that the major product of the enamine-Michael cascade was the unusual tricyclic hydroquinium salt. This can subsequently be engaged in a diastereoselective inter- or intramolecular reduction to afford either a trans-perhydro-[1,2-a]-quinoline or a tetracyclic aminal in high overall yield depending on the C1 oxygen substituent.

The aims of this project were to develop a cascade approach towards decahydroquinoline frameworks (Scheme I) and apply this to the synthesis of decahydroquinoline-containing natural products such as lycopodine, cermizine B and lepadin D (Scheme I). Scheme I. Several linear precursors were synthesized via a modular strategy. For example, lycopodine linear precursor i was synthesized in a total of 12 steps (Scheme II). Scheme II. Conditions for cyclization and hydrogenation were tested, with the diastereoselectivity examined in each system. For example, the lepadin linear precursor ii produced two decahydroquinolines iii and iv upon cyclization (Scheme III). Scheme III. It was found that the diastereoselectivity was dependent on the ring substituents and variation of the hydrogenation conditions could change the facial selectivity of enamine reduction.

Understanding physiological responses to threat can inform therapeutic interventions for phobias, anxieties, and PTSD. The defense cascade is reviewed as a theoretical model that predicts behavioral and physiological responses to threats. Nineteen undergraduates (five male), average age 19.4 experienced a novel virtual reality (VR) threat scenario while their physiology was measured. The Subjective Units of Distress Scale (SUDS) was used as a self-report indicator of distress in the research setting. Averaged SUDS reports suggested that the VR stimulus was experienced as threatening for most participants, but their autonomic response patterns did not fit those predicted by the defense cascade. Participants who had scored high on adaptive response questionnaires tended to show uncoupled ANS activation during baseline, but varied across the stimulus condition. Nearly all participants showed either coactivation or reciprocal activation during the stimulus period except those reporting the most dissociative trauma experiences, who mostly showed uncoupled ANS activation.<br>M.S.<br>The more we understand about how people’s bodies and their energies act when they feel threatened, the better we can find help for folks who struggle with anxiety, trauma or other challenging conditions. This research uses a theoretical model called the defense cascade to explore how people respond mentally and physically to threatening situations. Nineteen undergraduates went through a virtual reality (VR) experience that was designed to feel threatening while their body and its energy systems were measured. A scale was introduced called the Subjective Units of Distress Scale (SUDS) and was used to help the researchers understand how distressed people felt while they were in the VR experience. Averaged SUDS reports suggested that the VR stimulus was experienced as threatening for most participants, but their body response patterns did not fit those predicted by the defense cascade. Participants whose questionnaire responses suggested they were not anxiety-prone or traumatized, tended to show bodily activation that uncoupled their two autonomic bodily systems during a baseline period before the threatening stimulus. However, their autonomic responses during the stimulus period varied. Nearly all participants showed either both autonomic systems acting together or only one system acting in a mutually exclusive way to the other system during the stimulus period. This was the case for most participants except those reporting the most trauma involving dissociative experiences. This latter group mostly showed uncoupled autonomic bodily patterns.

Mestrado em Engenharia Física<br>In this thesis we study analytically and numerically the bootstrap percolation process in random uncorrelated directed complex networks. We formulate and analyze the bootstrap percolation process on both unweighted and weighted networks and also study a probability based percolation process. The considered percolation process has an associated activation threshold k where a node only gets active if it has at least k active neighboring nodes. We compare our results with analytical and numerical results obtained for undirected complex networks. We also analyze how topological properties of the directed network components, such as the giant strongly connected component and the periphery, influence on the bootstrap percolation process. We apply our theoretical approach for studying the bootstrap percolation on real complex networks. We show that our theoretical approach developed for the case of random uncorrelated directed networks is in a good agreement with numerical simulations of the bootstrap percolation process on real complex networks which actually are correlated and clustered.<br>Nesta tese estudamos analiticamente e numericamente o processo de bootstrap percolation em redes complexas direcionadas. Formulamos e analisamos o processo de bootstrap percolation em ambas redes com pesos e sem pesos e também estudamos um processo de bootstrap percolation baseado em probabilidades. O processo de bootstrap percolation considerado tem um parâmetro de ativação associado k onde um nó é ativado se tiver pelo menos k nó vizinhos ativos. Comparamos os nossos resultados com resultados analíticos e numéricos obtidos para redes complexas não direcionadas. Analisamos também como as propriedades topológicas dos componentes das redes complexas direcionadas, como o giant strongly connected component e a periferia, influenciam o processo de bootstrap percolation. Aplicamos a nossa teoria no estudo do processo de bootstrap percolation em redes complexas reais. Mostramos que a nossa teoria desenvolvida para redes complexas aleatórias e não correlacionadas está em bom acordo com simulações numéricas do processo de bootstrap percolation em redes complexas reais que são correlacionas e agrupadas.

This project aims to combine radical reactions, initiated by organoboranes under mild conditions, with polar reactions to form industrially useful small molecules and polymers in one-pot processes. In Chapter 2, it is shown that commercially available organoborole, 2-propyl-1,3,2-benzodioxaborole (PBD), can be used to initiate radical reactions at room temperature, including the addition of organophosphorus hydride radical mediators to alkenes. Also, the polymerisation of maleimides has been achieved using PBD at room temperature. The hydroboration of alkenes with catecholborane can be used to form β-alkylcatecholborane radical initiators in situ, and these reagents have been used for the initiation of radical polymerisation reactions at room temperature. In Chapter 3, it is shown that the β-alkylcatecholboranes, formed in situ through the hydroboration of alkenes, react with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) to initiate conjugate addition-aminoxylation reactions with maleimides, to form alkoxyamines. It was observed that heating the alkoxyamines leads to the elimination of TEMPO-H, to re-form the maleimide C=C double bond. Thus, a new, mild, one-pot procedure for the synthesis of 3-substituted maleimides has been developed. Alternatively, the alkoxyamine can be heated with Zn in AcOH to afford an alcohol, which offers a new route to hydroxylated-succinimides. In Chapter 4, the newly developed method for the synthesis of 3-substituted maleimides was applied to the preparation of carbocyclic analogues of the natural product D-(+)-showdomycin. It was found that this procedure could be used directly with maleimide to produce carbocyclic analogues of D-(+)-showdomycin in reasonable overall yield, from cheap starting materials, and without the need for protecting groups (for the maleimide nitrogen or the hydroxyl group). In Chapter 5, the synthesis and reactivity of N-(2,3-epoxypropyl)maleimide (EPM) is explored, with respect to the use of this molecule as an additive in the polymer industry. It is shown that EPM can be prepared in an overall yield of 50%, at room temperature, from cheap starting materials maleic anhydride and N-allylamine. The reaction of EPM with primary amines is investigated, and it is shown that the amine reacts preferentially with the C=C double bond of the maleimide, rather than the epoxide ring. However, it is shown that the use of Lewis acid additives to the reaction can reverse the chemoselectivity, leading to preferential opening of the epoxide ring.