published_or_final_version / Physics / Master / Master of Philosophy

The topic of this thesis is about experimental study of phenomena which are associated with light propagation in nonlinear dielectric media. In the first part of this work, we study experimentally the snake instability of the bright soliton stripe of the (2+1)-dimensional hyperbolic nonlinear Schrödinger equation. The instability is observed, through spectral measurements, on spatially extended femtosecond pulses propagating in a normally dispersive self-defocusing semiconductor planar waveguide. The second part of this thesis is about light propagation in nonlinear periodic media. We experimentally observe a stationary spatial gap (or Bragg) soliton in a periodic semiconductor planar waveguide. Based on the interference pattern of the soliton beam, we measure the power parameter of the soliton which is related to the position of the spatial spectrum in the linear band gap. Cette thèse de doctorat a pour sujet l’étude expérimentale de phénomènes associés à la propagation de la lumière dans les milieux diélectriques non linéaires. La première partie porte sur la démonstration expérimentale de l’instabilité serpentine d’une bande solitonique dans un système décrit par une équation de Schrödinger non linéaire à (2+1)-dimensions. L’instabilité est observée sur base de mesures du spectre spatial ainsi que du profil spatio-fréquentiel d’une impulsion femtoseconde après propagation dans un guide plan semi-conducteur qui présente une dispersion normale et une non-linéarité défocalisante. Le second thème abordé concerne la propagation de la lumière dans les milieux non linéaires périodiques. Les expériences réalisées ont montré l’existence du soliton de Bragg spatial stationnaire sous forme de faisceaux se propageant dans des guides plans semi-conducteurs périodiquement gravés. Sur base du profil de la distribution modale en intensité du faisceau soliton, il a été possible de mesurer le paramètre de puissance du soliton de Bragg qui détermine la position du spectre spatial dans la bande interdite linéaire.

Gap junctions are intercellular channels that permit passage of electrical and chemical signals between neighbouring cells. Vascular endothelium typically co-expresses Cx37 and Cx40, but may downregulate its expression of Cx37 (and upregulate Cx43) in response to changes in flow. The specific regulatory roles mediated by vascular endothelial connexins, and the consequences of altered connexin expression, remain unclear. In this study, we hypothesized that Cx37 and Cx40 regulate distinct vascular responses. We further hypothesize that Cx37 is predominantly involved in vascular growth control, whereas vascular growth is not affected by ablation of Cx40 expression. We show herein that Cx37, but not Cx40 or Cx43, suppresses growth of a highly-proliferative cancer cell line by inducing G1 cell cycle accumulation. We further show that Cx37-deficient mice, lacking Cx37's putative growth inhibitory effect on the vasculature, exhibit a more extensive native and post-ischemic collateral circulation, and greater ischemia-induced microvascular density. In addition, Cx37-/- mice demonstrate a functional improvement in recovery over wild-type animals in two models of hindlimb ischemia. By contrast, Cx40-/- mice fail to recover distal limb flow following unilateral hindlimb ischemia, resulting in necrosis. Long-term angiotensin II antagonism normalized post-ischemic hindlimb bloodflow, reduced macrophage infiltration, and delayed (but did not reverse) the necrotic phenotype of these animals. In summary, we show a distinct role for each of the endothelial connexins, Cx37 and Cx40, in regulating post-ischemic vascular responses.

For decades, researchers have sought greater understanding of the educational achievement gap between Blacks and Whites in the U.S. Past studies have concentrated heavily on K-12 attainment, and more recently on that of minority paths to higher education as well as obstacles to academic achievement. Often unnoticed are the interactions between social forces and the individual level psycho-social and cultural factors that may place a significant role; the stigmatization and resultant marginalization of Black students by negative stereotypes that classify them as intellectually inferior. This study explores African American and Black African highs school students' perceptions of negative stereotypes placed upon them through the conceptual frameworks of critical race theory (CRT) and the multidimensional model of racial identity (MMRI). Examining differences by immigrant status, this study seeks to uncover the intersection between the socially constructed images assigned to stigmatized groups differently influenced by negative stereotypes of Blacks and the subsequent influence on the students' perceptions and expectations of higher education. The narratives of this study illustrate the complexity of and interplay between external forces, minority youth social identities and pathways to academic attainment. This study finds that African American and Black African youth have multiple social identities that are not always reflective of the most accessible one of race. This study finds that salient social identities, personal or vicarious experiences of discrimination and being negatively stereotyped shape Black youths' individual aspirations and strategies for achievement. The present study calls into question the claim that Black youth process and respond to negative stereotypes of Blacks in a predictable manner and that these students respond to them independently of other social forces such as their families and communities in which they reside.

There is a widespread intuition that physicalist theories of consciousness are importantly incomplete. But the psychological facts give us reason to think that the gap-intuition does not justify the belief that physical theories of consciousness always leave out some facts about consciousness. I target this belief, and aim to establish that it is not epistemically justified by the gap-intuition. I begin by making a case for thinking that a purely psychological analysis of the "explanatory gap" is not only a viable one, but is in many ways preferable to the standard modal-epistemological analysis. Then I marshal a body of empirical findings in support of the view that various sub-personal psychological processes play a key role in producing the gap-intuition. The most crucial of these processes is the agent-detector, a cognitive system specifically dedicated to detecting other conscious agents in the third-person mode. Leveraging this account, I argue that while the relevant sub-personal processes are generally accurate, in the case of the gap-intuition they are "tricked" in a manner analogous to a visual blind spot or a bias in decision-making. Thus gap-intuitions are not trustworthy and do not confer justification upon belief in a "real" gap. I conclude by situating my account within the context of existing literature on the explanatory gap. My account naturally complements various physicalist accounts of the gap, and also deserves consideration as an outright replacement for such accounts. The overall lesson is that the gap-intuition would arise whether or not physicalist theories of consciousness really do leave something out, and would persist even if we came to accept a true physicalist theory of consciousness. Thus anti-physicalist arguments that are based on the gap-intuition pose no serious threat to physicalist theories of consciousness.

Labor economists have persistently observed a gender gap in both wages and employment within the STEM fields. The prevalence of sexism and the lack of female representation within the technology industry, in particular, has recently been the subject of much media attention. This study seeks to determine the extent of the wage and employment gender gap in this field by examining the labor market outcomes of people who graduated college with a degree in computer science. Using data from the American Community Survey (2009-2011), I compare the aforementioned gender gaps among people with a computer science degree to those among people with a male-dominated, female-dominated, or gender-mixed degree. I also attempt to ascertain if there is an age cohort effect on these gender gaps. I find that, for almost all labor market outcomes, the gender gap among those with a computer science degree is smaller than the gender gap among those with another degree. Furthermore, I find that the gender disparity for those with a computer science degree improves across age cohorts. I thus conclude that women who choose to pursue a degree in computer science actually experience greater equality in terms of wages and employment relative to women who choose to pursue a degree in another area of study. This may be attributed to the high workplace flexibility offered to those working in computer science occupations.

This thesis describes a novel device, the Virtual Air Gap Variable Reactor (VAG-VR), which is capable of producing a continuously variable reactance by locally saturating a small section of the reactor core via an embedded dc control winding. Variable Reactors have many applications in the power industry such as control of line power flow, voltage regulation, reactive line compensation and limiting inrush currents. A variable reactor is most commonly implemented as a thyristor controlled reactor (TCR) by switching in and out a constant reactance to achieve an averaged variable reactance. By using a virtual air gap, a continuously variable reactance is possible. The VAG-VR offers a better dynamic response, without introducing the harmonics created by the thyristor switching of a TCR. The VAG-VR gives low triplen harmonics and therefore allows control of reactive power in single phase or unbalanced three phase systems as would be required in the distribution system. An experimental prototype VAG-VR was developed to investigate three main performance measures: steady state performance, dynamic response and harmonic performance. Over the operating range of the VAG-VR inductance was varied from 100% to 9% of its original value. The dynamic response of the VAG-VR is approximately one tenth of a cycle. This compares favorably to a TCR which responds in approximately half a cycle. Harmonics are also shown to be significantly reduced in the VAG-VR compared to the TCR. A dynamic model of the VAG-VR, suitable for incorporation into power system simulations, was developed and validated. Parameters were determined both experimentally and through finite element method (FEM) simulations. Both experimental and simulation results indicate that the VAG-VR offers a technically viable alternative to the TCR.

This thesis presents the design of a two-dimensional phononic band gap crystal simulator, and phononic crystal analysis. <br /><br /> Phononic crystals and their application to microwave acoustic filters are studied. Wave mechanics is introduced. A two-dimensional phononic crystal simulator is developed. Simulator operation is validated through comparison with published data. Design parameters for phononic crystal band gap engineering are outlined. Digital signal processing and wave mechanics are utilized to analyze fractal and circular inclusion based phononic crystals. Topics for further study are given. <br /><br /> Phononic crystal band structure is found to be sensitive to inclusion boundary geometry. Fractal inclusion based crystals provide multiple pass band characteristics. The evolution of a fractal inclusion in a phononic crystal may cause band gap widening and the formation of new band gaps. Circular inclusion based phononic crystals have piecewise-linear phase characteristics and quality factors up to 600.

A revised lithostratigraphy and glacial history of north Hertfordshire and south Bedfordshire is based upon detailed textural data in the clay to fine gravel fraction, carbonate content, small clast lithological data and macrofabrics, derived from laboratory and field analyses of tills from 30 sites. These include four deep boreholes sunk within the Hitchin Gap. A range of statistical procedures was used, including multivariate analysis of the petrographic properties, enabling the identification of tills from two separate incursions into the Gap. A further till-type was identified in south Bedfordshire indicating an ice advance from the northwest/NNW extending at least as far east as Milton Bryan. Statistical comparison with tills in the neighbouring Vale of St. Albans suggested the presence of the Ware Member till within the Gap. Two hypotheses are suggested to explain variations in lithological content of tills north of the Chalk scarp. The first envisages ice entering the study area along the different trajectories suggested by Fish and Whiteman (2001). During the early part of the glaciation, ice reaching the west of the study area would approach from the north, crossing a shorter distance over Chalk bedrock and collecting less chalk and flint than ice moving into the eastern part of the study area. The second hypothesis invokes an incursion of ice from a northwest - NNW direction into the west of the study area, depositing a chalk-free till. This is subsequently assimilated by ice from the northeast, resulting in the final deposition of a homogeneous mixture of debris from the two advances, with a lower chalk content than tills found to the east. The outcome of either of these scenarios is a till with a low acid-soluble content and low flint/quartz ratio in the west of the study area, as found during this work. Within the Hitchin Gap, a lobe of ice, probably an early part of the northeasterly advance, deposited a lower till. This is considered to be earlier than the Ware Member till and has more variable lithological characteristics and a finer matrix that the higher tills. The latter are mainly melt-out, flow or slumped tills with occasional instances of lodgement and deformation. They represent in situ wasting of dead ice within the Gap. Surface tills in the Gap form a continuum with tills to the north and comprise mainly deformation tills, deposited by the final northeasterly re-advance of ice responsible for widespread coverage of the region, with the exception of the Chiltern Hills southwest of Hitchin. No evidence is found of more than one lithostratigraphic unit of till outside the Hitchin Gap.

Continuous Cover Forestry (CCF) has become increasingly popular since the early 1990s. CCF utilises several silvicultural techniques in order to promote and enhance forest structural diversity and favours natural regeneration. As CCF is relatively new to the UK there are still areas of knowledge regarding management interventions that need to be improved upon. This study utilises simple models, seedling physiology and a hybrid gap model and applies them to the Glentress Trial Area which has been under transformation from even-aged forestry since 1952. These efforts have led to an improved understanding of thinning interventions and the effects they may have on future stand structure. Since the formation of the Forestry Commission in 1919, clearfell-replant forestry has been the main form of management practiced in the UK. CCF management differs in several respects and is commonly practiced using expert knowledge in Continental Europe. In the UK the knowledge-base is still growing and therefore simple models can prove useful for guiding management. This study investigated the use of the idealised reverse-J and the Equilibrium Growing Stock (EGS). This study found that the reverse-J shaped diameter distribution is maintained at the Trial, Block and sub- Block scale indicating that an irregular structure is being approached. In addition, the diminution coefficient, a parameter of the reverse-J distribution, falls within values typical of continental Europe. Comparison of the actual diameter-frequency distribution against an ideal reverse-J distribution can inform both thinning intensity and which diameter classes to target. The EGS, which is a volume–diameter distribution, examines standing volume and how that volume is distributed across three broad diameter classes. Typical distributions from the Swiss Jura indicate that percentage volume should be split 20:30:50 across diameter classes. The EGS analysis showed that standing volume in the Trial Area is much lower than European values at just 174 m3 ha-1. In addition, the classic 20:30:50 percent split was not observed. The 1990 data set showed a 49:43:8 distribution but by 2008 it was 40:41:19. As natural regeneration is favoured in CCF a better understanding of seedling physiology is essential. This study established open (15-35 m2 ha-1) and closed canopy plots (>35 m2 ha-1). Plot characteristics were recorded and then seedlings were selected for physical measurements, chlorophyll fluorescence and gas-exchange measurements. There were clear differences between the physical characteristics with a mean Apical Dominance Ratio (APR) of 1.41 for the open plots and 0.9 for the closed plots which is consistent with previous studies suggesting an APR of 1 is needed for successful regeneration. The chlorophyll fluorescence measurements showed a linear relationship with PAR. However, although the results of the gas-exchange measurements showed an increase in photosynthetic rates with PAR for open plots, there was no obvious relationship in the closed plots. As a result, the study did not find a linear relationship between photosynthetic rate and chlorophyll fluorescence. Finally a complex, hybrid gap model was used to investigate the effects of management on predicted future stand structure. The hybrid gap model, PICUS v1.41, was parameterised for Sitka spruce. The model was used to explore different management scenarios on stand structure over two time periods; 1954-2008 and 1952- 2075. The output from the group selection with underplanting scenario, which resembled the actual management, produced realistic output that was comparable to the stand characteristics measured during the 2008 assessment. The output from the 1952-2075 runs suggested that thinning to a residual basal area suitable to allow natural regeneration (<30 m2 ha-1) or a group selection with underplanting were the best management options for maintaining structural diversity.