GaInNAs-based Hellish-vertical cavity semiconductor optical amplifier for 1.3 μm operation

Hot electron light emission and lasing in semiconductor heterostructure (Hellish) devices are surface emitters the operation of which is based on the longitudinal injection of electrons and holes in the active region. These devices can be designed to be used as vertical cavity surface emitting laser or, as in this study, as a vertical cavity semiconductor optical amplifier (VCSOA). This study investigates the prospects for a Hellish VCSOA based on GaInNAs/GaAs material for operation in the 1.3-μm wavelength range. Hellish VCSOAs have increased functionality, and use undoped distributed Bragg reflectors; and this coupled with direct injection into the active region is expected to yield improvements in the gain and bandwidth. The design of the Hellish VCSOA is based on the transfer matrix method and the optical field distribution within the structure, where the determination of the position of quantum wells is crucial. A full assessment of Hellish VCSOAs has been performed in a device with eleven layers of Ga0.35In0.65N0.02As0.08/GaAs quantum wells (QWs) in the active region. It was characterised through I-V, L-V and by spectral photoluminescence, electroluminescence and electro-photoluminescence as a function of temperature and applied bias. Cavity resonance and gain peak curves have been calculated at different temperatures. Good agreement between experimental and theoretical results has been obtained

Overfeeding, Autonomic Regulation and Metabolic Consequences

The autonomic nervous system plays an important role in the regulation of body processes in health and disease. Overfeeding and obesity (a disproportional increase of the fat mass of the body) are often accompanied by alterations in both sympathetic and parasympathetic autonomic functions. The overfeeding-induced changes in autonomic outflow occur with typical symptoms such as adiposity and hyperinsulinemia. There might be a causal relationship between autonomic disturbances and the consequences of overfeeding and obesity. Therefore studies were designed to investigate autonomic functioning in experimentally and genetically hyperphagic rats. Special emphasis was given to the processes that are involved in the regulation of peripheral energy substrate homeostasis. The data revealed that overfeeding is accompanied by increased parasympathetic outflow. Typical indices of vagal activity (such as the cephalic insulin release during food ingestion) were increased in all our rat models for hyperphagia. Overfeeding was also accompanied by increased sympathetic tone, reflected by enhanced baseline plasma norepinephrine (NE) levels in both VMH-lesioned animals and rats rendered obese by hyperalimentation. Plasma levels of NE during exercise were, however, reduced in these two groups of animals. This diminished increase in the exercise-induced NE outflow could be normalized by prior food deprivation. It was concluded from these experiments that overfeeding is associated with increased parasympathetic and sympathetic tone. In models for hyperphagia that display a continuously elevated nutrient intake such as the VMH-lesioned and the overfed rat, this increased sympathetic tone was accompanied by a diminished NE response to exercise. This attenuated outflow of NE was directly related to the size of the fat reserves, indicating that the feedback mechanism from the periphery to the central nervous system is altered in the overfed state.

Association of Spermatogenic Failure with the b2/b3 Partial AZFc Deletion

Infertility affects around 1 in 10 men and in most cases the cause is unknown. The Y chromosome plays an important role in spermatogenesis and specific deletions of this chromosome, the AZF deletions, are associated with spermatogenic failure. Recently partial AZF deletions have been described but their association with spermatogenic failure is unclear. Here we screened a total of 339 men with idiopathic spermatogenic failure, and 256 normozoospermic ancestry-matched men for chromosome microdeletions including AZFa, AZFb, AZFc, and the AZFc partial deletions (gr/gr, b1/b3 and b2/b3)

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

Energy and momentum relaxation of hot electrons in GaN/AlGaN

We report the experimental studies of hot-electron energy and momentum relaxation in the steady state in GaN/AlGaN HEMT structures with a high two-dimensional electron density of n = 1.5 x 10(13) cm(-2). From the LO-phonon-scattering-limited component of the mobility we obtain for the LO phonon the energy of homega similar to 90 meV and the momentum relaxation time of tau(m) similar to 4 fs. Drift velocity versus electric field characteristics obtained from the pulsed I-V measurements show that, at T-L = 77 K, the drift velocity saturates at upsilon(d) = 1.0 X 10(7) cm s(-1) at electric fields in excess of E similar to 7.5 kV cm(-1), and at T-L = 300 K it saturates at upsilon(d) similar to 5 x 10(6) cm s(-1), at an electric field of around E similar to 10 kV cm(-1). Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility mu(E) at a fixed lattice temperature, with the lattice temperature dependence of the mobility at a fixed low electric field. The electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. The effect of hot-phonon production on the observed momentum and energy relaxation of hot electrons is discussed within the framework of a theoretical model, which was originally developed for III-V material systems and has been adapted for a two-dimensional electron gas in GaN, and in which phonon drift is neglected

Hot electron capture and power loss in 2D GaN

We report the experimental studies of hot electron energy relaxation and capture in a low electron mobility GaN/ GaAIN HEMT structure grown with MBE on sapphire substrate. Pulsed I- V characteristics indicate that at elevated electric fields, current pulse decays exponentially with time. The threshold electric field (F(th)) above which the current pulse exhibits the decay, increases with temperature (E(th)similar to200V/cm at T = 77K, E(th)similar to350V/cm at T = 300K). The magnitude of the decay decreases with increasing temperature and increases with applied field. Time constant associated with this decay decreases rapidly with increasing applied field. These observations indicate the presence of hot electron capture over potential barriers. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility mu(E) at a low lattice temperature, with the lattice temperature dependence of the mobility at a low electric field. Electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. Our results show that the experimental power loss has an exponential factor, hw/k which is much smaller than the value expected from LO phonon emission. Furthermore, the magnitude of the experimental power loss is also much lower than the theoretical value. This observation is explained in terms of capture of hot electrons. (C) 2002 Elsevier Science B.V. All rights reserved

Effect of fast thermal annealing on the optical spectroscopy in MBE- and CBE-grown GaInNAs/GaAs QWs: blue shift versus red shift

An investigation is presented of thermal annealing effects on spectral photoconductivity and photoluminescence in sequentially grown GaInNAs/GaAs and GaInAs/GaAs quantum well structures. Experiments have been carded out at temperatures between 30 K and 300 K. The results indicate that thermal annealing improves the optical quality of GaInNAs, but may cause either a blue shift, as commonly observed by other groups, or a red shift depending on the growth technique. The anneal-induced blue-shift behaviour can be explained in terms of two competing mechanisms involving the redistribution of nearest-neighbour configuration and the change of quantum well profile. The red shift is explained in terms of hydrogen-induced chemical effects. © IEE, 2004

Comparison of theoretical models for interband transitions in dilute nitrides and experimental measurement

We have developed a model for the rapid calculation of interband transitions in dilute nitride quantum well structures. The model assumes parabolic bands with the conduction band modified using the band anticrossing approach. The model is compared to results from k . p models, which while being more accurate and flexible than our model, require extensive computational time. The model predictions are compared to photoluminescence and photomodulated reflectance measurements of interband transitions in a number of GaNAs quantum well samples. (C) 2002 Elsevier Science B.V. All rights reserved