2,908 research outputs found
Large tunable photonic band gaps in nanostructured doped semiconductors
A plasmonic nanostructure conceived with periodic layers of a doped
semiconductor and passive semiconductor is shown to generate spontaneously
surface plasmon polaritons thanks to its periodic nature. The nanostructure is
demonstrated to behave as an effective material modeled by a simple dielectric
function of ionic-crystal type, and possesses a fully tunable photonic band
gap, with widths exceeding 50%, in the region extending from mid-infra-red to
Tera-Hertz.Comment: 6 pages, 4 figures, publishe
Optimal design of the fiber-reinforcement to strengthen existing structures
An original approach is proposed to define the optimal design of any unidirectional fiber–reinforcement to improve the structural performance of existing structural elements. A problem of topology optimization is formulated, simultaneously searching for the regions to be strengthened and the optimal local fiber
orientation. The maximum equivalent stress in the underlying material is minimized, for a given amount
of reinforcement. The Tsai–Wu strength criterion is employed, to take into account the different strength properties of the material in tension and compression and the possible material anisotropy. Tensile stresses along the fiber direction are not allowed in the reinforcement. The resulting multi–constrained min–max problem is solved by mathematical programming. A numerical example is presented to discuss the features of the achieved optimal layouts, along with their possible application to the preliminary design of any fiber reinforcement
Spin-Exchange Interaction in ZnO-based Quantum Wells
Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure
Development and first operation of a Cavity Ring Down Spectroscopy diagnostic in the negative ion source SPIDER
The Neutral Beam Injectors of the ITER experiment will rely on negative ion
sources to produce 16.7 MW beams of H/D particles accelerated at 1 MeV. The
prototype of these sources was built and is currently operated in the SPIDER
experiment (Source for the Production of Ions o Deuterium Extracted from an RF
plasma), part of the Neutral Beam Test Facility of Consorzio RFX, Padua. In
SPIDER, the H-/D- ion source is coupled to a three grids 100 kV acceleration
system. One of the main targets of the experimentation in SPIDER is to
uniformly maximize the extracted current density; to achieve this it is
important to study the density of negative ions available in proximity of the
ion acceleration system. In SPIDER, line-integrated measurements of negative
ion density are performed by a Cavity Ring Down Spectroscopy (CRDS) diagnostic.
Its principle of operation is based on the absorption of the photons of a laser
beam pulse by H-/D- photo-detachment; the absorption detection is enhanced by
trapping the laser pulse in an optical cavity, containing the absorbing medium
(i.e. negative ions). The paper presents and discusses the CRDS diagnostic
setup in SPIDER, including the first measurements of negative ion density,
correlated to the main source parameters.Comment: 5 pages, 7 figures. Contributer paper for the HTPD 2020 conference.
Accepted manuscrip
Dynamic behaviour analysis of an english-bond masonry prototype using a homogenized-based discrete FE model
Full Finite Element strategies (the so called micro- and macro- models) are still nowadays the most used ones for the study of large masonry structures. However, macro-modelling still lacks accuracy at a meso-scale in terms of damage localization. On the other hand, micro-models are rather computational demanding and require a cumbersome modelling stage. Thus, homogenization-based frameworks give considerable advantages. Moreover, the study of English bond masonry appears to be disregarded in comparison to the running bond one. On this behalf, a two-step procedure based on homogenization theory is herein presented for the dynamic study of English-bond masonry structures. The presented homogenization approach uses two models at a micro-scale: (i) a plane-stress FE discretization within the concepts of Kirchhoff-Love plate theory; and (ii) a three-dimensional micro-model accounting with the mortar joint discontinuity existent at the thickness direction. Bricks are meshed with elastic elements with linear interpolation and joints are reduced to interfaces which obey to the nonlinear behaviour described by the so-called combined cracking-shearing-crushing model. The procedure allows obtaining homogenized bending moment/torque curvature relationships to be used at a structural level within a FE discrete model implemented in a commercial code. The model relies in rigid quadrilateral elements interconnected by homogenized bending/torque nonlinear springs. The framework is used to study the dynamic behaviour of an English-bond masonry wall benchmark. A macroscopic strategy is also considered to enrich the study. The numerical results are compared with the experimental data and a good agreement has been found.FCT (Portuguese Foundation for Science and Technology), within ISISE, scholarship SFRH/BD/95086/2013. This work was also partly financed by FEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT – Foundation for Science and Technology within the scope of the project POCI-01-0145-FEDER-00763
Electromagnetic filaments and edge modifications induced by electrode biasing in the RFX-mod tokamak
Optical detection and spatial modulation of mid-infrared surface plasmon polaritons in a highly doped semiconductor
Highly doped semiconductors (HDSCs) are promising candidates for plasmonic applications in the mid-infrared (MIR) spectral range. This work examines a recent addition to the HDSC family, the dilute nitride alloy In(AsN). Post-growth hydrogenation of In(AsN) creates a highly conducting channel near the surface and a surface plasmon polariton detected by attenuated total reflection techniques. The suppression of plasmonic effects following a photo-annealing of the semiconductor is attributed to the dissociation of the N-H bond. This offers new routes for direct patterning of MIR plasmonic structures by laser writing
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