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To examine the influence of radiative transitions on the performance of nonequilibrium seed-free argon plasma magnetohydrodynamic (MHD) generators, we developed MHD numerical simulation technique with a collisional-radiative (C-R) model, where neutral argon atoms with 45 discrete effective electronic excitation levels, singly ionized argon ions, and free electrons were considered under a two-temperature, low-magnetic Reynolds number MHD approximation. Furthermore, a so-called escape factor was implemented in the C-R model to consider the effects of radiation trapping by ground-state neutral atoms. Using the developed technique, we computed the performance characteristics of a nonequilibrium seed-free argon plasma MHD generator. The numerical results showed that the generator performance gets higher as the escape factor decreases, i.e., the optical thickness for radiative transitions to the ground state of neutral atoms increases. The numerical results also suggested that when the escape factor is $1.0\times 10^{-3}$ or less, the generator performance is almost the same as that for the escape factor of 0 corresponding to completely optical-thick plasma case for the radiative transitions to the ground state of neutral atoms. Furthermore, realistic escape factor values for resonance lines in the power generation channel were evaluated using a well-known estimation model. Consequently, the estimated values had the order of $10^{-4}$. We also showed that under such relatively optically thick plasma cases, a collisional (C) model and a widely used global ionization-recombination rate model with no explicit consideration of electronically excited-state neutral atoms can reproduce the generator performance predicted by the C-R model with relatively good accuracy. [ABSTRACT FROM AUTHOR]

We report on the development of Mo/Au based transition edge sensors (TESs) aimed at soft X-ray detection. TESs of different sizes with Tc ∼ 100 mK and very narrow transitions have been fabricated. Dark characterization based on I–V, complex impedance, and noise measurements has allowed us to obtain their basic functional parameters at different bath temperatures and operating points. Electrodeposited Bi films, to be used as X-ray absorbers, have been developed and characterized. [ABSTRACT FROM PUBLISHER]

This paper will investigate the effect of layered snow in alpine regions on microwave emission at Ku and Ka bands, using both experimental data and model simulations. A multilayer dense-media radiative transfer model (DMRT), was implemented under the quasi-crystalline approximation (ML-QCA), to account for the effects of snow layers on the emission from dry snow covers. The model then evaluated the sensitivity of two microwave indices, based on frequency and polarization combinations, to snow parameters. Model simulations were compared to radiometric dual frequency/polarization measurements of snow covers, collected during long-term experiments carried out over three winter seasons between 2007 and 2011 in the Eastern Italian Alps. This comparison has the twofold purpose of validating the model with experimental data and verifying the influence of snow layering on microwave emission and related frequency and polarization indices. The wide variations in snow characteristics over several winter seasons allowed for an extended validation of the model, which was demonstrated to account for the complex stratigraphy (up to 15 layers) of snow. The measured brightness temperatures at Ku and Ka bands were compared to those simulated through the multi (ML-QCA) and single-layer (SL-QCA) models, by using the observed snow parameters as inputs. In the case of SL, we used the average value of all layers weighted for the layer thickness. The results showed that the ML-QCA model was better correlated to the radiometric measurements than the SL-QCA. A direct comparison of measured and simulated data showed that the slope of correlation for the single-layer ranged between 0.4 and 0.5, with determination coefficient lower than 0.3; whereas the slope in the multi-layer approach ranged between 0.7 and 1.0, with determination coefficients between 0.51 and 0.75 and Root Mean Square Error (RMSE) between 11K and 15K. [ABSTRACT FROM PUBLISHER]

For relativistic Cherenkov devices, we investigate the process of high-power microwave pulse generation with its phase correlating to the sharp edge of an e-beam current pulse. Our theoretical consideration is referred to quasi-stationary and superradiative (SR) generation regimes when spontaneous emission of the e-beam edge serves as the seed for the development of further coherent oscillations. Phase correlation of the excited microwave pulses with the characteristics of the current pulse front and/or an initial external electromagnetic pulse has been additionally confirmed by particle-in-cell simulations. Pulse-to-pulse stability of the radiation phase within several percents of the oscillation period makes it possible to arrange multichannel schemes producing mutually coherent microwave pulses. In the experiments that have been carried out, the cathodes of independent generators were powered by identical accelerating pulses from strictly synchronized voltage modulators, or by splitting the pulse from a single powerful modulator. For the 2-ns regime with the power of each Ka-band backward-wave oscillator about 100 MW, we demonstrate quadratic growth of the power density in the interference maximum of the directional diagram. In a short pulse SR regime, with the peak power of 600 MW in a single channel, for a four-channel 2-D array, we attained a 16-fold radiation intensity gain. [ABSTRACT FROM PUBLISHER]

We have decomposed light-emitting diode (LED) current into component currents by identifying the ideality factor of each component and by fitting the measured and simulated current–voltage ( I–V ), I(dV/dI)-I and the shape of the relative luminescence efficiency–current characteristics. Nearly unity ideality factor of the radiative recombination current is found from luminescence–voltage (L– V) characteristics in AlGaInP red and GaInN green and blue LEDs. The current-component analysis enables an estimation of the internal quantum efficiency (IQE) as a function of current. Compared with red LEDs, blue LEDs suffer from lower radiative recombination efficiency and much larger efficiency droop to result in lower IQE. [ABSTRACT FROM PUBLISHER]

The segmented contact technique has been used to study the effects on nonradiative recombination of stacking multiple quantum-dot layers. As the number of stacked layers is increased a shift in the balance of dots in the bimodal dot distribution is observed due to a reduction in the number in the smaller dot size subset. This is attributed to an increase in the density of defect islands, as the number of layers is increased, that preferentially take material from the smaller dots, and lead to an increased level of nonradiative recombination per layer at low injection level. A second nonradiative process is apparent at higher injection level, which is related to the population of the small dot size subset. Spontaneous radiative efficiency was improved in a five-layer sample where the large to small dot size energy separation and relative density of the large dot size subset were increased. [ABSTRACT FROM AUTHOR]

Recently, we observed quasi-periodic frequency oscillations in CSF1, which is the first cesium fountain clock at Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. These oscillations could be traced back to Majorana transitions after the second Ramsey interaction. A change in field configuration eliminated the Majorana transitions and with it the frequency oscillations. [ABSTRACT FROM AUTHOR]

In this paper, we report the observation of the higher order frequency shift due to the trapping field in a 87Sr optical lattice clock. We show that at the magic wavelength of the lattice, where the first order term cancels, the higher order shift will not constitute a limitation to the fractional accuracy of the clock at a level of 10-is. We also report an accurate frequency measurement of the clock transition. The frequency is determined to be V¹S0–³P0 = 429 228 004 229 879 (5) Hz with a fractional uncertainty that is comparable to state-of-the-art optical clocks with neutral atoms in free fail. [ABSTRACT FROM AUTHOR]

The potential of 1.3-μm AlGaInAs quantum-well (MQW) laser diodes for uncooled operation in high-speed optical communication systems is experimentally evaluated by characterizing the temperature dependence of key parameters such as the threshold current, transparency current density, optical gain and carrier lifetime. Detailed measurements performed in the 20°C-100°C temperature range indicate a localized T0 value of 68 K at 98°C for a device with a 208 μm ridge width and 700μm cavity length. The transparency current density is measured for temperatures from 20°C to 60°C and found to increase at a rate of 7.7 A.cm-2 · °C-1. Optical gain characterizations show that the peak model gain at threshold is independent of temperature, whereas the differential gain decreases linearly with temperature, whereas the differential gain decreases linearly with temperature at a rate of 3 × 10-4 A-1 · °C-1. The differential carrier lifetime is determined from electrical impedance measurements and found to decrease with temperature. From the measured carrier lifetime we derive the monomolecular (A), radiative (B), and nonradiative Auger (C) recombination coefficients and determine their temperature dependence in the 20°C-80°C range. Our study shows that A is temperature independent, B decreases with temperature. The experimental observations are discussed and compared with theoretical predictions and measurements performed on other material systems. [ABSTRACT FROM AUTHOR]

When dealing with electromagnetic problems of small three-dimensional (3-D) elements, such as dipole, microstrip patches and slots, in the vicinity of two-dimensional (2-D) structures, it is very efficient to find the radiation characteristics of the 3-D elements by using a spectrum of 2-D solutions (S2DS). Only one 2-D solution in the spectral domain is needed to obtain the radiation pattern in the transverse plane defined by θ = 90°. For the radiation in a longitudinal plane defined by a fixed azimuth angle φ, one 2-D problem has to be solved for each value of the polar angle θ. This paper presents a fast algorithm by which the radiation pattern in the longitudinal plane can be computed more efficiently. The algorithm applies the method of asymptotic waveform evaluation (AWE) in the 2-D spectral domain. For the cases shown in the paper, the algorithm needs only three 2-D solutions to determine the complete radiation pattern in the longitudinal plane. [ABSTRACT FROM AUTHOR]

Studies the dependence of the intersubband absorption on the number of quantum wells (QW). Effect of the changes in the number of wells, the angle of incidence, and the barrier thickness between adjacent cells on QW radiative coupling; Effects of radiative coupling on intersubband absorption spectrum.