UWB Monocycle Generator based on the non-linear effects of an SOA-integrated structure

In this letter, we propose and experimentally demonstrate a novel and single structure to generate ultra-wideband (UWB) pulses by means of the cross-phase modulation present in a semiconductor optical amplifier unified structure. The key components of this system is an integrated Mach-Zehnder interferometer with two semiconductor optical amplifiers and an optical processing unit. The fusion of these two components permits the generation and customization of UWB monocycle pulses. The polarity of the output pulses is easily modified through the single selection of a specific input port. Moreover, the capacity of transmitting several data sequences is demonstrated and the potentiality to adapt the system to different modulation formats is analyzed.

Nonlinear normal modes in mechanical systems: concept and computation with numerical continuation

The purpose of this Project is, first and foremost, to disclose the topic of nonlinear vibrations and oscillations in mechanical systems and, namely, nonlinear normal modes NNMs to a greater audience of researchers and technicians. To do so, first of all, the dynamical behavior and properties of nonlinear mechanical systems is outlined from the analysis of a pair of exemplary models with the harmonic balanced method. The conclusions drawn are contrasted with the Linear Vibration Theory. Then, it is argued how the nonlinear normal modes could, in spite of their limitations, predict the frequency response of a mechanical system. After discussing those introductory concepts, I present a Matlab package called 'NNMcont' developed by a group of researchers from the University of Liege. This package allows the analysis of nonlinear normal modes of vibration in a range of mechanical systems as extensions of the linear modes. This package relies on numerical methods and a 'continuation algorithm' for the computation of the nonlinear normal modes of a conservative mechanical system. In order to prove its functionality, a two degrees of freedom mechanical system with elastic nonlinearities is analized. This model comprises a mass suspended on a foundation by means of a spring-viscous damper mechanism -analogous to a very simplified model of most suspended structures and machines- that has attached a mass damper as a passive vibration control system. The results of the computation are displayed on frequency energy plots showing the NNMs branches along with modal curves and time-series plots for each normal mode. Finally, a critical analysis of the results obtained is carried out with an eye on devising what they can tell the researcher about the dynamical properties of the system.

Improving Optical Performance of Concentrator Cells by Means of a Deposited Nanopattern Layer

Multijunction solar cells (MJSC) use anti-reflective coatings (ARC) to minimize Fresnel reflection losses for a family of light incidence angles. These coatings adapt the refractive index of the cell to that of the surrounding medium. Patterns with sizes in the range of the light wavelength can be used to further reduce reflections through diffraction. Transparent nanopatterns with a gradual profile, called moth-eye nanostructures, can adapt the refractive index of the optical interfaces (often with n∼1.5) used to encapsulate concentrator solar cells to that of the air (n air∼1). Here we show the effect of a nanometric moth-eye ARC with a round motif deposited on commercial MJSC that achieves short-circuit current (I SC) gains greater than 2% at normal incidence and even higher in the case of tilted illumination. In this work, MJSC with different moth-eye ARC are characterized under quantum efficiency (QE) as well as under concentrated illumination I-V in order to assess their potential. Simulations based on coupled wave analysis (RCWA) are used to fit the experimental results with successful results.

Stranski-Krastanov InN/InGaN quantum dots grown directly on Si(111)

The authors discuss and demonstrate the growth of InN surface quantum dots on a high-In-content In0.73Ga0.27N layer, directly on a Si(111) substrate by plasma-assisted molecular beam epitaxy. Atomic force microscopy and transmission electron microscopy reveal uniformly distributed quantum dots with diameters of 10–40 nm, heights of 2–4 nm, and a relatively low density of ∼7 × 109 cm−2. A thin InN wetting layer below the quantum dots proves the Stranski-Krastanov growth mode. Near-field scanning optical microscopy shows distinct and spatially well localized near-infrared emission from single surface quantum dots. This holds promise for future telecommunication and sensing devices.

Implications of low breakdown voltage of component subcells on external quantum efficiency measurements of multijunction solar cells

The electrical and optical coupling between subcells in a multijunction solar cell affects its external quantum efficiency (EQE) measurement. In this study, we show how a low breakdown voltage of a component subcell impacts the EQE determination of a multijunction solar cell and demands the use of a finely adjusted external voltage bias. The optimum voltage bias for the EQE measurement of a Ge subcell in two different GaInP/GaInAs/Ge triple-junction solar cells is determined both by sweeping the external voltage bias and by tracing the I–V curve under the same light bias conditions applied during the EQE measurement. It is shown that the I–V curve gives rapid and valuable information about the adequate light and voltage bias needed, and also helps to detect problems associated with non-ideal I–V curves that might affect the EQE measurement. The results also show that, if a non-optimum voltage bias is applied, a measurement artifact can result. Only when the problems associated with a non-ideal I–V curve and/or a low breakdown voltage have been discarded, the measurement artifacts, if any, can be attributed to other effects such as luminescent coupling between subcells.