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Abstract Multi-junction solar cells made by assembling semiconductor materials with different bandgap energies have hold the record conversion efficiencies for many years and are currently approaching 50%. Theoretical efficiency limits make use of optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0–1.15 eV material lattice-matched to GaAs/Ge. Nevertheless, the lack of suitable semiconductor materials is hindering the achievement of the predicted efficiencies, since the only candidates were up to now complex quaternary and quinary alloys with inherent epitaxial growth problems that degrade carrier dynamics. Here we show how the use of strain-balanced GaAsSb/GaAsN superlattices might solve this problem. We demonstrate that the spatial separation of Sb and N atoms avoids the ubiquitous growth problems and improves crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness and provide a type-II band alignment with long carrier lifetimes. All this leads to a strong enhancement of the external quantum efficiency under photovoltaic conditions with respect to bulk layers of equivalent thickness. Our results show that GaAsSb/GaAsN superlattices with short periods are the ideal (pseudo)material to be integrated in new GaAs/Ge-based multi-junction solar cells that could approach the theoretical efficiency limit.

Using the mechano-optical stress sensor technique, we observe a counter-intuitive reduction of the compressive stress when InAs is deposited on GaAs (001) during the growth of quantum posts. Through modelling of the strain fields, we find that such anomalous behaviour can be related to the strain-driven detachment of In atoms from the crystal and their surface diffusion towards the self-assembled nanostructures.

High transaction costs deter entry of small farmers into the market. With the data from 240 smallholder cassava farmers in Central Madagascar, this study identified strategies to promote successful smallholder commercialization. The coefficients for membership of cooperatives, native of community and farming experience, have a direct relationship with decision to participate in the market and which is significant at 1% level and road condition to the nearest town is good at 10% level. The coefficients for age, distance to the nearest town and distance from the farm to the market have an indirect relationship with decision to participate in the market and significant at 5, 10 and 1% levels, respectively. The results also show that the coefficients for personal means of transportation and marketing experience have a direct relationship with decision to sell cassava off-farm and at 10 and 1% level of significance, respectively, while distance to the nearest town and distance from the farm to the market had an indirect relationship with decision to sell off-farm at 5% level of significance each and cost of transportation at 1% level. The study raises policy issues which might reduce these transaction costs by providing more market outlets, better rural infrastructure and also bulking centres.

This study employed the Cobb–Douglas stochastic frontier production function to measure the level of technical efficiency among smallholder cassava farmers in Central Madagascar. A multi-stage random sampling technique was used to select 180 cassava farmers in the region and from this sample, input–output data were obtained using the cost route approach. The parameters of the stochastic frontier production function were estimated using the maximum likelihood method. The results of the analysis showed that individual farm-level technical efficiency was about 79%. The study found education, gender and age to be indirectly and significantly related to technical efficiency at a 1% level of probability, and to household size at a 5% level. The coefficient for occupational status was positive and highly significant at a 1% level. The results show that the study’s cassava farmers are not fully technically efficient, showing a mean score of .79%, and suggesting that opportunities still exist for increasing efficiency among the farmers. There is a need, therefore, to ensure that these farmers have access to the appropriate inputs, especially land and capital. The results also call for land reform policies to be introduced, aimed at making more land available, especially to the younger and full-time female farmers.

Arrhythmia is a common finding in heart failure patients. It has a greater impact on prognosis in those patients, especially ventricular arrhythmia type. Although there are some studies about PVCs and NSVT in heart failure, few data are available regarding sustained VT/VF in heart failure. In this study, our aim was to investigate the incidence of sustained VT/VF, the factors associated with its occurrence and prognosis in hospitalized heart failure patients.We hypothesized that heart failure patients are prone to develop ventricular arrhythmia which can adversely affect the prognosis.Prospective study of 2610 patients admitted in 18 government hospitals with HF between October 2009 and December 2010. Patients were categorized as having ventricular arrhythmia (VA) if they experienced either sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) or both during hospitalization.Of 2610 patients with HF enrolled in the HEARTS registry, 110 (4.2%) were diagnosed with VA. The vast majority (97%) occurred in patients with heart failure with reduced ejection fraction (HFrEF). Factors associated with an increased risk of developing VA during hospitalization for HF are arrhythmia (OR 8.2; 95% CI 3.3-20.4), STEMI (OR 2.6; 95% CI 1.1–6.3) as precipitating factors for HF, and systolic blood pressure

Multi-junction solar cells made by assembling semiconductor materials with different bandgap energies have hold the record conversion efficiencies for many years and are currently approaching 50%. Theoretical efficiency limits make use of optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0-1.15 eV material lattice-matched to GaAs/Ge. Nevertheless, the lack of suitable semiconductor materials is hindering the achievement of the predicted efficiencies, since the only candidates were up to now complex quaternary and quinary alloys with inherent epitaxial growth problems that degrade carrier dynamics. Here we show how the use of strain-balanced GaAsSb/GaAsN superlattices might solve this problem. We demonstrate that the spatial separation of Sb and N atoms avoids the ubiquitous growth problems and improves crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness and provide a type-II band alignment with long carrier lifetimes. All this leads to a strong enhancement of the external quantum efficiency under photovoltaic conditions with respect to bulk layers of equivalent thickness. Our results show that GaAsSb/GaAsN superlattices with short periods are the ideal (pseudo)material to be integrated in new GaAs/Ge-based multi-junction solar cells that could approach the theoretical efficiency limit.

Electrical contacts on the top surface of solar cells and light emitting diodes cause shadow losses. The phenomenon of extraordinary optical transmission through arrays of subwavelength holes suggests the possibility of engineering such contacts to reduce the shadow using plasmonics, but resonance effects occur only at specific wavelengths. Here we describe instead a broadband effect of enhanced light transmission through arrays of subwavelength metallic wires, due to the fact that, in the absence of resonances, metal wires asymptotically tend to invisibility in the small size limit regardless of the fraction of the device area taken up by the contacts. The effect occurs for wires more than an order of magnitude thicker than the transparency limit for metal thin films. Finite difference in time domain calculations predict that it is possible to have high cloaking efficiencies in a broadband wavelength range, and we experimentally demonstrate contact shadow losses less than half of the geometric shadow.

We investigate L7 photonic crystal microcavities (PCMs) fabricated by epitaxial re-growth of GaAs pre-patterned substrates, containing InAs quantum dots. The resulting PCMs show hexagonal shaped nano-holes due to the development of preferential crystallographic facets during the re-growth step. Through a careful control of the fabrication processes, we demonstrate that the photonic modes are preserved throughout the process. The quality factor (Q) of the photonic modes in the re-grown PCMs strongly depends on the relative orientation between photonic lattice and crystallographic directions. The optical modes of the re-grown PCMs preserve the linear polarization and, for the most favorable orientation, a 36% of the Q measured in PCMs fabricated by the conventional procedure is observed, exhibiting values up to ~6000. The results aim to the future integration of site-controlled QDs with high-Q PCMs for quantum photonics and quantum integrated circuits.

The spontaneous emission rate and Purcell factor of self-assembled quantum wires embedded in photonic crystal micro-cavities are measured at 80 K by using micro-photoluminescence, under transient and steady state excitation conditions. The Purcell factors fall in the range 1.1 - 2 despite the theoretical prediction of ≈15.5 for the figure of merit. We explain this difference by introducing a polarization dependence on the cavity orientation, parallel or perpendicular with respect to the wire axis, plus spectral and spatial detuning factors for the emitters and the cavity modes, taking in account the finite size of the quantum wires.

We present laser emission of a compact surface-emitting micro laser, optical pumped and operating at 1.5 microm at room temperature. A two-dimensional photonic crystal lattice conformed in a hybrid triangular-graphite configuration is designed for vertical emission. The structures have been fabricated in an InP slab, including four InAsP quantum wells as active layer, on the top of a Si substrate SiO(2) wafer bonded. Laser emission with thresholds around 70 microW and quality factors (Qs) up to 12000 have been measured. The Bloch mode selected for the emission keeps a high Q (>or= 2 x 10(5)) around the Gamma point for a wide range of in-plane values k(||)

We present continuous wave laser emission in a photonic crystal microcavity operating at 1.5 microm at room temperature. The structures have been fabricated in an InP slab including a single layer of self-assembled InAs/InP quantum wires (QWrs) as active material. Laser emission in air suspended membranes with thresholds of effective optical pump power of 22 microW and quality factors up to 55000 have been measured., ((c) 2009 Optical Society of America)