Ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting laser devices

Spin-polarized lasers offer new encouraging possibilities for future devices. We investigate the polarization dynamics of electrically pumped vertical-cavity surface-emitting lasers after additional spin injection at room temperature. We find that the circular polarization degree exhibits faster dynamics than the emitted light. Moreover the experimental results demonstrate a strongly damped ultrafast circular polarization oscillation due to spin injection with an oscillation frequency of approximately 11GHz depending on the birefringence in the VCSEL device. We compare our experimental results with theoretical calculations based on rate-equations. This allows us to predict undamped long persisting ultrafast polarization oscillations, which reveal the potential of spin-VCSELs for ultrafast modulation applications

Spin induced gigahertz polarization oscillations in vertical-cavity surface-emitting laser devices

Spin-controlled vertical-cavity surface-emitting lasers (VCSELs) have been intensively studied in recent years because of the low threshold feasibility and the nonlinearity above threshold, which make spin-VCSELs very promising for spintronic devices. Here we investigate the circular polarization dynamics of VCSELs on a picosecond time scale after pulsed optical spin injection at room temperature. A hybrid excitation technique combining continuous-wave (cw) unpolarized electrical excitation slightly above threshold and pulsed polarized optical excitation is applied. The experimental results demonstrate ultrafast circular polarization oscillations with a frequency of about 11 GHz. The oscillations last inside the first undulation of the intensity relaxation oscillations. Via theoretical calculations based on a rate equation model we analyze these oscillations as well as the underlying physical mechanisms

PnuT uses a facilitated diffusion mechanism for thiamine uptake

Membrane transporters of the bacterial pyridine nucleotide uptake (Pnu) family mediate the uptake of various B-type vitamins. For example, the PnuT transporters have specificity for vitamin B1 (thiamine). It has been hypothesized that Pnu transporters are facilitators that allow passive transport of the vitamin substrate across the membrane. Metabolic trapping by phosphorylation would then lead to accumulation of the transported substrates in the cytoplasm. However, experimental evidence for such a transport mechanism is lacking. Here, to determine the mechanism of thiamine transport, we purify PnuTSw from Shewanella woodyi and reconstitute it in liposomes to determine substrate binding and transport properties. We show that the electrochemical gradient of thiamine solely determines the direction of transport, consistent with a facilitated diffusion mechanism. Further, PnuTSw can bind and transport thiamine as well as the thiamine analogues pyrithiamine and oxythiamine, but does not recognize the phosphorylated derivatives thiamine monophosphate and thiamine pyrophosphate as substrates, consistent with a metabolic trapping mechanism. Guided by the crystal structure of the homologous nicotinamide riboside transporter PnuC, we perform mutagenesis experiments, which reveal residues involved in substrate binding and gating. The facilitated diffusion mechanism of transport used by PnuTSw contrasts sharply with the active transport mechanisms used by other bacterial thiamine transporters

Best practice for motor imagery: a systematic literature review on motor imagery training elements in five different disciplines

<p>Abstract</p> <p>Background</p> <p>The literature suggests a beneficial effect of motor imagery (MI) if combined with physical practice, but detailed descriptions of MI training session (MITS) elements and temporal parameters are lacking. The aim of this review was to identify the characteristics of a successful MITS and compare these for different disciplines, MI session types, task focus, age, gender and MI modification during intervention.</p> <p>Methods</p> <p>An extended systematic literature search using 24 databases was performed for five disciplines: Education, Medicine, Music, Psychology and Sports. References that described an MI intervention that focused on motor skills, performance or strength improvement were included. Information describing 17 MITS elements was extracted based on the PETTLEP (physical, environment, timing, task, learning, emotion, perspective) approach. Seven elements describing the MITS temporal parameters were calculated: study duration, intervention duration, MITS duration, total MITS count, MITS per week, MI trials per MITS and total MI training time.</p> <p>Results</p> <p>Both independent reviewers found 96% congruity, which was tested on a random sample of 20% of all references. After selection, 133 studies reporting 141 MI interventions were included. The locations of the MITS and position of the participants during MI were task-specific. Participants received acoustic detailed MI instructions, which were mostly standardised and live. During MI practice, participants kept their eyes closed. MI training was performed from an internal perspective with a kinaesthetic mode. Changes in MI content, duration and dosage were reported in 31 MI interventions. Familiarisation sessions before the start of the MI intervention were mentioned in 17 reports. MI interventions focused with decreasing relevance on motor-, cognitive- and strength-focused tasks. Average study intervention lasted 34 days, with participants practicing MI on average three times per week for 17 minutes, with 34 MI trials. Average total MI time was 178 minutes including 13 MITS. Reporting rate varied between 25.5% and 95.5%.</p> <p>Conclusions</p> <p>MITS elements of successful interventions were individual, supervised and non-directed sessions, added after physical practice. Successful design characteristics were dominant in the Psychology literature, in interventions focusing on motor and strength-related tasks, in interventions with participants aged 20 to 29 years old, and in MI interventions including participants of both genders. Systematic searching of the MI literature was constrained by the lack of a defined MeSH term.</p

Evaluation of cell-free protein synthesis for the crystallization of membrane proteins – a case study of the glutamate transporter family from Staphylothermus marinus

Cell-free in vitro synthesis of proteins using coupled transcription/translation is considered to be a powerful alternative to the use of traditional cellbased expression systems. Recently, promising developments have been reported applying cell-free production to membrane proteins for structural biology and in particular for NMR spectroscopy. However, the general applicability of this system to produce large amounts of stable, functional and homogeneous membrane proteins as required for X-ray crystallography remains to be determined. Here, we present a systematic study comparing structural and functional properties of membrane proteins produced using Escherichia coli derived in vitro and in vivo expression systems. The function of the target membrane protein, a previously uncharacterized bacterial glutamate transporter homolog from Staphylothermus marinus, was analyzed using ligand binding and transport assays. In addition, the protein structure was investigated with respect to its overall fold and oligomeric state in different detergents. We found that the protein synthesized in vitro is highly stable and monodisperse. However, in contrast to the protein produced using an in vivo system, it was not able to assemble into the native trimeric state nor to bind substrate. We thus conclude that cell-free expression systems can compromise folding and function of such complex secondary active transporters. The expression product has to be carefully characterized prior to biophysical investigations like crystallography of membrane proteins

Structural and Functional Characterization of NadR from Lactococcus lactis

NadR is a bifunctional enzyme that converts nicotinamide riboside (NR) into nicotinamide mononucleotide (NMN), which is then converted into nicotinamide adenine dinucleotide (NAD). Although a crystal structure of the enzyme from the Gram-negative bacterium Haemophilus influenzae is known, structural understanding of its catalytic mechanism remains unclear. Here, we purified the NadR enzyme from Lactococcus lactis and established an assay to determine the combined activity of this bifunctional enzyme. The conversion of NR into NAD showed hyperbolic dependence on the NR concentration, but sigmoidal dependence on the ATP concentration. The apparent cooperativity for ATP may be explained because both reactions catalyzed by the bifunctional enzyme (phosphorylation of NR and adenylation of NMN) require ATP. The conversion of NMN into NAD followed simple Michaelis-Menten kinetics for NMN, but again with the sigmoidal dependence on the ATP concentration. In this case, the apparent cooperativity is unexpected since only a single ATP is used in the NMN adenylyltransferase catalyzed reaction. To determine the possible structural determinants of such cooperativity, we solved the crystal structure of NadR from L. lactis (NadRLl). Co-crystallization with NAD, NR, NMN, ATP, and AMP-PNP revealed a ‘sink’ for adenine nucleotides in a location between two domains. This sink could be a regulatory site, or it may facilitate the channeling of substrates between the two domains

Gigahertz circular polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers

We analyze ultrafast circular polarization oscillations in a commercial vertical-cavity surface-emitting laser after spin injection at room temperature. The circular polarization exhibits faster dynamics than the intensity and longer persistence than the spin relaxation time

Ultrafast spin dynamics in spin-polarized vertical-cavity surface-emitting laser devices

Spin-polarized lasers offer new encouraging possibilities for future devices. We compare time-resolved luminescence measurements with theoretical models for spin dynamics in spin-polarized lasers and demonstrate ultrafast polarization switching during one short single-mode laser pulse. (C) 2009 Optical Society of Americ