Polarization fluctuations in vertical cavity surface emitting lasers: a key to the mechanism behind polarization stability

We investigate the effects of the electron-hole spin dynamics on the polarization fluctuations in the light emitted from a vertical cavity surface emitting laser (VCSEL). The Langevin equations are derived based on a rate equation model including birefringence, dichroism, and two carrier density pools seperately coupled to right and left circular polarization. The results show that the carrier dynamics phase lock the polarization fluctuations to the laser mode. This is clearly seen in the difference between fluctuations in ellipticity and fluctuations in polarization direction. Seperate measurements of the polarization fluctuations in ellipticity and in polarization direction can therefore provide quantitative information on the non-linear contribution of the carrier dynamics to polarization stability in VCSELs.Comment: 6 pages RevTex and 3 figures, to be published in Quantum and Semiclassical Optics, minor changes to the discussion of timescale

Results of Skylab medical experiment M171: Metabolic activity

The experiment was conducted to establish whether man's ability to perform mechanical work would be progressively altered as a result of exposure to the weightless environment of space flight. The Skylab crewmen exercised on a bicycle ergometer at workloads approximating 25, 50, and 75 percent of their maximum aerobic capacity. The physiological parameters monitored were respiratory gas exchange, blood pressure, and vectorcardiogram/heart rate. The results of these tests indicate that the crewmen had no significant decrement in their responses to exercise during their exposure to zero gravity. The results of the third manned Skylab mission (Skylab 4) are presented and a comparison is made of the overall results obtained from the three successively longer Skylab manned missions. The Skylab 4 crewmembers' 84-day in-flight responses to exercise were no worse and were probably better than the responses of the crewmen on the first two Skylab missions. Indications that exercise was an important contributing factor in maintaining this response are discussed

Shallow carrier traps in hydrothermal ZnO crystals

Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140-300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

The uncertainties on the EFT coupling limits for direct dark matter detection experiments stemming from uncertainties of target properties

Direct detection experiments are still one of the most promising ways to unravel the nature of dark matter. To fully understand how well these experiments constrain the dark matter interactions with the Standard Model particles, all the uncertainties affecting the calculations must be known. It is especially critical now because direct detection experiments recently moved from placing limits only on the two elementary spin independent and spin dependent operators to the complete set of possible operators coupling dark matter and nuclei in non-relativistic theory. In our work, we estimate the effect of nuclear configuration-interaction uncertainties on the exclusion bounds for one of the existing xenon-based experiments for all fifteen operators. We find that for operator number 13 the $\pm1\sigma$ uncertainty on the coupling between the dark matter and nucleon can reach more than 50% for dark matter masses between 10 and 1000 GeV. In addition, we discuss how quantum computers can help to reduce this uncertainty.Comment: 12 pages, 6 figures; submitted to Phys. Rev. D, May 17, 202

Reconstructing palaeoclimate and hydrological fluctuations in the Fezzan Basin (southern Libya) since 130 ka: a catchment-based approach

We propose a novel method to evaluate regional palaeoclimate that can be used to alleviate the problems caused by the discontinuous nature of palaeoenvironmental data found in deserts. The technique involves processing satellite imagery and DEM’s to map past rivers, catchments and evaluate the areas and volumes of palaeolakes. This information is used to determine the new Lake Evaluation Index (LEI) that allows a qualitative estimate of the amount of sediment received by lakes and how long-lived those lakes are. Lakes with considerable longevity and large sediment stores are selected for study. Validation is performed using image interpretation of remote sensing data, UltraGPR surveys and fieldwork. These techniques are also used to identify and study spring deposits and fluvial landforms that provide valuable palaeoclimate information. The method is applied to the Fezzan Basin in southern Libya focusing on the Wadi ash Shati and Wadi el-Agial catchments. Results indicate that the palaeohydrology is accurately mapped except within dune fields. We analysed the sedimentology of the key deposits identified by this methodology, developing a chronology using optically stimulated luminescence (OSL) and radiocarbon dating. We find evidence for relatively humid conditions during MIS 5c/d and e, as well as during the early to middle Holocene. Larger lakes and more extensive river systems were present during MIS 5 than are found during the Holocene, suggestive of greater humidity. The Holocene humid period started at ~11 ka and continued until ~5 ka being interrupted by abrupt periods of aridity at ~8.2 ka and ~6 ka that coincide with North Atlantic cooling. After each of these arid events the climate was less humid than previously, suggesting that they were superimposed upon an overall drying trend. The termination of the Holocene humid period in the Sahara has received much scrutiny in recent years, and sediments of Palaeolake Shati provide a continuous record of this. We do not find evidence to support the hypothesis of either sudden or gradual aridification of the Sahara at ~5 ka, instead we find that that aridity started to develop at ~6.5 ka, whereupon the lake levels oscillated until finally drying-up by 5.3 ka. Most of the other lakes in the Fezzan also dried up at ~ 5ka. We suggest that thousands of years of aridification prior to 5 ka shrunk these lakes so that additional aridity at this time led to their final desiccation. Because lakes are prodigious dust sources this mechanism potentially explains the rapid rise in dust flux to the Atlantic at 5 ka, with this final drying being the culmination of longer term aridity, albeit overprinted with considerable climate variability

Comparison of Bioimpedance Analysis and Dual Energy X-Ray Absorptiometry in Division III Football Athletes

Body composition is an integral component of physiology and pathology, as well as an indication of how the musculoskeletal system changes over time in response to training and/or nutritional modifications. Two common methods of estimating body composition include bioelectrical impedance analysis (BIA) and dual-energy X-ray absorptiometry (DXA). There is a debate regarding the agreement between these two methods, and it is thought that the population being tested may influence the similarity of estimates obtained by these technologies. Limited data comparing these two methodologies in athletic populations are available. PURPOSE: The purpose of this study was to take examine the agreement between BIA and DXA for estimates of body mass (BM) and body fat percentage (BF%). METHODS: Forty-three division III football players (mean ± SD; age: 19.9 ± 1.7 y; height: 179.7 ± 7.4 cm; BM: 100.3 ± 21.9 kg; DXA BF%: 21.8 ± 7.1%) reported to the Human Performance Lab after abstaining from exercise for the previous 24 hours and fasting from food/beverages (except water) for 12 hours. After an initial height assessment, BM and BF% were assessed using both BIA (Inbody 770) and DXA (Hologic Horizon W). BIA was performed in the standing position using an 8-point electrode arrangement. The electrode panels were placed at the hands and feet. DXA was performed with the participants in a supine position and hips internally rotated. Differences in BM and BF% from DXA and BIA were compared using paired-samples t-tests, Bland-Altman analysis, and validity metrics. RESULTS: BM detected by DXA was significantly lower than BM obtained from the BIA scale (DXA: 98.2 ± 20.8 kg, BIA: 100.3 ± 21.9 kg; p \u3c 0.001). Additionally, Bland-Altman analysis indicated significantly greater underestimation of BM by DXA in individuals with higher BM values (slope: 0.053; p \u3c 0.001). BF% did not significantly differ between DXA and BIA (DXA: 21.8 ± 7.1%, BIA: 21.8 ± 8.4%; p = 0.86). However, Bland-Altman analysis indicated significant proportional bias, with underestimations of BF% by BIA in leaner individuals but overestimations of BF% in those with higher BF% (slope: 0.176; p = 0.005). The total error and 95% limits of agreement for BF% were 3.2% and ± 6.4%, respectively. CONCLUSION: This study demonstrates that although DXA and BIA exhibit good group-level agreement for BF%, several inconsistencies between these methodologies are present. Estimates of BM differed between technologies. Additionally, proportional bias for BF% was observed indicating different levels of agreement between DXA and BIA depending on the body fat of the individuals being tested. Based on these findings, caution should be employed when interpreting data from DXA and BIA assessments in athletic populations