Solar energy collection by antennas

The idea of collecting solar electromagnetic radiation with antenna-rectifier (rectenna) structures was proposed three decades ago but has not yet been achieved. The idea has been promoted as having potential to achieve efficiency approaching 100% but thermodynamic considerations imply a lower limit of 85.4% for a non-frequency-selective rectenna and 86.8% for one with infinite selectivity, assuming maximal concentration in each case. This paper reviews the history and technical context of solar rectennas and discusses the major issues: thermodynamic efficiency limits, rectifier operation at optical frequencies, harmonics production and electrical noise

Epitaxially grown GaAsN random laser

We have studied the photoluminescence properties of as-grown GaAs1-xNx epitaxial layers grown on GaAs containing 0.6%, 1.77%, and 2.8% nitrogen. We found laser emission from thick (d>200 nm) GaAs0.972N0.028 layers exhibiting the characteristic lasing properties of random lasers. This is unusual because random lasers have so far only been associated with highly disordered or random media. We believe that high gain in combination with structural inhomogeneities that are evident in these GaAs0.972N0.028 layers, can explain the random lasing in such epitaxial layers. (C) 2003 American Institute of Physics

Spin wave scattering and interface magnetism in superconducting-ferromagnet-superconducting hybrid structures

In this work we report on interlayer spin-polarized transport studies carried out on YBa2Cu3O7-δ/La2/3Sr1/3MnO3/YBa2Cu3O7-δ thin film multilayers. Anisotropic YBa2Cu3O7-δ high-temperature superconductors (YBCO) and the manganite La2/3Sr1/3MnO3 (LSMO) are layered metallic systems where the c-axis interlayer current transport occurs via sequential tunneling of charge carriers. The temperature dependence of the resistance R(T) for different applied fields shows that these heterostructures are clearly influenced by both the insulator-to-metal transition of the LSMO layers at ∼ 320 K and the stripe fluctuations in the conductivity of the YBCO layers at the onset of electronic inhomogeneity at ∼ 210 K. Furthermore, the tunneling of spin-polarized charge carriers between the YBCO layers through LSMO showed sharp switching between the high and low resistance states, which we assume corresponds to antiparallel and parallel alignments of the magnetic (spin) moments of these hybrid structures. The investigation of these promising hybrid structures is aimed at understanding the science of the new spin-based devices, laying the foundation for a new generation of ultrafast, nonvolatile electronics

Numerical Study of the Wave-Wave Interaction by Viscous Flow Simulations with OpenFOAM\uae

This study is part of the two research projects \u201cOpenSHIP - Highly reliable computational fluid-dynamics (CFD) simulations for the prediction of the hydrodynamic performance of the hull-propeller system in OpenSource background\u201d and \u201cOpenViewSHIP \u2013 Development of an integrated computational ecosystem for the hydrodynamic design of the hull-propeller system\u201d, co-financed by Friuli Venezia Giulia Region [see acknowledgements], carried out at the Dept. of Engineering and Architecture of the University of Trieste with the aim to establish a strong link between the shipbuilding industry needs and the academic expertise in the field of open-source CFD and High Performance Computing. In this work the wave-wave interaction problem is tackled, mainly focused on the nonlinear aspects related to the energy exchange between Fourier components of steep breaking or non-breaking waves, specifically in the presence of focusing. The problem has been studied in the past by the authors in the frame of inviscid flow fully nonlinear simulations. Even though the comparison with experimental results showed extremely good results, the limitations of that work are here overcome. The computations are carried out with OpenFOAM\uae, adopting a VOF technique at the free surface

Subterahertz Josephson plasma emission in layered high-T-C superconducting tunnel junctions

We investigated the emission of subterahertz frequency electromagnetic radiation from high-TC superconducting c-axis NdBa2Cu3O7−8/PrBa2Cu3O7−8/NdBa2Cu3O7−8 trilayer thin film tunneling junctions when external electric and magnetic fields are applied. The current-voltage characteristics under applied ab-plane magnetic fields H (up 8 T) exhibit well defined steps, Vn, such that eVn -hwp / (2n), where the plasma frequency wp-0.4 THz and n=1,2,3, . . . .. These steps may be interpreted using Josephson plasma dynamics. The applied voltage creates oscillating currents via the Josephson coupling energy EJ (EJ=hIJ /4e, where IJ is the Josephson current and h is Planck’s constant) and the charge energy EC (EC=e2 /2C, where C is the junction capacitance). Thus the Josephson plasma becomes excited by the tunneling current, with some of the energy being emitted as subterahertz frequency radiation. Our results provide a new insight into a solid-state quantum system with considerable potential for new solid-state terahertz emission sources

Fabrication and optical characterisation of planar silica lens pair

The fabrication and experimental results on the performance of a planar silica lens pair that facilitates low-loss optical beam propagation in free space between opposing planar silica waveguides/singlemode fibres are presented. Each lens uses the parabolic graded refractive index vertically and the convex curvature horizontally to focus the optical beam in free space. The lens pair, designed for 200 mu m of free space propagation distance with ideal zero loss, is fabricated and measured for net loss of 1.6 dB which is 12.4 dB less than the coupling loss without a lens pair at 633 nm wavelength

Characterization of UHV E-beam Evaporated Low-Stress Thick Silicon Film for MEMS Application

AbstractThis paper investigates various deposition and subsequent processing conditions on UHV e-beam evaporated silicon to obtain low stress film. They include substrate temperature, deposition rate, annealing, thermal oxidation and post-oxidation annealing. Film stress is measured for each condition and cantilever beams made from the films are released for evaluating stress-gradient. Films are also deposited on sloped step structures to observe step and corner coverage. The results indicate that as-deposited evaporated silicon exhibits tensile stress at substrate temperatures below 400̊C and compressive stress as substrate temperature is increased above 400̊C for a 100nm/min deposition rate. For evaporated amorphous silicon films, performing thermal oxidation at 900̊C and annealing at elevated temperatures has been found to be effective in reducing film stress. For fully crystallized poly-silicon films, however, annealing at 1000̊C without thermal oxidation seems to be the more effective way of reducing stress in the film

VIV analysis of a single elastically-mounted 2D cylinder: Parameter Identification of a single-degree-of-freedom multi-frequency model

A novel single-degree-of-freedom multi-frequency model (sdof-mf) for the prediction of the Vortex Induced Vibrations (VIV) of an elastically mounted circular cylinder in two-dimensional cross flow is presented. The proposed model treats the total hydrodynamic force as sum of conventional Morison-like inertia and drag terms related to the cylinder motion in still fluid plus additional harmonics that account for the lift force induced by vortex shedding. Amplitudes, frequencies and phase lags of these harmonics are identified using a Parameter Identification (PI) procedure applied to time domain data of vortex induced forces, here obtained via CFD simulations. The proposed sharp identification via PI of the independent frequencies of the vortex shedding fluid force is the peculiarity of the proposed method. The model is assessed considering a wide range of flow regimes, including lock-in conditions. From the overall results, the proposed sdof-mf model exhibits promising but consistent capabilities in the reproduction of the vortex shedding forces and cylinder motion, in terms of both amplitudes and frequencies