Journal YBCO Thin Film Formation near the Stability Line by Resistive Evaporation of BaF2, Cu, and Y

A single resistively heated source was used to deposit a mixture of BaF₂, Cu, and Y to form precursor films onto MgO substrates held at room temperature. Different heat treatment conditions were applied to study the stability diagram of YBa₂Cu₃O_7−x. It was found that the stability line is shifted toward the higher oxygen partial pressure and lower temperature side of the pure YBCO line in the presence of fluorine and fluorides in the precursor. Films of good quality were obtained when the annealing conditions were in close proximity to this line.     

Submilliwatt optical bistability in wafer fused vertical cavity at 1.55-/spl mu/m wavelength

We report on low-power optical bistability in a vertical cavity structure at 1.55-/spl mu/m wavelength due to the bandgap resonant dispersive optical nonlinearity of InGaAsP. In this structure, a GaAs-AlAs Bragg reflector grown on GaAs and the nonlinear medium grown on InP are bonded through wafer-fusion, leading to a high-quality vertical cavity after deposition of a top dielectric mirror. This device shows interesting characteristics for optical switching applications at fiber communication wavelengths, such as a switching contrast higher than 8:1 in the reflective mode and a bistability threshold power as low as 0.6 mW. True steady-state memory effect is observed with continuous-wave input, and the device switching time is in the ns range.     

Methodology of a study on the French population exposure to 50 Hz magnetic fields

The characterisation of population exposure to a 50-Hz magnetic field (MF) is important for assessing health effects of electromagnetic fields. With the aim of estimating and characterising the exposure of the French population to 50-Hz MFs, two representative samples of the population were made. A random selection method based on the distribution of households in different regions of France was used. The samples were carried out starting from a random polling of telephone numbers of households (listed, unlisted fixed phones and cell phones only). A total of 95,362 telephone numbers were dialed to have 2148 volunteers (1060 children and 1088 adults). They all agreed to carrying an EMDEX II meter, measuring and recording MFs, and to filling out a timetable for a 24-hour period. In this article, the methodology of the sample selection and the collection of all necessary information for the realisation of this study are presented.     

Elucidating the Detectivity Limits in Shortwave Infrared Organic Photodiodes

While only few organic photodiodes have photoresponse past 1 µm, novel shortwave infrared (SWIR) polymers are emerging, and a better understanding of the limiting factors in narrow bandgap devices is critically needed to predict and advance performance. Based on state‐of‐the‐art SWIR bulk heterojunction photodiodes, this work demonstrates a model that accounts for the increasing electric‐field dependence of photocurrent in narrow bandgap materials. This physical model offers an expedient method to pinpoint the origins of efficiency losses, by decoupling the exciton dissociation efficiency and charge collection efficiency in photocurrent–voltage measurements. These results from transient photoconductivity measurements indicate that the main loss is due to poor exciton dissociation, particularly significant in photodiodes with low‐energy charge‐transfer states. Direct measurements of the noise components are analyzed to caution against using assumptions that could lead to an overestimation of detectivity. The devices show a peak detectivity of 5 × 10¹⁰ Jones with a spectral range up to 1.55 µm. The photodiodes are demonstrated to quantify the ethanol–water content in a mixture within 1% accuracy, conveying the potential of organics to enable economical, scalable detectors for SWIR spectroscopy.     

Efficient bidding strategies for Cliff-Edge problems

In this paper, we propose an efficient agent for competing in Cliff-Edge (CE) and simultaneous Cliff-Edge (SCE) situations. In CE interactions, which include common interactions such as sealed-bid auctions, dynamic pricing and the ultimatum game (UG), the probability of success decreases monotonically as the reward for success increases. This trade-off exists also in SCE interactions, which include simultaneous auctions and various multi-player ultimatum games, where the agent has to decide about more than one offer or bid simultaneously. Our agent competes repeatedly in one-shot interactions, each time against different human opponents. The agent learns the general pattern of the population’s behavior, and its performance is evaluated based on all of the interactions in which it participates. We propose a generic approach which may help the agent compete against unknown opponents in different environments where CE and SCE interactions exist, where the agent has a relatively large number of alternatives and where its achievements in the first several dozen interactions are important. The underlying mechanism we propose for CE interactions is a new meta-algorithm, deviated virtual learning (DVL), which extends existing methods to efficiently cope with environments comprising a large number of alternative decisions at each decision point. Another competitive approach is the Bayesian approach, which learns the opponents’ statistical distribution, given prior knowledge about the type of distribution. For the SCE, we propose the simultaneous deviated virtual reinforcement learning algorithm (SDVRL), the segmentation meta-algorithm as a method for extending different basic algorithms, and a heuristic called fixed success probabilities (FSP). Experiments comparing the performance of the proposed algorithms with algorithms taken from the literature, as well as other intuitive meta-algorithms, reveal superiority of the proposed algorithms in average payoff and stability as well as in accuracy in converging to the optimal action, both in CE and SCE problems.     

Optical contrast in apertureless microscopy

We report on optical image contrast for a specific apertureless near-field optical microscope. We demonstrate that the main part of the optical image's contrast results from the sample's topography. The coupling mechanism is analyzed, and we show that the microscope can be regarded as an interferometer that sensitively detects near-field components. However, in the basic configuration the reference field of the interferometer is coupled to the topography. Finally, it is demonstrated that, by controlling the phase of the reference field, one can largely decorrelate the optical image from the topography.     

Selective growth of GaAs and GaAlAs by Cl-assisted OMVPE at atmospheric pressure

Selective epitaxy of GaAs is generally obtained at low pressure, when the diffusion length of the active species over the dielectric mask is large enough. Another way is to use an organochloride as a precursor, which suppresses the deposition over the dielectric. This method is also used at reduced pressure. In this work, we have obtained selective area epitaxy of GaAs and GaAIAs for a wide range of aluminium composition by adding a controlled flow of AsCl₃ during a conventional OMVPE growth at atmospheric pressure. This method is called Cl-assisted OMVPE. When no AsCl₃ is used, a finely structured polycrystal is deposited on the mask. When AsCl₃ is introduced into the reactor, with otherwise the same growth conditions, complete selectivity was obtained over a 100μm wide Si₃N₄ mask. We have studied the influence of the growth temperature, the TMG/ AsCl₃ and the AsH₃/TMG ratios on the growth selectivity. We have also studied the evolution of the limiting planes of the selectively grown ridges with different growth conditions. Finally, on a selectively grown GaAs/GaAlAs double heterostructure, room temperature photoluminescence topography was performed.