Properties of III-Nitride-Based Polariton and Spin Polariton Diode Lasers

The cavity electrodynamic regime of strong coupling of emitter-photon interactions in a semiconductor microcavity gives rise to new light-matter entangled quasiparticles, also known as exciton-polaritons. The non-linear nature of the energy-momentum dispersions of these composite bosons has been suitably engineered and efficiently utilized to demonstrate inversionless coherent emission, or polariton lasing in submicron-scale optical cavities. Previous theoretical as well as experimental work on Gallium Arsenide and Cadmium Telluride-based systems operated at cryogenic temperatures, have shown the central importance of the nature of the output polarization of the emitted light originating from the radiative decomposition of these polaritons. Room-temperature operation of these lasers necessitates the use of wide-band gap semiconductors such as Gallium Nitride, because of their large free excitonic binding energies and oscillator strengths, which consequently lead to stronger and more robust exciton-photon strong coupling. Thus, the steady state output polarization characteristics of Gallium Nitride-based microcavity polariton lasers operated with unpolarized electrical injection, have been examined at room temperature. The output is essentially unpolarized below the nonlinear threshold injection current and is linearly polarized above it with a maximum degree of polarization of ∼ 22%. Besides other advantages, a spin-polarized laser offers inherent control of the output circular polarization. Electrical spin injection in a bulk Gallium Nitride-based microcavity polariton diode laser enables the realization of an electrically modulated low-energy circularly-polarized coherent light source. Successful electrical spin injection in bulk Gallium Nitride, which is the active layer of the polariton diode laser, has been independently confirmed from room-temperature four-terminal Hanlè spin precession measurements made on Gallium Nitride-based spin valves, and observation of hysteretic circular polarization in III-nitride-based light-emitting diodes. The optical selection rules governing the operation of the latter have also been elucidated. Electrical injection of spin polarized electrons is accomplished in all the above-mentioned devices via a n-type Cobalt Iron alloy/Magnesium Oxide spin injector contact. The output polarization characteristics of this polariton diode laser have been examined at room temperature. A degree of output circular (linear) polarization of ~ 25 (33) % is recorded under remanent magnetization. The helicity as well as the degree of the steady-state circular polarization is deterministically governed by the magnetizing field used to magnetize the ferromagnetic contacts. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed employing two distinct spin-dependent rate equation models, and there is good agreement between measured and calculated data in both cases. The present work also theoretically explores other optoelectronic properties of these spin polariton lasers. Optical effects arising from spin-induced gain anisotropy such as threshold reduction and emission intensity enhancement have been theoretically predicted for these diode lasers. An electrical excitation mechanism has also been formulated, which can potentially magnify the degree of a deterministic circular polarization of the output emission by an order of magnitude, compared to the injected electron spin polarization. The dissertation concludes with the discussion of the observation of a non-linear enhancement in the excitation-dependent photocurrent characteristics of the microcavity diodes with a threshold, which is consistent with the polariton lasing threshold. This is explained in the framework of an Auger-like process of excitonic dissociation into its constituent electron-hole pairs, which can be stimulated by the occupation of the polariton lasing states and the observed effect is therefore a unique manifestation of the bosonic final-state stimulation effect in polariton lasers.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145800/1/anirudb_1.pd

Early clinical exposure to first year medical students through case-based learning in endocrine physiology

Background:As per the Medical Council of India’s Vision-2015 document, coordinated inter-departmental efforts should be undertaken to provide early clinical exposure and to develop communication skills among medical students during the first year of Bachelor of Medicine, Bachelor of Surgery (MBBS) course. For this reason, this inter-departmental study was undertaken, to provide early clinical exposure while teaching endocrine physiology to first-year medical students. Case based learning (CBL) was used as an add-on to traditional didactic lectures (TDLs) for teaching endocrine physiology to first-year undergraduate medical students to compare the cognitive domain scores in pre CBL and post CBL tests.Methods: After getting permissions from the Institutional Ethics Committee (IEC) and other institutional authorities for conducting the study, the objectives of the study were explained to first year MBBS students. Written informed consent was taken from students (n=59) who were willing to participate in the study. After delivering regular TDLs on endocrine physiology as per topics in the syllabus, a pre CBL test was conducted. Subsequently, the participating students were simultaneously exposed to case scenarios related to the endocrine system by the same facilitator. A post CBL test (identical to pre CBL test) was administered to compare the cognitive domain scores.Results: Statistically significant differences were found in the student wise and question wise scores in the pre and post tests.Conclusions: Early clinical exposure of a relatively larger batch of students to CBL by the same facilitator was found to increase cognitive domain scores.

WW-triviality of low dimensional manifolds

A space $X$ is $W$-trivial if for every real vector bundle $\alpha$ over $X$ the total Stiefel-Whitney class $w(\alpha)=1$. It follows from a result of Milnor that if $X$ is an orientable closed smooth manifold of dimension $1,2,4$ or $8$, then $X$ is not $W$-trivial. In this note we completely characterize $W$-trivial orientable closed smooth manifolds in dimensions $3,5$ and $6$. In dimension $7$, we describe necessary conditions for an orientable closed smooth $7$-manifold to be $W$-trivial.Comment: 10 page

Effect of case-based learning in reproductive physiology on cognitive domain scores of first-year medical students in Western India

Background: The Medical Council of India has recommended early clinical exposure, problem-oriented approach and case-based learning throughout the graduate medical curriculum. Case-based learning is a teaching-learning model that helps effective use of student and faculty time.Methods: This complete-enumeration, before-and-after type of educational intervention study (without controls) was conducted in a municipal medical college in Western India. After explaining the objectives of the study to first-year medical students, aged 18 years and above, of either sex, written informed consent was obtained from students (n=55) who were willing to participate in the study. The pre-test was conducted after lectures by faculty from the Departments of Physiology and Community Medicine. An identical post-test was administered after case-based learning, which was conducted by the same faculty in two sub-groups. The outcome studied was the difference in cognitive domain scores after attending lectures (by a pre-test) and case-based learning (by a post-test).Results: The overall mean score increased from 5.36 ± 0.97 (95% CI: 5.11 - 5.62) in the pre-test to 6.49 ± 1.14 (95% CI: 6.19 - 6.79) in the post-test. The differences between the pre- and post-test correct responses were statistically significant for two questions.Conclusions: The participating first-year medical students had adequate basic knowledge of reproductive physiology. Gender differences in correct responses were statistically significant for few questions. Use of case scenarios enhanced cognitive domain scores

Studies on Rain Structure Based on Ground Based Dropsize Distribution and Rain Attenuation Measurements over an Earth Space Path

Abstract The drop size distribution (DSD) of rain in tropical region shows the characteristic features of different types of rain, namely, stratiform, transitional and convective. This is also indicated in behaviors of the rain decay parameter, obtained from the measurements of rain rate and attenuation over an earth-space path. The DSD parameters and rain decay parameter give similar clustering indicating the feature of convective and stratiform rain. To predict rain attenuation from rain rate, the decay parameter is modeled for different rain rate regions that provides a good matching between the predicted and measured values