Quantum dot polarized light sources

The design, operation and performance of quantum dot spin-polarized vertical cavity surface emitting lasers (VCSELs) and single-photon sources are described and discussed. The effects of spin-induced gain anisotropy on output polarization and threshold current reduction have been studied along with the high-frequency response in a spin-polarized VCSEL. While the output circular polarization in a VCSEL follows the out-of-plane magnetization characteristics of the ferromagnetic spin injector, the output polarization of the spin-polarized single-photon source shows a switching behavior which is explained by invoking the exciton fine structure in the quantum dots and the effects of electron–hole exchange splitting due to in-plane quantum dot rotational asymmetry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90780/1/0268-1242_26_1_014002.pd

Distribution of dwell times of a ribosome: effects of infidelity, kinetic proofreading and ribosome crowding

Ribosome is a molecular machine that polymerizes a protein where the sequence of the amino acid residues, the monomers of the protein, is dictated by the sequence of codons (triplets of nucleotides) on a messenger RNA (mRNA) that serves as the template. The ribosome is a molecular motor that utilizes the template mRNA strand also as the track. Thus, in each step the ribosome moves forward by one codon and, simultaneously, elongates the protein by one amino acid. We present a theoretical model that captures most of the main steps in the mechano-chemical cycle of a ribosome. The stochastic movement of the ribosome consists of an alternating sequence of pause and translocation; the sum of the durations of a pause and the following translocation is the time of dwell of the ribosome at the corresponding codon. We derive the analytical expression for the distribution of the dwell times of a ribosome in our model. Whereever experimental data are available, our theoretical predictions are consistent with those results. We suggest appropriate experiments to test the new predictions of our model, particularly, the effects of the quality control mechanism of the ribosome and that of their crowding on the mRNA track.Comment: This is an author-created, un-copyedited version of an article accepted for publication in Physical Biology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher authenticated version is available online at DOI:10.1088/1478-3975/8/2/02600

High Temperature Spintronic Devices.

Spintronics is a rapidly developing multidisciplinary field which investigates avenues of exploiting the spin degree of freedom in charge carriers and the nucleus of atoms to design novel devices that would outperform existing ones based on complementary-metal-oxide-semiconductor (CMOS) technology. Spin-polarized semiconductor based coherent light sources promise an ability to predict, stabilize and control orthogonal polarization states with reduced input power requirements. This doctoral thesis investigates different aspects of design, epitaxial growth, fabrication, characterization and modeling of spin based electronic and optoelectronic devices working near room temperature. We have demonstrated the modulation of magnetoresistance in an InAs/ In0.53Ga0.47As/ In0.52Al0.48As lateral spin valve with a gate electrode placed alongside the MnAs polarizer contact and outside the current transport channel. The results indicate that the change in magnetoresistance is caused, in part, by Rashba spin-orbit coupling due to the gate bias. In order to achieve higher working temperatures devices for real world applications MnAs/GaAs/MnAs based vertical spin valves have been realized. We have attributed the near room temperature operation of these devices to valence band electron tunneling of spin polarized carriers in and out of a heavily p-doped GaAs:Mn layer. Peak magnetoresistance of 40 % and 1 % have been observed at 10 K and 300 K respectively. The continuous wave, transient and high frequency dynamics of spin-polarized carriers and photons in a spin laser have been studied. Besides lowering of threshold currents, it has been theoretically estimated that these devices can show larger small-signal modulation bandwidth and 100% output polarization, independent of the injected carrier spin polarization, under appropriate biasing conditions. Measurements were done at 230 K on a InAs/GaAs quantum dot spin vertical cavity surface emitting laser (VCSEL). A time-averaged output polarization of 55% is measured with an active region spin polarization of 5-6%. A peak threshold current reduction of 4.5 % and a polarization modulation index of 0.6 have been measured in these devices under continuous wave bias. The measured output characteristics, both DC and transient, match very well with those calculated from theory.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78952/1/debasu_1.pd

Stochastic theory of protein synthesis and polysome: ribosome profile on a single mRNA transcript

The process of polymerizing a protein by a ribosome, using a messenger RNA (mRNA) as the corresponding template, is called {\it translation}. Ribosome may be regarded as a molecular motor for which the mRNA template serves also as the track. Often several ribosomes may translate the same (mRNA) simultaneously. The ribosomes bound simultaneously to a single mRNA transcript are the members of a polyribosome (or, simply, {\it polysome}). Experimentally measured {\it polysome profile} gives the distribution of polysome {\it sizes}. Recently a breakthrough in determining the instantaneous {\it positions} of the ribosomes on a given mRNA track has been achieved and the technique is called {\it ribosome profiling} \cite{ingolia10,guo10}. Motivated by the success of these techniques, we have studied the spatio-temporal organization of ribosomes by extending a theoretical model that we have reported elsewhere \cite{sharma11}. This extended version of our model incorporates not only (i) mechano-chemical cycle of individual ribomes, and (ii) their steric interactions, but also (iii) the effects of (a) kinetic proofreading, (b) translational infidelity, (c) ribosome recycling, and (d) sequence inhomogeneities. The theoretical framework developed here will serve in guiding further experiments and in analyzing the data to gain deep insight into various kinetic processes involved in translation.Comment: Minor revisio

Gate control and amplification of magnetoresistance in a three-terminal device

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98686/1/ApplPhysLett_99_152503.pd

Provincial screening rates for chronic diseases of lifestyle, cancers and HIV in a health-insured population

Background. Screening for asymptomatic diseases can reduce the burden of morbidity and mortality in all population groups. There is widespread geographical variation in the quality of care. Few data are available on national screening rates in South Africa and how these vary across the provinces. Objective. To examine screening rates for chronic diseases of lifestyle (CDL), HIV and cancer in a privately insured population for a single insurer across all nine provinces in South Africa, and to determine whether or not there are any differences between the provinces. Method. Screening rates were calculated as the proportion of eligible members who had received screening tests during 2011 in each province. Mean screening rates were compared between Gauteng and the other eight provinces. Results. Nationwide screening rates were 20.5% for CDL, 8.2% for HIV and 31.9% for cancer. Despite similar insurance coverage, screening rates ranged from 0.3% to 0.95% lower in other provinces compared with Gauteng. Of all the provinces, Gauteng had the highest annual screening rates for CDL, breast cancer, prostate cancer and HIV (

Basic science232. Certolizumab pegol prevents pro-inflammatory alterations in endothelial cell function

Background: Cardiovascular disease is a major comorbidity of rheumatoid arthritis (RA) and a leading cause of death. Chronic systemic inflammation involving tumour necrosis factor alpha (TNF) could contribute to endothelial activation and atherogenesis. A number of anti-TNF therapies are in current use for the treatment of RA, including certolizumab pegol (CZP), (Cimzia ®; UCB, Belgium). Anti-TNF therapy has been associated with reduced clinical cardiovascular disease risk and ameliorated vascular function in RA patients. However, the specific effects of TNF inhibitors on endothelial cell function are largely unknown. Our aim was to investigate the mechanisms underpinning CZP effects on TNF-activated human endothelial cells. Methods: Human aortic endothelial cells (HAoECs) were cultured in vitro and exposed to a) TNF alone, b) TNF plus CZP, or c) neither agent. Microarray analysis was used to examine the transcriptional profile of cells treated for 6 hrs and quantitative polymerase chain reaction (qPCR) analysed gene expression at 1, 3, 6 and 24 hrs. NF-κB localization and IκB degradation were investigated using immunocytochemistry, high content analysis and western blotting. Flow cytometry was conducted to detect microparticle release from HAoECs. Results: Transcriptional profiling revealed that while TNF alone had strong effects on endothelial gene expression, TNF and CZP in combination produced a global gene expression pattern similar to untreated control. The two most highly up-regulated genes in response to TNF treatment were adhesion molecules E-selectin and VCAM-1 (q 0.2 compared to control; p > 0.05 compared to TNF alone). The NF-κB pathway was confirmed as a downstream target of TNF-induced HAoEC activation, via nuclear translocation of NF-κB and degradation of IκB, effects which were abolished by treatment with CZP. In addition, flow cytometry detected an increased production of endothelial microparticles in TNF-activated HAoECs, which was prevented by treatment with CZP. Conclusions: We have found at a cellular level that a clinically available TNF inhibitor, CZP reduces the expression of adhesion molecule expression, and prevents TNF-induced activation of the NF-κB pathway. Furthermore, CZP prevents the production of microparticles by activated endothelial cells. This could be central to the prevention of inflammatory environments underlying these conditions and measurement of microparticles has potential as a novel prognostic marker for future cardiovascular events in this patient group. Disclosure statement: Y.A. received a research grant from UCB. I.B. received a research grant from UCB. S.H. received a research grant from UCB. All other authors have declared no conflicts of interes

Traffic of interacting ribosomes: Effects of single-machine mechanochemistry on protein synthesis

Many ribosomes simultaneously move on the same messenger RNA (mRNA), each separately synthesizing the protein coded by the mRNA. Earlier models of ribosome traffic represent each ribosome by a ``self-propelled particle'' and capture the dynamics by an extension of the totally asymmetric simple exclusion process (TASEP). In contrast, here we develope a theoretical model that not only incorporates the mutual exclusions of the interacting ribosomes, but also describes explicitly the mechano-chemistry of each of these individual cyclic machines during protein synthesis. Using analytical and numerical techniques of non-equilibrium statistical mechanics, we analyze this model and illustrate its power by making experimentally testable predictions on the rate of protein synthesis in real time and the density profile of the ribosomes on some mRNAs in E-Coli.Comment: 4 pages including 3 EPS figure