Multiple Quantum Well Structures As Optical Waveguides

This thesis is concerned with the design, fabrication and characterisation of semiconductor optical waveguides in which the high index guiding layer is a multiple quantum well structure (MQWS), consisting of alternate layers of high and low band gap semiconductors with the electrons and holes in the MQWS being confined to the low band gap material. This confinement in two dimensions alters greatly the electronic and optical properties of the MQWS in comparison to the bulk properties of the constituent layers. The basic concepts involved in MQW waveguides are introduced using an elementary quantum mechanical analysis of quantum wells together with a brief description of the properties of dielectric waveguides, A more detailed treatment of the electronic and optical properties of MQWS and a review of published experimental work is used to show that the fundamental absorption edge is much more abrupt than that in the corresponding bulk material with strong excitonic characteristics being evident even at room temperature. In addition, the absorption edge is seen to be anisotropic with the fundamental energy gap being larger for light polarised perpendicular to the MQW layers. This anisotropic absorption edge, together with the layered dielectric nature of MQWS, makes them birefringent with a smaller refractive index for light polarised perpendicular to the MQW layers. The quantum confinement of carriers in MQWS also enhances their electroabsorption and electro-optic properties through the quantum confined Stark effect. Standard techniques used in the design, fabrication and analysis of bulk semiconductor waveguides are developed for application to MQW waveguides. These include analytical and numerical techniques for the design of dielectric waveguides; dry etching and metallisation processes for the fabrication of devices; and a laser/optics system to analyse the waveguide devices. To verify these techniques they are first applied to the well-understood case of n/n+ GaAs waveguides and are used to successfully fabricate and analyse single-mode, passive, rib waveguides at l=1.15mum. The electro-optic coefficient is also measured in an active, planar n/n+ waveguide and found to be close to that reported by other workers. The design techniques are then applied to MOWS waveguides resulting in the design of a MQW double heterostructure (MQW-DH), p-i-n diode which was predicted to produce the required Quantum properties (strong, room temperature, excitonic behaviour), waveguide properties (single-mode propagation up to the fundamental absorption edge) and electronic properties (a high reverse bias breakdown voltage and uniform applied electric field). Most of the theoretical work and all the experimental work included is devoted to MQWS in the (Al,Ga)As, III-V semiconductor alloy system. Accordingly, the methods available for growing MQWS in this system are reviewed with Molecular Beam Epitaxy (MBE) being found the most likely method to satisfactorily reproduce the desired structure. MQW-DH were grown at two establishments and are initially studied by photoluminescence and scanning electron microscopy before their planar optical waveguide characteristics are checked using the laser system. Only one sample is found to satisfy all the design requirements, and then only partially. Detailed analysis of the properties of MQW waveguides is therefore limited to this structure. Passive MQW-DH waveguides are demonstrated to exhibit an anisotropic absorption edge as predicted, and it is shown that the design and fabrication techniques developed can be successfully used to obtain single, double and multi-mode strip loaded waveguides. Single-mode waveguides are also used to fabricate passive directional couplers with coupling lengths in good agreement with theoretically predicted values. A semi-empirical model is put forward to describe the band edge electro-absorption of MQWS. Although simple, the model is in qualitative and approximate quantitative agreement with published results. (Abstract shortened by ProQuest.)

Can Authority be Sustained while Balancing Accessibility and Formality?

Economics has developed into a quantitative discipline that makes extensive use of mathematical and statistical concepts. When writing a dictionary for economics undergraduates it has to be recognised that many users will not have sufficient training in mathematics to benefi t from formal definitions of mathematical and statistical concepts. In fact, it is more than likely that the user will want the dictionary to provide an accessible version of a definition that avoids mathematical notation. Providing a verbal description of a mathematical concept has the risk that the outcome is both verbose (compared to a definition using appropriate mathematical symbols) and imprecise. For the author of a dictionary this raises the question of how to resolve this conflict between accessibility and formal correctness. We use a range of examples from the Oxford Dictionary of Economics to illustrate this conflict and to assess the extent to which a non-formal definition can be viewed as authoritative

Lake Erie 1993, Western, West Central and Eastern Basins: Change in Trophic Status, and Assessment of the Abundance, Biomass and Production of the Lower Trophic Levels

The western and west central basins were mesotrophic and the eastern basin was oligotrophic, based on many biological and chemical parameters measured in 1993. Gradients were observed for most parameters, with chlorophyll a, nitrogen, phosphorus, silica, and light extinction decreasing from west to east. In the western basin, phytoplankton biomass declined by 51% from 1983-85. Phytoplankton photosynthesis (g C·m-2), predicted from total phosphorus (TP) using a relationship developed in other offshore productivity studies in Lake Ontario, declined by 35% in 1993, without a corresponding decline in phosphorus (P) loading or TP. Diatoms decreased and there was a shift towards smaller phytoplankton species. These changes were attributed to zebra mussel filtration, but were not of sufficient magnitude to reduce zooplankton biomass. In the west central basin, the reductions in phytoplankton biomass were modest. Photosynthesis (g C·m-2) in 1993, was in line with that predicted by TP and the empirical relationship developed in other offshore studies. Limited mussel populations in the west central basin, resulting from low hypolimnetic oxygen concentrations, caused little change in the phytoplankton. There also were no reductions in mean biomass of zooplankton from 1984-87. In the eastern basin, phytoplankton biomass declined by 49% from 1983-85. Photosynthesis (g C·m-2) declined by 50% from the value predicted, from TP and the empirical relationship developed for other studies, for 1983-85, without a decline in P-leading. TP was lower in 1993 and was attributed to filtering by Dreissena and subsequent redirection of pelagic material to the sediments. Phytoplankton species indicative of eutrophy were reduced and there was an overall shift towards smaller species. Zooplankton biomass was also reduced. Mean zooplankton community size and the loss of Daphnia sp. suggest that predation by planktivores as well as a reduced food supply, affected zooplankton biomass in 1993. The Dreissena population also affected the benthic community structure as Diporeia were virtually eliminated from the eastern basin and Gammarus increased in all basins. Benthic biomass was 40% higher on average than in 1979. Dreissena dominated benthic production at all stations except offshore in the west-central basin

Tax Evasion, Social Customs and Optimal Auditing

Abstract: The optimal audit policy is analysed for an independent revenue service when a social custom exists that rewards honest tax-paying. The implication of the existence of the social custom is that in equilibrium the income level of a taxpayer cannot always be inferred exactly from their report. The structure of the optimal audit policy is determined both for a fixed (report-invariant) audit probability and for when the audit probability is a function of the income report. For the constant probability of audit it is shown that an interior solution exists to the decision problem of the revenue service and comparative statics results are given. When the audit probability can vary, the audit function is proved to be a decreasing function of the income report which reaches zero at the highest income report of a tax evader. Increases in the fine for evading and in the tax rate raise the optimal audit probability

A Strategic Approach to Public Health Workforce Development and Capacity Building

In February 2010, CDC’s National Center for HIV/AIDS, Viral Hepatitis, Sexually Transmitted Disease (STD), and Tuberculosis (TB) Prevention (NCHHSTP) formally institutionalized workforce development and capacity building (WDCB) as one of six overarching goals in its 2010–2015 Strategic Plan. Annually, workforce team members finalize an action plan that lays the foundation for programs to be implemented for NCHHSTP’s workforce that year. This paper describes selected WDCB programs implemented by NCHHSTP during the last 4 years in the three strategic goal areas: (1) attracting, recruiting, and retaining a diverse and sustainable workforce; (2) providing staff with development opportunities to ensure the effective and innovative delivery of NCHHSTP programs; and (3) continuously recognizing performance and achievements of staff and creating an atmosphere that promotes a healthy work–life balance. Programs have included but are not limited to an Ambassador Program for new hires, career development training for all staff, leadership and coaching for mid-level managers, and a Laboratory Workforce Development Initiative for laboratory scientists. Additionally, the paper discusses three overarching areas—employee communication, evaluation and continuous review to guide program development, and the implementation of key organizational and leadership structures to ensure accountability and continuity of programs. Since 2010, many lessons have been learned regarding strategic approaches to scaling up organization-wide public health workforce development and capacity building. Perhaps the most important is the value of ensuring the high-level strategic prioritization of this issue, demonstrating to staff and partners the importance of this imperative in achieving NCHHSTP’s mission

The nature of the pandemic:Exploring the negative impacts of the COVID-19 pandemic upon recreation visitor behaviors and experiences in parks and protected areas

The COVID-19 pandemic dramatically affected parks and protected areas and overall recreation visitation across the United States. While outdoor recreation has been demonstrated to be beneficial, especially during a pandemic, the resulting increase in recreation visitation raises concerns regarding the broader influence of social, situational, ecological, and behavioral factors upon overall visitor experiences. This study investigated the extent to which recreation visitors’ behaviors and experiences have been impacted by the COVID-19 pandemic within the White Mountain National Forest (WMNF). A modified drop-off pick-up survey method was employed to collect population-level data from WMNF visitors from June to August of 2020 (n=317), at the height of the pandemic. Results from this mixed-method study suggest social factors (e.g., crowding and conflict), situational factors (e.g., access and closures), ecological factors (e.g., vegetation damage), behavioral factors (e.g., substitution), and sociodemographic factors (e.g., gender and income) significantly influenced overall visitor decision-making and experience quality within the WMNF. For example, more than one-third of visitors indicated the pandemic had either a major or severe impact upon their WMNF recreation experience. A more nuanced investigation of qualitative data determined that the majority of pandemic-related recreation impacts revolved around the themes of social impacts, general negative recreation impacts, situational and ecological impacts, and behavioral adaptation impacts. Moreover, historically marginalized populations (e.g., low-income households and females) within the sample reported significantly higher recreation experience impacts during the pandemic. This study demonstrates the influence of the pandemic upon outdoor recreation visitor experiences and behaviors and considers outdoor recreation as a central component within the broader social-ecological systems framework. This study demonstrates the influence of the pandemic upon outdoor recreation visitor experiences and behaviors and considers resource users a central component within the broader social-ecological systems conceptual framework. MANAGEMENT IMPLICATIONS: This study found that during the peak of the COVID-19 pandemic, social, situational, ecological, behavioral, and sociodemographic factors significantly influenced overall visitor decision-making andexperience quality: · Social and general recreation impacts were most common, with approximately 56% of the sample reporting these issues. · Results suggest significant crowding and conflict impacts stemmed from interactions between in-state and out-of-state visitors, largely based upon perceived violations of pandemic protocols. · Moreover, historically marginalized populations stated unique recreation impacts during the pandemic. For instance, visitors from low-income households reported significantly less substitution options as opposed to high-income visitors. · Female visitors perceived significantly more pandemic-related conflict than male visitors. Study findings suggest visitor crowding and conflict should be prioritized by resource managers, especially amongst historically marginalized populations. Resource managers should consider adopting a broader social-ecological systems approach to parks and protected areas management, particularly during a global pandemic

The projected impact of the COVID-19 lockdown on breast cancer deaths in England due to the cessation of population screening: a national estimation

BACKGROUND: Population breast screening services in England were suspended in March 2020 due to the COVID-19 pandemic. Here, we estimate the number of breast cancers whose detection may be delayed because of the suspension, and the potential impact on cancer deaths over 10 years. METHODS: We estimated the number and length of screening delays from observed NHS Breast Screening System data. We then estimated additional breast cancer deaths from three routes: asymptomatic tumours progressing to symptomatically diagnosed disease, invasive tumours which remain screen-detected but at a later date, and ductal carcinoma in situ (DCIS) progressing to invasive disease by detection. We took progression rates, prognostic characteristics, and survival rates from published sources. RESULTS: We estimated that 1,489,237 women had screening delayed by around 2–7 months between July 2020 and June 2021, leaving 745,277 outstanding screens. Depending on how quickly this backlog is cleared, around 2500–4100 cancers would shift from screen-detected to symptomatic cancers, resulting in 148–452 additional breast cancer deaths. There would be an additional 164–222 screen-detected tumour deaths, and 71–97 deaths from DCIS that progresses to invasive cancer. CONCLUSIONS: An estimated 148–687 additional breast cancer deaths may occur as a result of the pandemic-related disruptions. The impact depends on how quickly screening services catch up with delays

National critical incident reporting systems relevant to anaesthesia: a European survey

Background Critical incident reporting is a key tool in the promotion of patient safety in anaesthesia. Methods We surveyed representatives of national incident reporting systems in six European countries, inviting information on scope and organization, and intelligence on factors determining success and failure. Results Some systems are government-run and nationally conceived; others started out as small, specialty-focused initiatives, which have since acquired a national reach. However, both national co-ordination and specialty enthusiasts seem to be necessary for an optimally functioning system. The role of reporting culture, definitional issues, and dissemination is discussed. Conclusions We make recommendations for others intending to start new systems and speculate on the prospects for sharing patient safety lessons relevant to anaesthesia at European leve

Cognition in schizophrenia improves with treatment of severe obstructive sleep apnoea: a pilot study

Previous studies have shown that people with schizophrenia have high rates of Obstructive Sleep Apnoea (OSA). Despite this, intervention studies to treat OSA in this population have not been undertaken. The ASSET (Assessing Sleep in Schizophrenia and Evaluating Treatment) pilot study investigated Continuous Positive Airway Pressure (CPAP) treatment of severe OSA in participants recruited from a clozapine clinic in Adelaide. Participants with severe untreated OSA (Apnoea-Hypopnoea Index (AHI) > 30), were provided with CPAP treatment, and assessed at baseline and six months across the following domains: physical health, quality of sleep, sleepiness, cognition, psychiatric symptoms and CPAP adherence. Six of the eight ASSET participants with severe OSA accepted CPAP. At baseline, half of the cohort had hypertension, all were obese with a mean BMI of 45, and they scored on average 1.47 standard deviations below the normal population in cognitive testing. The mean AHI was 76.8 and sleep architecture was markedly impaired with mean rapid eye movement (REM) sleep 4.1% and mean slow wave sleep (SWS) 4.8%. After six months of treatment there were improvements in cognition (BACS Z score improved by an average of 0.59) and weight loss (mean weight loss 7.3 ± 9 kg). Half of the participants no longer had hypertension and sleep architecture improved with mean REM sleep 31.4% of the night and mean SWS 24% of the night. Our data suggests CPAP may offer novel benefits to address cognitive impairment and sleep disturbance in people with schizophrenia.Hannah Myles, Nicholas Myles, Ching Li Chai Coetzer, Robert Adams, Madhu Chandratilleke, Dennis Liu, Jeremy Mercer, Andrew Vakulin, Andrew Vincent, Gary Wittert, Cherrie Galletl