Optical Precursors in Rubidium Vapor and Their Relation to Superradiance

Optical precursor is the sharp optical pulse front that does not show delay in absorptive media. In this thesis, optical precursor behavior in rubidium (Rb) vapor was investigated in the picoseconds regime. An amplified femtosecond laser was shaped to a 7-ps square pulse with sharp rising and trailing edges. This pulse was then sent into a hot rubidium vapor, and the center frequency of the laser pulse was absorbed. The output pulses were measured by a fast streak camera with 2-picosecond resolution. By varying the temperature of the Rb vapor, the measured pulse shapes showed the progression of formation of optical precursors. The measured pulses shapes showed good agreement with theory. On the other hand, a connection between optical precursors and femtosecond laser pumped 3-photon superradiance was investigated in this thesis. Maxwell-Bloch equations were numerically solved in two steps with commercial software Mathematica 8. A good agreement was found between simulation and experiment. It was confirmed that, at low excitation regime, superradiance generated from hot rubidium vapor, which were pumped by a femtosecond laser, can be understood as the formation of optical precursors

Exploring diastereoselectivity mechanism of L-threonine aldolase

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A step-by-step modelling approach for SiC half-bridge modules considering temperature characteristics

In this paper, a detailed step-by-step modellingapproach is proposed for Silicon Carbide (SiC) MOSFET half-bridge power modules. The drain-to-source current, anti-paralleldiode and parasitic capacitors are accurately modelled consider-ing temperature dependency. A step-by-step parameter extractionmethod based on datasheet is introduced. A SPICE model is builtbased on the proposed modelling approach for a commercialpower module. The model is verified by comparing experimentand PSpice simulation results of the same double pulse tester(DPT), which proves the effectiveness of the modelling approachfor analysing switching losses and converter design. The proposedmodelling approach can help the converter designers quickly andaccurately develop their own models for SiC MOSFET powermodules

Topological Photonic Phase in Chiral Hyperbolic Metamaterials

Recently the possibility of achieving one-way backscatter immune transportation of light by mimicking the topological order present within certain solid state systems, such as topological insulators, has received much attention. Thus far however, demonstrations of non-trivial topology in photonics have relied on photonic crystals with precisely engineered lattice structures, periodic on the scale of the operational wavelength and composed of finely tuned, complex materials. Here we propose a novel effective medium approach towards achieving topologically protected photonic surface states robust against disorder on all length scales and for a wide range of material parameters. Remarkably, the non-trivial topology of our metamaterial design results from the Berry curvature arising from the transversality of electromagnetic waves in a homogeneous medium. Our investigation therefore acts to bridge the gap between the advancing field of topological band theory and classical optical phenomena such as the Spin Hall effect of light. The effective medium route to topological phases will pave the way for highly compact one-way transportation of electromagnetic waves in integrated photonic circuits.Comment: 11 pages, 3 figures. To appear in PR

Three dimensional photonic Dirac points in metamaterials

Topological semimetals, representing a new topological phase that lacks a full bandgap in bulk states and exhibiting nontrivial topological orders, recently have been extended to photonic systems, predominantly in photonic crystals and to a lesser extent, metamaterials. Photonic crystal realizations of Dirac degeneracies are protected by various space symmetries, where Bloch modes span the spin and orbital subspaces. Here, we theoretically show that Dirac points can also be realized in effective media through the intrinsic degrees of freedom in electromagnetism under electromagnetic duality. A pair of spin polarized Fermi arc like surface states is observed at the interface between air and the Dirac metamaterials. These surface states show linear k-space dispersion relation, resulting in nearly diffraction-less propagation. Furthermore, eigen reflection fields show the decomposition from a Dirac point to two Weyl points. We also find the topological correlation between a Dirac point and vortex/vector beams in classic photonics. The theoretical proposal of photonic Dirac point lays foundation for unveiling the connection between intrinsic physics and global topology in electromagnetism.Comment: 15 pages, 5 figure

A Likelihood Approach to Incorporating Self-Report Data in HIV Recency Classification

Estimating new HIV infections is significant yet challenging due to the difficulty in distinguishing between recent and long-term infections. We demonstrate that HIV recency status (recent v.s. long-term) could be determined from the combination of self-report testing history and biomarkers, which are increasingly available in bio-behavioral surveys. HIV recency status is partially observed, given the self-report testing history. For example, people who tested positive for HIV over one year ago should have a long-term infection. Based on the nationally representative samples collected by the Population-based HIV Impact Assessment (PHIA) Project, we propose a likelihood-based probabilistic model for HIV recency classification. The model incorporates both labeled and unlabeled data and integrates the mechanism of how HIV recency status depends on biomarkers and the mechanism of how HIV recency status, together with the self-report time of the most recent HIV test, impacts the test results, via a set of logistic regression models. We compare our method to logistic regression and the binary classification tree (current practice) on Malawi, Zimbabwe, and Zambia PHIA data, as well as on simulated data. Our model obtains more efficient and less biased parameter estimates and is relatively robust to potential reporting error and model misspecification

WebGIS-based Natural Disaster Information System of China

We have constructed China natural disaster information system utilizing open source WebGIS framework. Through comparisons and researches on different WebGIS system structures, implementation patterns, spatial database management patterns and several popular software, open source WebGIS platform MapServer, PostgreSQL+PostGIS (spatial database and spatial data engine), client-side browser framework openLayer were used. This system have basic function of web information publishing, spatial data storage and management, and it collects natural disaster information that have been publishing on the internet through grabing internet information, and based on the analysis to the text information, different kinds of natural disaster information databases were constructed. Additionally, locations of the disaster occurred were combined to provide map services, and the system is able to meet multi-user access issues, and have functions of thematic mapping, query and statistics, charting and output, et.al

Users’ Perception and Utility of Health Information Based on WeChat

Objective: To explore the ability of users to identify the health information spread on WeChat platform, and then to discuss the utility of such information on them. Methods: Questionnaire survey and descriptive statistical methods were used to collect and analyze the data. The process of “get health related information, judge true or false, production related behavior” was used to design the survey problem, finally, the paper and network surveys were investigated in users. Results: The proportion of accurate recognition, ambiguous recognition, none recognition and none classification were 21.55%, 63.26%, 4.41%, 10.78% respectively. The average frequency of identification, transmission, guidance and none behavior of originally true health information were 52.00, 78.40, 45.20, 31.80 respectively, and originally false health information were 21.30, 27.70, 14.10, 5.50 respectively.Conclusions: Most WeChat users surveyed lack the ability to accurately identify the authenticity of health information, and improvement of citizens’ health literacy has long way to go. WeChat is an effective platform for the dissemination of health information, but also provides a fertile soil for the spread of false health information. Additionally, users are not aware of the problem of their ability to identify information, even if produced a false judgment of false health information, most of the active users will also lead this kind of information still widely spread

A four-leg buck inverter for three-phase four-wire systems with the function of reducing DC-bus ripples

Three-phase four-wire inverters are usually used to feed unbalanced three-phase loads with neutral currents. The unbalanced three-phase loads also bring to second-order ripples in the DC bus, which should be mitigated by bulky DC-bus capacitors to improve the system performance. In this case, the DC capacitance is designed for the second-order ripple frequency instead of the switching frequency, so it can not be reduced even when SiC MOSFETs are adopted to achieve high switching frequency. Although various topologies of three-phase four-wire inverters has been proposed to provide the path for neutral currents, they cannot handle the second-order ripples. Also, some active power decoupling solutions can be adopted, but they require additional active swithes and components, which increases the cost of the system. In this paper, a four-leg buck inverter is proposed, which consists of four DC-DC buck converters. Each buck converter is independently controlled. This topology can not only provide neutral currents, but also reduce the second-order ripples in the DC bus with active power decoupling control. The proposed topology doesn't require any additional active switches comparing to the conventional topologies with neutral legs. The effectiveness of proposed topology is verified by the simulation in MATLAB/Simulink