Letteratura scientifica selezionata sul tema "Semi-conducteur à large bande"
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Articoli di riviste sul tema "Semi-conducteur à large bande"
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Baili, G., M. Alouini, D. Dolfi e F. Bretenaker. "Laser semi-conducteur de classe A et de très faible bruit sur une large bande passante". Journal de Physique IV (Proceedings) 135, n. 1 (ottobre 2006): 165–66. http://dx.doi.org/10.1051/jp4:2006135042.
Testo completo
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Naudan, F., C. Nicolas e J. P. Huignard. "Filtrage des modes latéraux d'un laser semi-conducteur à cavité large par rétroinjection conjuguée". Annales de Physique 20, n. 5-6 (1995): 599–600. http://dx.doi.org/10.1051/anphys:199556030.
Testo completo
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Pan, Hengxing, Danail Obreschkow, Cullan Howlett, Claudia del P. Lagos, Pascal J. Elahi, Carlton Baugh e Violeta Gonzalez-Perez. "Multiwavelength consensus of large-scale linear bias". Monthly Notices of the Royal Astronomical Society 493, n. 1 (25 gennaio 2020): 747–64. http://dx.doi.org/10.1093/mnras/staa222.
Testo completoAbstract (sommario):
ABSTRACT We model the large-scale linear galaxy bias bg(x, z) as a function of redshift z and observed absolute magnitude threshold x for broad-band continuum emission from the far-infrared to ultraviolet, as well as for prominent emission lines, such as the H α, H β, Ly a, and [O ii] lines. The modelling relies on the semi-analytic galaxy formation model galform, run on the state-of-the-art N-body simulation surfs with the Planck 2015 cosmology. We find that both the differential bias at observed absolute magnitude x and the cumulative bias for magnitudes brighter than x can be fitted with a five-parameter model: bg(x, z) = a + b(1 + z)e(1 + exp [(x − c)d]). We also find that the bias for the continuum bands follows a very similar form regardless of wavelength due to the mixing of star-forming and quiescent galaxies in a magnitude-limited survey. Differences in bias only become apparent when an additional colour separation is included, which suggest extensions to this work could look at different colours at fixed magnitude limits. We test our fitting formula against observations, finding reasonable agreement with some measurements within 1σ statistical uncertainties, and highlighting areas of improvement. We provide the fitting parameters for various continuum bands, emission lines, and intrinsic galaxy properties, enabling a quick estimation of the linear bias in any typical survey of large-scale structure.
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Partamies, Noora, James M. Weygand e Liisa Juusola. "Statistical study of auroral omega bands". Annales Geophysicae 35, n. 5 (7 settembre 2017): 1069–83. http://dx.doi.org/10.5194/angeo-35-1069-2017.
Testo completoAbstract (sommario):
Abstract. The presence of very few statistical studies on auroral omega bands motivated us to test-use a semi-automatic method for identifying large-scale undulations of the diffuse aurora boundary and to investigate their occurrence. Five identical all-sky cameras with overlapping fields of view provided data for 438 auroral omega-like structures over Fennoscandian Lapland from 1996 to 2007. The results from this set of omega band events agree remarkably well with previous observations of omega band occurrence in magnetic local time (MLT), lifetime, location between the region 1 and 2 field-aligned currents, as well as current density estimates. The average peak emission height of omega forms corresponds to the estimated precipitation energies of a few keV, which experienced no significant change during the events. Analysis of both local and global magnetic indices demonstrates that omega bands are observed during substorm expansion and recovery phases that are more intense than average substorm expansion and recovery phases in the same region. The omega occurrence with respect to the substorm expansion and recovery phases is in a very good agreement with an earlier observed distribution of fast earthward flows in the plasma sheet during expansion and recovery phases. These findings support the theory that omegas are produced by fast earthward flows and auroral streamers, despite the rarity of good conjugate observations.
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Marcati, Carmen Regina, Veronica Angyalossy e Ray Franklin Evert. "SEASONAL VARIATION IN WOOD FORMATION OF CEDRELA FISSILIS (MELIACEAE)". IAWA Journal 27, n. 2 (2006): 199–211. http://dx.doi.org/10.1163/22941932-90000149.
Testo completoAbstract (sommario):
Cambial activity and periodicity of secondary xylem formation in Cedrela fissilis, a semi-ring-porous species, were studied. Wood samples were collected periodically from 1996 to 2000. The phenology was related to climate data of the region. The cambium has one active and one dormant period per year. The active period coincides with the wet season when trees leaf-out. The dormant period coincides with the dry season when trees lose their leaves. Growth rings are marked by parenchyma bands that begin to be formed, together with the small latewood vessels, just before the cambium becomes dormant at the beginning of the dry season. These bands are added to when the cambium reactivates in the wet season. At this time, the large earlywood vessels of the growth rings are also formed. As these bands consist of both terminal and initial parenchyma, we suggest the general term marginal bands be used to describe them. The growth layers vary in width among and within the trees.
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Li, Zongrui, Jun Pan, Zhuoer Zhang, Mi Wang e Likun Liu. "MTCSNet: Mean Teachers Cross-Supervision Network for Semi-Supervised Cloud Detection". Remote Sensing 15, n. 8 (12 aprile 2023): 2040. http://dx.doi.org/10.3390/rs15082040.
Testo completoAbstract (sommario):
Cloud detection methods based on deep learning depend on large and reliable training datasets to achieve high detection accuracy. There will be a significant impact on their performance, however when the training data are insufficient or when the label quality is low. Thus, to alleviate this problem, a semi-supervised cloud detection method, named the mean teacher cross-supervision cloud detection network (MTCSNet) is proposed. This method enforces both consistency and accuracy on two cloud detection student network branches, which are perturbed with different initializations, for the same input image. For each of the two student branches, the respective teacher branches, used to generate high-quality pseudo labels, are constructed using an exponential moving average method (EMA). A pseudo one-hot label, produced by one teacher network branch, supervises the other student network branch with the standard cross-entropy loss, and vice versa. To incorporate additional prior information into the model, the presented method uses near-infrared bands instead of red bands as model inputs and injects strong data augmentations on unlabeled images fed into the student model. This induces the model to learn richer representations and ensure consistency constraints on the predictions of the same unlabeled image across different batches. To attain a more refined equilibrium between the supervised and semi-supervised loss in the training process, the proposed cloud detection network learns the optimal weights based on homoscedastic uncertainty, thus effectively exploiting the advantages of semi-supervised tasks and elevating the overall performance. Experiments on the SPARCS and GF1-WHU public cloud detection datasets show that the proposed method outperforms several state-of-the-art semi-supervised algorithms when only a limited number of labeled samples are available.
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Cabrera, Densil, Shuai Lu, Jonothan Holmes e Manuj Yadav. "Sound Reflections in Indian Stepwells: Modelling Acoustically Retroreflective Architecture". Acoustics 4, n. 1 (2 marzo 2022): 227–47. http://dx.doi.org/10.3390/acoustics4010014.
Testo completoAbstract (sommario):
Retroreflection is rarely used as a surface treatment in architectural acoustics but is found incidentally with building surfaces that have many simultaneously visible concave right-angle trihedral corners. Such surfaces concentrate reflected sound onto the sound source, mostly at high frequencies. This study investigated the potential for some Indian stepwells (stepped ponds, known as a kund or baori/baoli in Hindi) to provide exceptionally acoustically retroreflective semi-enclosed environments because of the unusually large number of corners formed by the steps. Two cases—Panna Meena ka Kund and Lahan Vav—were investigated using finite-difference time-domain (FDTD) acoustic simulation. The results are consistent with retroreflection, showing reflected energy concentrating on the source position mostly in the high-frequency bands (4 kHz and 2 kHz octave bands). However, the larger stepped pond has substantially less retroreflection, even though it has many more corners, because of the greater diffraction loss over the longer distances. Retroreflection is still evident (but reduced) with non-right-angle trihedral corners (80°–100°). The overall results are sufficiently strong to indicate that acoustic retroreflection should be audible to an attuned visitor in benign environmental conditions, at least at moderately sized stepped ponds that are in good geometric condition.
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Ayres, J. F., R. D. Murison, A. D. Turner e S. Harden. "A rapid semi-quantitative procedure for screening hydrocyanic acid in white clover (Trifolium repens L.)". Australian Journal of Experimental Agriculture 41, n. 4 (2001): 515. http://dx.doi.org/10.1071/ea97069.
Testo completoAbstract (sommario):
The presence of cyanogenic glucosides in white clover (Trifolium repens L.) is an anti-nutritional factor due to the potential for cyanide to pre-dispose selenium deficiency in grazing animals. Considerable genotypic variation in cyanogenesis occurs in white clover and it is important that highly cyanogenic white clover lines are identified to ensure that germpasm used in breeding programs does not lead to the release of cultivars that exceed safe levels. A procedure for rapid semi-quantitative screening of large white clover germplasm collections is described. This procedure is based on the picrate assay and utilises computer imagery and calibration relationships between spectral intensity (red, green, blue bands) of the colour reaction on picrate paper with cyanide in standard solutions to predict hydrocyanic acid concentration in white clover leaf.
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Dagbovie, Ayawoa S., e Jonathan A. Sherratt. "Pattern selection and hysteresis in the Rietkerk model for banded vegetation in semi-arid environments". Journal of The Royal Society Interface 11, n. 99 (6 ottobre 2014): 20140465. http://dx.doi.org/10.1098/rsif.2014.0465.
Testo completoAbstract (sommario):
Banded vegetation is a characteristic feature of semi-arid environments. It occurs on gentle slopes, with alternating stripes of vegetation and bare ground running parallel to the contours. A number of mathematical models have been proposed to investigate the mechanisms underlying these patterns, and how they might be affected by changes in environmental conditions. One of the most widely used models is due to Rietkerk and co-workers, and is based on a water redistribution hypothesis, with the key feedback being that the rate of rainwater infiltration into the soil is an increasing function of plant biomass. Here, for the first time, we present a detailed study of the existence and stability of pattern solutions of the Rietkerk model on slopes, using the software package wavetrain ( www.ma.hw.ac.uk/wavetrain ). Specifically, we calculate the region of the rainfall–migration speed parameter plane in which patterns exist, and the sub-region in which these patterns are stable as solutions of the model partial differential equations. We then perform a detailed simulation-based study of the way in which patterns evolve when the rainfall parameter is slowly varied. This reveals complex behaviour, with sudden jumps in pattern wavelength, and hysteresis; we show that these jumps occur when the contours of constant pattern wavelength leave the parameter region giving stable patterns. Finally, we extend our results to the case in which a diffusion term for surface water is added to the model equations. The parameter regions for pattern existence and stability are relatively insensitive to small or moderate levels of surface water diffusion, but larger diffusion coefficients significantly change the subdivision into stable and unstable patterns.
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Oliveira Santos, Victor, Bruna Monallize Duarte Moura Guimarães, Iran Eduardo Lima Neto, Francisco de Assis de Souza Filho, Paulo Alexandre Costa Rocha, Jesse Van Griensven Thé e Bahram Gharabaghi. "Chlorophyll-a Estimation in 149 Tropical Semi-Arid Reservoirs Using Remote Sensing Data and Six Machine Learning Methods". Remote Sensing 16, n. 11 (24 maggio 2024): 1870. http://dx.doi.org/10.3390/rs16111870.
Testo completoAbstract (sommario):
It is crucial to monitor algal blooms in freshwater reservoirs through an examination of chlorophyll-a (Chla) concentrations, as they indicate the trophic condition of these waterbodies. Traditional monitoring methods, however, are expensive and time-consuming. Addressing this hindrance, we conducted a comprehensive investigation using several machine learning models for Chla modeling. To this end, we used in situ collected water sample data and remote sensing data from the Sentinel-2 satellite, including spectral bands and indices, for large-scale coverage. This approach allowed us to conduct a comprehensive analysis and characterization of the Chla concentrations across 149 freshwater reservoirs in Ceará, a semi-arid region of Brazil. The implemented machine learning models included k-nearest neighbors, random forest, extreme gradient boosting, the least absolute shrinkage, and the group method of data handling (GMDH); in particular, the GMDH approach has not been previously explored in this context. The forward stepwise approach was used to determine the best subset of input parameters. Using a 70/30 split for the training and testing datasets, the best-performing model was the GMDH model, achieving an R2 of 0.91, an MAPE of 102.34%, and an RMSE of 20.4 μg/L, which were values consistent with the ones found in the literature. Nevertheless, the predicted Chla concentration values were most sensitive to the red, green, and near-infrared bands.
Tesi sul tema "Semi-conducteur à large bande"
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Forster, Simon. "Nouveau matériau semi-conducteur à large bande interdite à base de carbures ternaires - Enquête sur Al4SiC4". Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAI095.
Testo completoAbstract (sommario):
Les matériaux semi-conducteurs à large bande interdite sont capables de résister aux environnements difficiles et de fonctionner dans une large plage de températures. Celles-ci sont idéales pour de nombreuses applications telles que les capteurs, la haute puissance et les radiofréquences.Cependant, des matériaux plus nouveaux sont nécessaires pour atteindre une efficacité énergétique significative dans diverses applications ou pour développer de nouvelles applications destinées à compléter les semi-conducteurs à bande interdite tels que le GaN et le SiC.Dans cette thèse, trois méthodes différentes sont utilisées pour étudier l’un de ces nouveauxmatériaux, carbure d'aluminium et de silicium (Al4SiC4): (1) simulations d'ensemble de Monte Carloafin d'étudier les propriétés de transport d'électrons du nouveau carbure ternaire, (2)études expérimentales pour déterminer ses propriétés matérielles et (3) simulations de dispositifsd'un dispositif à hétérostructure rendu possible par ce carbure ternaire. Toutes ces méthodesinterconnecter les uns avec les autres. Les données de chacun d’eux peuvent alimenter l’autre pour acquérir de nouvelles connaissances.résultats ou affiner les résultats obtenus conduisant ainsi à des propriétés électriques attrayantes telles qu’une bande interdite de 2,78 eV ou une vitesse de dérive maximale de 1,35 × 10 cm s.Ensemble Monte Carlo, développé en interne pour les simulations de Si, Ge, GaAs,AlxGa1-xAs, AlAs et InSb; est adopté pour les simulations du carbure ternaire en ajoutant untransformation de la nouvelle vallée pour tenir compte de la structure hexagonale de Al4SiC4. Nous prédisonsune vitesse maximale de dérive des électrons de 1,35 × 107 cm-1 à un champ électrique de 1400 kVcm-1 et une mobilité maximale des électrons de 82,9 cm V s. Nous avons vu une constante de diffusion de 2,14 cm2s-1 à un champ électrique faible et de 0,25 cm2s-1 à un champ électrique élevé. Enfin nousmontrer que Al4SiC4 a un champ critique de 1831 kVcmOn utilise des cristaux semi-conducteurs qui avaient été cultivés auparavant à l’IMGP, l’un par la croissance en solution et l’autre par la fusion en creuset. Trois expériences différentes sont effectuées sur eux; (1) spectroscopie UV, IR et visuelle, (2) spectroscopie photographique à rayons X, et (3) mesures à deux et à quatre sondes dans lesquelles un contact métallique est formé sur les cristaux. Nous avons trouvé ici une bande interdite de spectroscopie UV, IR et Vis de 2,78 ± 0,02 eV et une couche d’oxyde épaisse sur les échantillons en utilisant du XPS. Malheureusement, les mesures à deux et à quatre sondes n'ont donné aucun résultat autre que le bruit, probablement en raison de l'épaisse couche d'oxyde trouvée sur les échantillons.Dans les simulations de dispositifs, le logiciel commercial Atlas de Silvaco est utilisé pour prédire les performances des dispositifs à hétérostructure, avec des longueurs de grille de 5, 2 et 1 µm, rendues possibles par le carbure ternaire en combinaison avec du SiC. Le transistor à hétérostructure SiC / Al4SiC4 d'une longueur de grille de 5 µm délivre un courant de drain maximal de 1,68 × 10−4 A / µm, qui passe à 2,44 × 10−4 A / µm et à 3,50 × 10−4 A / µm pour des longueurs de grille de 2 µm et 1 µm, respectivement. La tension de claquage de l'appareil est de 59,0 V, ce qui réduit à 31,0 V et à 18,0 V les transistors mis à l'échelle des longueurs de grille de 2 µm et de 1 µm. Le dispositif à longueur de grille réduite de 1 μm bascule plus rapidement en raison de la transconductance supérieure de6,51 × 10−5 S / μm par rapport à une fois par an1,69 × 10−6 S / μm pour le plus grand périphérique.Enfin, une pente inférieure au seuil des dispositifs mis à l'échelle est égale à 197,3 mV / dec, 97,6 mV / dec et 96,1 mV / dec pour des longueurs de grille de 5 µm, 2 µm et 1 µm, respectivementWide bandgap semiconductor materials are able to withstand harsh environments and operate over a wide range of temperatures. These make them ideal for many applications such as sensors, high-power and radio-frequencies to name a few.However, more novel materials are required to achieve significant power efficiency of various applications or to develop new applications to complement current wide bandgap semiconductors such as GaN and SiC.In this dissertation, three different methods are used to study one of these novelmaterials, aluminium silicon carbide (Al4SiC4): (1) ensemble Monte Carlo simulationsin order to study the electron transport properties of the novel ternary carbide, (2)experimental studies to determine its material properties, and (3) device simulationsof a heterostructure device made possible by this ternary carbide. All these methodsinterlink with each other. Data from each of them can feed into the other to acquire newresults or refine obtained results thus leading way to attractive electrical properties such as a bandgap of 2.78 eV or a peak drift velocity of 1.35×10 cm s .Ensemble Monte Carlo toolbox, developed in-house for simulations of Si, Ge, GaAs,AlxGa1−xAs, AlAs, and InSb; is adopted for simulations of the ternary carbide by adding anew valley transformation to account for the hexagonal structure of Al4SiC4. We predicta peak electron drift velocity of 1.35×107 cms−1 at electric field of 1400 kVcm−1 and a maximum electron mobility of 82.9 cm V s . We have seen a diffusion constant of 2.14 cm2s−1 at a low electric field and of 0.25 cm2s−1 at a high electric field. Finally, weshow that Al4SiC4 has a critical field of 1831 kVcmsemiconductor crystals are used that had previously been grown at IMGP, one by solution grown and the other by crucible melt. Three different experiments are performed on them; (1) UV, IR and Vis Spectroscopy, (2) X-ray Photo Spectroscopy, and (3) Two- and four-probe measurements where metal contact are grown on the crystals. Here we have found a bandgap of 2.78 ± 0.02 eV UV, IR and Vis Spectroscopy and a thick oxide layer on the samples using XPS. Unfortunately the Two- and four-probe measurements failed to give any results other than noise, most likely due to the thick oxide layer that was found on the samples.In the device simulations, a commercial software Atlas by Silvaco is utilized to predict performance of heterostructure devices, with gates lengths of 5 μm, 2 μm and 1 μm, made possible by the ternary carbide in a combination with SiC. The 5 μm gate length SiC/Al4SiC4 heterostructure transistor delivers a maximum drain current of 1.68×10−4 A/μm, which increases to 2.44×10−4 A/μm and 3.50×10−4 A/μm for gate lengths of 2 μm and 1 μm, respectively. The device breakdown voltage is 59.0 V which reduces to 31.0 V and to 18.0 V for the scaled 2 μm and the 1 μm gate length transistors. The scaled down 1 μm gate length device switches faster because of the higher transconductance of6.51×10−5 S/μmcomparedtoonly1.69×10−6 S/μmforthelargestdevice.Finally,a sub-threshold slope of the scaled devices is 197.3 mV/dec, 97.6 mV/dec, and 96.1 mV/dec for gate lengths of 5 μm, 2 μm, and 1 μm, respectively
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Pacuski, Wojciech. "Spectroscopie optique de semi-conducteurs magnétiques dilués à large bande interdite, à base de ZnO et GaN". Phd thesis, Grenoble 1, 2007. http://www.theses.fr/2007GRE10310.
Testo completoAbstract (sommario):
Ce travail porte sur la spectroscopie magnéto-optique de semi-conducteurs magnétiques dilués (DMS) : ZnO et GaN dopés manganese, fer et cobalt. Les deux semi-conducteurs hôtes, ZnO et GaN ont une grande bande interdite, une structure wurtzite, une faible interaction spin - orbite et une forte interaction d'échange excitonique entre trous et électrons. En présence de champs magnétiques, les ions magnétiques induisent un effet Zeeman géant dont l'interprétation est complexe : les excitons s'anti-croisent et leurs énergies de transition et leurs forces d'oscillateur sont fortement influencées par l'effet Zeeman géant. On a mesuré expérimentalement le splitting Zeeman géant des excitons A et B avec des couches epitaxiées sur saphir (0001) et une propagation de la lumiere parallele a l'axe c du cristal et au champ magnétique (configuration Faraday). Le splitting Zeeman géant diminue avec la température et augmente non linéairement avec le champ magnétique en accord avec l'aimantation calculée des spins isolés. Une analyse quantitative nous a permis d'analyser les propriétés magnétiques et de mesurer les intégrales d'échange pour l'ensemble des matériaux étudiés. Pour des ions avec une configuration d5 (Mn2+ et Fe3+), l'aimantation suit une fonction de Brillouin, mais pour les configurations d7 et d4 (Co2+ ou Mn3+) l'interaction spin-orbite et le champ cristallin trigonal induisent une aimantation anisotrope, en accord avec l'analyse des transitions internes des ions mesurées en spectroscopie infrarouge. Pour (Ga,Mn)N, et (Ga,Fe)N, nous avons trouvé un signe positive pour l'intégrale d'échange entre trous et spins localisés (beta). En supposant une symétrie de la bande de valence dans ZnO correspondant a une interaction spin-orbite positive (Gamma_9, Gamma_7, Gamma_7), nous trouvons un signe négative de beta pour (Zn,Co)O, et beta est de pres de zéro pour (Zn,Mn)O. Toutefois, dans l'hypothese avec spin-orbite négative, nous trouvons un signe positif de beta. Les signes et les valeurs des intégrales d'échange déterminées a partir de nos mesures magnéto-optiques ne peuvent pas etre expliqués par des tendances matérielles et des modeles basés sur l'approximation de cristal virtuel. Ceci suggere que l'échange p-d en DMS a large bande interdite, soit dans le régime de couplage fort, et la nature de splitting Zeeman géant observé est différente qu'en semi-conducteurs magnétiques dilués classiquesThis work presents a magnetooptical study of diluted magnetic semiconductors (DMS) based on ZnO and GaN, doped with manganese, iron, and cobalt. Both host materials, ZnO and GaN, are wide band gap semiconductors with a wurtzite structure, a weak spin-orbit coupling and a strong electron-hole exchange interaction within the excitons. In the presence of a magnetic field, the magnetic ions induce in such materials a giant Zeeman effect with a complex behavior: excitons anti-cross, and not only the transition energies, but also the oscillator strengths are strongly affected by the giant Zeeman effect. On thin epitaxial layers grown on (0001) sapphire, we observed the giant Zeeman splitting of the A and B excitons, which are optically active in the Faraday configuration when the propagating light is parallel to the c-axis. The Zeeman splitting decreases with the temperature and increases non-linearly with the magnetic field, demonstrating a dependence on the magnetization of the localized spins. A quantitative analysis allows us to discuss the detailed behavior of the magnetization and to estimate the p-d exchange integral beta for the studied wide bandgap DMS. For the d^5 electronic configuration (Mn2+, and Fe3+) the magnetization follows a Brillouin function, whereas, for d7 or d4 of Co2+, and Mn3+, respectively, the spin orbit coupling and the trigonal crystal field lead to an anisotropic magnetization, consistent with that deduced independently from the analysis of intra-ionic optical transitions. We find a positive sign of beta for (Ga,Mn)N, and (Ga,Fe)N. In ZnO, the sign of the spin-orbit interaction, which determines the structure of the valence band, has been a matter of debate. Assuming that the valence band ordering in ZnO is Gamma_9, Gamma_7, Gamma_7 (this corresponds to usual, positive sign of the spin-orbit coupling), we find beta to be negative for (Zn,Co)O, and to be close to zero in (Zn,Mn)O. However, assuming the reversed valence band ordering, we find beta to be positive in both ZnO based DMS. The sign and the value of p-d exchange integrals determined from our magnetooptical measurements do not follow material trends in DMS and cannot be explained by models based on the virtual crystal approximation. This suggests that the p-d coupling in wide gap DMS is in the strong coupling regime, so that the nature of the observed giant Zeeman splitting is different from that in classical diluted magnetic semiconductors
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Pacuski, Wojciech. "Spectroscopie optique de semi-conducteurs magnétiques dilués à large bande interdite, à base de ZnO et GaN". Phd thesis, Université Joseph Fourier (Grenoble), 2007. http://tel.archives-ouvertes.fr/tel-00296634.
Testo completoAbstract (sommario):
Ce travail porte sur la spectroscopie magnéto-optique de semi-conducteurs magnétiques dilués (DMS) : ZnO et GaN dopés manganese, fer et cobalt. Les deux semi-conducteurs hôtes, ZnO et GaN ont une grande bande interdite, une structure wurtzite, une faible interaction spin - orbite et une forte interaction d'échange excitonique entre trous et électrons. En présence de champs magnétiques, les ions magnétiques induisent un effet Zeeman géant dont l'interprétation est complexe : les excitons s'anti-croisent et leurs énergies de transition et leurs forces d'oscillateur sont fortement influencées par l'effet Zeeman géant.On a mesuré expérimentalement le splitting Zeeman géant des excitons A et B avec des couches epitaxiées sur saphir (0001) et une propagation de la lumiere parallele a l'axe c du cristal et au champ magnétique (configuration Faraday). Le splitting Zeeman géant diminue avec la température et augmente non linéairement avec le champ magnétique en accord avec l'aimantation calculée des spins isolés. Une analyse quantitative nous a permis d'analyser les propriétés magnétiques et de mesurer les intégrales d'échange pour l'ensemble des matériaux étudiés. Pour des ions avec une configuration d5 (Mn2+ et Fe3+), l'aimantation suit une fonction de Brillouin, mais pour les configurations d7 et d4 (Co2+ ou Mn3+) l'interaction spin-orbite et le champ cristallin trigonal induisent une aimantation anisotrope, en accord avec l'analyse des transitions internes des ions mesurées en spectroscopie infrarouge. Pour (Ga,Mn)N, et (Ga,Fe)N, nous avons trouvé un signe positive pour l'intégrale d'échange entre trous et spins localisés (beta). En supposant une symétrie de la bande de valence dans ZnO correspondant a une interaction spin-orbite positive (Gamma_9, Gamma_7, Gamma_7), nous trouvons un signe négative de beta pour (Zn,Co)O, et beta est de pres de zéro pour (Zn,Mn)O. Toutefois, dans l'hypothese avec spin-orbite négative, nous trouvons un signe positif de beta. Les signes et les valeurs des intégrales d'échange déterminées a partir de nos mesures magnéto-optiques ne peuvent pas etre expliqués par des tendances matérielles et des modeles basés sur l'approximation de cristal virtuel. Ceci suggere que l'échange p-d en DMS a large bande interdite, soit dans le régime de couplage fort, et la nature de splitting Zeeman géant observé est différente qu'en semi-conducteurs magnétiques dilués classiques.
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Fisne, Christophe. "Métasurfaces actives pour applications large bande". Thesis, Toulouse, INPT, 2020. http://www.theses.fr/2020INPT0086.
Testo completoAbstract (sommario):
Les métasurfaces offrent des propriétés électromagnétiques particulières permettant de générer des indices de réfraction inhabituels, produire des bandes électromagnétiques interdites, ou encore créer des surfaces à haute-impédance. Ces dernières, aussi nommées « Artificial Magnetic Conductor (AMC) » sont particulièrement intéressantes dans le domaine antennaire. En effet elles permettent de réduire les couplages de divers éléments rayonnants, mais aussi de miniaturiser les antennes comportant un plan réflecteur. Cependant, leur bande de fonctionnement inférieure à 10%, les rend souvent incompatibles avec des applications large bande. Pour pallier cette limitation en bande passante, l’intégration de circuits actifs de type « non- Foster » ouvre une voie permettant d’élargir la bande passante des surfaces à haute-impédance. Dans ce contexte, l’objectif de la thèse est de concevoir un réflecteur haute impédance largebande grâce à l’intégration de circuits actifs de type « non- Foster ». La bande passante visée est [0,5 GHz ; 1,5 GHz] soit 100% de bande passante relative. Dans cette thèse, une méthodologie de synthèse d’AMC large bande est proposée : un réflecteur AMC sous incidence normale est conçu à partir d’une métasurface connectée à un circuit non-Foster qui est lui-même chargé par une impédance optimisée. Les relations analytiques permettant d’exprimer le coefficient de réflexion de la métasurface en fonction du circuit non-Foster et de sa charge sont explicitées. Dans un premier temps, une métasurface fonctionnant en polarisation rectiligne permettant le report d’un circuit non Foster est proposée. Cette topologie permet de protéger le circuit électronique des ondes électromagnétiques incidentes.De plus, une étude permettant d’étendre le fonctionnement de la métasurface en polarisation circulaire est amorcée. Ensuite, un circuit Non- Foster de type « Negative Impedance Converter (NIC) » est conçu. La topologie du circuit est choisie dans un souci de simplification technologique. La réalisation de ce circuit a été faite en utilisant des composants standards et l’utilisation de potentiomètres permet un contrôle de l’impédance en entrée du circuit. Enfin, la charge de ce circuit NIC est déterminée afin de permettre le comportement AMC large bande en prenant en compte le comportement réel du circuit non FosterMetasurfaces have particular electromagnetic properties such as unusually refractive index, electromagnetic band gap and high impedance surface. Also named Artificial Magnetic Conductor (AMC), they are a focus of interest in the antennas field. Indeed, greater isolation between radiating elements and miniaturization of antenna with reflective plan can be achieved with those structures. Although they suffer from poor bandwidth (less than 10%), which make them inconsistent with wideband applications. To overcome this frequency limitation, implementation of non-Foster active circuits paves the way to extend the bandwidth of high impedance surfaces. In this respect, the thesis goal is to conceive a wideband high-impedance reflector with the integration of non-Foster circuits. The aimed bandwidth is [0.5 GHz; 1.5 GHz], that is to say 100% of the relative bandwidth. In this thesis, a synthesis methodology to realize a wideband AMC is proposed: an AMC reflector under normal incidence is conceived from a metasurface connected to a non-Foster circuit. The circuit is loaded with an optimized impedance. Analytic relationships between the reflection coefficient and the load impedance of the non-Foster circuit are given. Firstly, a metasurface working with linear polarizations and where the connection of the non-Foster circuit is offset. This topology protects the circuit against the perturbations due to the incident electromagnetic waves. Moreover, a study to extend metasurface functioning to circular polarization is under way. Then, a non-Foster circuit of type Negative Impedance Converter (NIC) is designed. A particular topology of circuit is selected in order to simplify the realization. It has been conceived using only components “off-the-shelf” and potentiometers which control the input impedance. Finally, the circuits load is calculated to obtain the attended wideband AMC behavior according with the real performance of the non-Foster circuit
5
Bouffaron, Renaud. "Modélisation et Réalisation de Réseaux Sub-Longueur d'Onde :Application au Contrôle de la Réflectivité Large Bande, Large Incidence". Phd thesis, Université Paul Cézanne - Aix-Marseille III, 2008. http://tel.archives-ouvertes.fr/tel-00353626.
Testo completoAbstract (sommario):
Les antireflets permettent d'accroître l'efficacité des cellules photovoltaïques, d'augmenter la sensibilité des détecteurs optroniques, et même d'améliorer l'extraction lumineuse des diodes électroluminescentes. Traditionnellement, des empilements de matériaux en couches minces sont utilisés pour les fabriquer. Nous avons étudié une technique alternative qui s'appuie sur la microstructuration de l'interface air-substrat. Il s'agit, plus précisément, de modéliser et de fabriquer des surfaces microstructurées bi-périodiques sur silicium et sur germanium présentant un effet antireflet très efficace dans l'infrarouge en bandes II et III respectivement. Ces structures nécessitent une description rigoureuse des phénomènes de propagation de la lumière. L'influence des paramètres opto-géométriques est examinée sous le point de vue des cristaux photoniques en utilisant les diagrammes de bandes. Pour réaliser ces structures, des techniques à bas coût, basées sur une gravure humide anisotrope du semi-conducteur cristallin à travers un masque obtenu par photolithographie, ont été utilisées. Nous obtenons expérimentalement sur silicium un facteur de réflexion inférieur à 4% sur l'ensemble du spectre IR II. Un très bon accord calcul/mesure permet de valider les résultats numériques obtenus précédemment.
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Regnat, Guillaume. "Onduleur à forte intégration utilisant des semi-conducteurs à grand gap". Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAT050/document.
Testo completoAbstract (sommario):
Les composants semi-conducteurs à base de matériaux à grand gap (SiC et GaN) présentent des caractéristiques intéressantes pour la réalisation de convertisseurs d’électronique de puissance toujours plus intégrés. Cependant, le packaging des composants traditionnels en silicium ne semble plus adapté pour ces nouveaux composants et apparaît même comme un facteur limitant. Le développement d’un packaging adapté aux caractéristiques des composants à grand gap est alors nécessaire. Les travaux développés dans cette thèse proposent un nouveau packaging tridimensionnel basé sur un procédé de fabrication de circuit imprimé. L’architecture du module est basé sur le concept « Power Chip On Chip » dont le principe de base permet de réduire les perturbations électromagnétiques. Le procédé de fabrication des circuits imprimés offre une grande flexibilité pour le routage en trois dimensions et permet de s’affranchir de l’interconnexion par fil de bonding entre le package et la puce. La démarche de conception du module s’appuie sur une approche multi-physique afin de qualifier le comportement électromagnétique et thermique du module puis de proposer des voies d’optimisation. Un prototype d’un module implémentant quatre cellules de commutation en parallèle, à base de MOSFET SiC, a été produit avec des moyens de production industriels. Les différents tests réalisés valident l’approche retenue dans ce projet mais soulignent également les aspects technologiques à approfondir pour la réalisation d’un module de puissance industrielWide-band-gap (WBG) semiconductors (SiC and Gan) offer interesting characteristics to realize high density power electronics converters. Conventional packaging used for silicon devices is no more adapted for those now components. Development of dedicated packaging for WBG devices is absolutely required. This PhD thesis presents a new 3D package based on Printed Circuit Board (PCB) industrial process. The module architecture is based on “Power Chip On Chip” concept which allows reducing electromagnetic perturbations. PCB fabrication process offers high design flexibility in three dimensions and allows removing wire bonding to interconnect power die and package. The power module design process is buit on multi-physics design tools in the aim to quantify electromagnetic and thermal behavior of the module. Furthermore, several optimization parameters are highlighted. A power module prototype, with four commutation cells in parallel based on SiC MOSFET, has been produced thanks to industrial facilities. Tests realized on new power module confirm the validity of the concept but furthermore to highlight critical technological parameters to realize an industrial power module
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Grelier, Michael. "Miniaturisation des antennes large bande à l'aide de matériaux artificiels". Phd thesis, Télécom ParisTech, 2011. http://pastel.archives-ouvertes.fr/pastel-00574620.
Testo completoAbstract (sommario):
Dans le domaine de la guerre électronique, la maîtrise du spectre électromagnétique est primordiale. Les antennes doivent donc posséder une couverture fréquentielle extrêmement large. Leur bande de fréquences de fonctionnement peut dépasser la décade, i.e. un rapport 10 entre fréquences haute et basse. De plus, la fréquence basse peut être voisine de la centaine de MHz augmentant de ce fait l'encombrement de l'antenne dans le sens de l'épaisseur. L'intégration d'antenne bande basse sur porteur constitue donc un enjeu majeur. Les antennes classiques dédiées a la guerre électronique font habituellement appel a des antennes dites " indépendantes de la fréquence " chargées par une cavité absorbante. Les travaux présentes dans ce manuscrit ont pour but de supprimer les absorbants présents dans les antennes pour a la fois augmenter leur efficacité et réduire leur épaisseur. Nos recherches se sont orientés vers les matériaux dits " artificiels ", les conducteurs magnétiques artificiels (CMA) et des matériaux a bandes interdites électromagnétiques (BIE), permettant la réalisation pratique de réflecteur magnétique. Apres un état de l'art sur les méthodes analytiques, numériques et expérimentales, nous étudions le comportement de l'antenne spirale d'Archimède en espace libre puis en présence de réflecteurs théoriques: conducteur électrique parfait et conducteur magnétique parfait. Nous avons proposé et validé expérimentalement une méthodologie innovante pour la conception d'antenne large bande faible épaisseur. Enfin, nous avons présenté un réflecteur hybride exploitant les propriétés des conducteurs électriques et magnétiques afin d'étendre la bande de fréquences des antennes.
8
Raboin, Jean-Christophe. "Complexes de métaux de transition pour la photosensibilisation de semi-conducteurs à large bande interdite". Metz, 1999. http://docnum.univ-lorraine.fr/public/UPV-M/Theses/1999/Raboin.Jean_Christophe.SMZ9940.pdf.
Testo completoAbstract (sommario):
Des ligands tridentates de la famille des terpyridines et des analogues cyclométallants ainsi que des ligands bidentates 5-(2-pyridyl)-tétrazoles sont préparés. Ces ligands sont fonctionnalisés par des acides carboxyliques vicinaux pour obtenir une bonne fixation et un transfert de photoélectron maximum sur TiO2 ou par des coumarines condensées pour augmenter l'efficacité de captation de la lumière et accéder à des états excités de plus basse énergie. Les ligands sont utilisés pour coordiner des cations de Ru, Fe et Rh. Des problèmes de décarboxylation sélective ou d'absence de cyclométallation lors de l'utilisation de phenylbipyridines à coumarines condensées sont résolus. Un cas de cyclométallation de pyridine par retournement de cycle est observé. Une corrélation entre les potentiels rédox et les groupes fonctionnels des complexes est établie ainsi que des effets auxo- et hyperchrome en spectroscopie d'absorption. La plupart des complexes sont émetteurs à température ambiante dont certains avec des rendements d'émission proches de Ru(bipy)3Cl2. Les complexes de ruthénium porteurs d'acides carboxyliques sont testés en photosensibilisation sur électrodes de TiO2. Des rendements de conversion de photon en courant jusqu'à plus de 70% sont atteints
9
Vigué, Florence. "Optoélectronique visible et ultraviolette à base de semi-conducteurs II-VI à large bande interdite". Nice, 2001. http://www.theses.fr/2001NICE5632.
Testo completoAbstract (sommario):
Depuis plusieurs années, un effort de recherche important est consacré aux semi-conducteurs à large bande interdite pour leur applications optoélectroniques. Les principaux objectifs sont la réalisation de détecteurs ultraviolet (UV) et la fabrication de diodes laser émettant dans le bleu ou l'UV. C'est avec cet objectif que nous avons étudié les composés II-VI à base de ZnSe et ZnO. Des photodétecteurs UV en ZnSe et (Zn,Mg,Be,Se) sont tout d'abord décrits. Trois types de structures ont été fabriquées avec de bonnes performances. Il s'agit des photodiodes p-i-n, à barrière Schottky et métal-semiconducteur-métal. Avec ces différents dispositifs, nous obtenons des réponses spectrales élevées, de forts contrastes de détection UV/visible et de faibles niveaux de bruit. Ce travail démontre le fort potentiel des composés II-VI ZnSe et (Zn,Mg,Be,Se) pour la détection ultraviolette. La croissance de ZnO sur substrat de saphir est parallèlement abordée. Deux modes de croissances sont obtenus selon les conditions expérimentales. La croissance bidimensionnelle conduit à la formation d'une structure mosai͏̈que de grains délimités par des dislocations verticales tandis que la croissance tridimensionnelle favorise de nombreuses interactions entre les dislocations et mène à une meilleure qualité structurale. Une densité totale de dislocations de 3-5x109 cm-2 est obtenue dans ce casOver the past few years, there has been a great deal of work focused on wide bandgap semiconductors for their applications in electronics and optoelectronics. Main goals are the fabrication of ultraviolet (UV) photodetectors and blue and UV laser diodes. In this work, II-VI compounds based on zinc selenide (ZnSe) and zinc oxyde (ZnO) are studied for these purposes. UV photodetectors based on ZnSe and especially on (Zn,Mg,Be,Se) quaternary alloys are first reported. Three types of structures have been studied which are p-i-n, Schottky barrier and metal-semiconductor-metal photodiodes. With these different devices, high maximum spectral responses, important UV/visible contrasts and low noise levels are achieved. This work thus demonstrates the potential of ZnSe and (Zn,Mg,Be,Se) compounds for UV detection. Growth of ZnO on sapphire substrate has also been studied. It is demonstrated that two different growth modes can be achieved depending on experimental conditions. Two-dimensional growth mode layers exhibit a mosaic structure with columnar subgrains delineated by vertical dislocations whereas the three-dimensional growth mode generates numerous interactions between dislocations and leads to higher structural quality films. A total dislocation density of 3-5x109 cm-2 is obtained in that case
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Kaddour, Darine. "Conception et réalisation de filtres RF passe-bas à structures périodiques et filtres Ultra Large Bande, semi localisés en technologie planaire". Phd thesis, Grenoble 1, 2007. http://www.theses.fr/2007GRE10095.
Testo completoAbstract (sommario):
La technologie planaire constitue une technologie attrayante pour la réalisation de filtres, en terme de coût, de volume, ainsi que des possibilités d'intégration. Dans ce contexte, le développement de filtres planaires sélectifs et peu encombrants est à l'heure actuelle un domaine d'activité d'un intérêt fondamental. Les travaux présentés dans ce manuscrit s'inscrivent dans cet axe de recherche. Après un état de l'art sur les différentes familles de filtres planaires, une nouvelle topologie de filtre passe-bas compact, constituée d'une ligne de propagation périodiquement chargée par des capacités CMS, est développée. Une méthode rigoureuse de conception est développée et des démonstrateurs hybrides en teclmologie CPW à 1 GHz sont réalisés, Les mesures, en très bon accord avec les simulations, valident des propriétés intéressantes en terme de l'éjection, d'adaptation et de remontées parasites, La montée en fréquence est également explorée par la réalisation d'un filtre passe-bas en bande K. Les résultats montrent la nécessité d'une caractérisation fine des capacités. Ensuite, une méthode de caractérisation des capacités est mise en oeuvre. Il s'avère que l'insertion des capacités dans des filtres passe-bas à structure périodique constitue un moyen précis et fiable de modélisation. Une topologie de filtre passe-bande à Ultra Large Bande, basée sur une imbrication d'un filtre passe-haut à stubs dans un, filtre passe-bas à lignes capacitivement chargées, est mise en place. Les mesures des prototypes en technologie micro- ' ruban, en bon accord avec les simulations, montrent des propriétés intéressantes de sélectivité, de miniaturisation et de rejet de lobes secondairesLn recent years, extensive research and development efforts have been put into exploiting planar filters for their potential qualities of low cost, tiny weight and high degree of integration. The aim of the research work presented in this manuscript is the design and the realization of selective and miniaturized planar filters. A new topology of compact low-pass filter with periodic structures, constituted by a transmission line loaded by SMT capacitors, is proposed. A rigorous design method for the low-pass filter is also implemented. The measurements of the low-pass filters, realized ne al' to 1 GHz in a CPW technology, in good agreement with simulations demonstrated interesting electrical proprieties of rejection, matching and spurious suppression. The validity of this topology is also investigated for higher frequencies with the realization of a low-pass filter in the K-band. Several capacitor's characterization approaches were also investigated. The implementation of capacitors in a low pass filter with periodic structure carried out constitutes the most precise way to characterize capacitors. A new topology of Ultra Wide Band band-pass filters, based on the combination of a high pass filter with short circuited stubs and a low pass filter with capacitively loaded lines, is proposed, The measurements of the prototype filters realized in a microstrip teclmology, in good agreement with simulations, show interesting proprieties of selectivity, miniaturization and spurious suppression
Libri sul tema "Semi-conducteur à large bande"
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1921-, Bhargava Rameshwar Nath, a cura di. Properties of wide bandgap II-VI semiconductors. London, U.K: INSPEC, Institute of Electrical Engineers, 1997.
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2
J, Pearton S., a cura di. Wide bandgap semiconductors: Growth, processing and applications. Park Ridge, N.J: Noyes Publications, 2000.
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Properties of Wide Bandgap Ii-VI Semiconductors (E M I S Datareviews Series). Institution of Electrical Engineers, 1997.
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4
Litvinov, Vladimir. Wide Bandgap Semiconductor Spintronics. Jenny Stanford Publishing, 2016.
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Wide Bandgap Semiconductor Spintronics. Taylor & Francis Group, 2016.
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Litvinov, Vladimir. Wide Bandgap Semiconductor Spintronics. Jenny Stanford Publishing, 2016.
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Wide Bandgap Semiconductor Spintronics. Jenny Stanford Publishing, 2024.
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Characterization of Wide Bandgap Power Semiconductor Devices. Institution of Engineering & Technology, 2018.
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Wang, Fei, Zheyu Zhang e Edward A. Jones. Characterization of Wide Bandgap Power Semiconductor Devices. Institution of Engineering & Technology, 2018.
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Ohshima, Takeshi, Hiroshi Yano, Kazuma Eto, Shinsuke Harada e Takeshi Mitani. Silicon Carbide and Related Materials 2019. Trans Tech Publications, Limited, 2020.
Cerca il testo completoCapitoli di libri sul tema "Semi-conducteur à large bande"
1
Benabdelouahab, Tarik, Hayat Lionboui, Rachid Hadria, Riad Balaghi, Abdelghani Boudhar e Bernard Tychon. "Support Irrigation Water Management of Cereals Using Optical Remote Sensing and Modeling in a Semi-Arid Region". In Geospatial Technologies for Effective Land Governance, 124–45. IGI Global, 2019. http://dx.doi.org/10.4018/978-1-5225-5939-9.ch008.
Testo completoAbstract (sommario):
Irrigated agriculture is an important strategic sector for Morocco, contributing to food security and employment. Nowadays, irrigation scheme managers shall ensure that water is optimally used. The main objective was to support the irrigation monitoring and management of wheat in the irrigated perimeter using optical remote sensing and crop modeling. The potential of spectral indices derived from SPOT-5 images was explored for quantifying and mapping surface water content changes at large scale. Indices were computed using the reflectance in red, near infrared, and shortwave infrared bands. A field crop model (AquaCrop) was adjusted and tested to simulate the grain yield and the temporal evolution of soil moisture status. This research aimed at providing a scientific and technical approach to assist policymakers and stakeholders to improve monitoring irrigation and mitigating wheat water stress at field and irrigation perimeter levels in semi-arid areas. The approach could lead to operational management tools for an efficient irrigation at field and regional levels.
2
Brocx, Margaret, Vic Semeniuk, Tom J. Casadevall e Dan Tormey. "Volcanoes: Identifying and Evaluating Their Significant Geoheritage Features from the Large to Small Scale". In Updates in Volcanology - Transdisciplinary Nature of Volcano Science. IntechOpen, 2021. http://dx.doi.org/10.5772/intechopen.97928.
Testo completoAbstract (sommario):
Across the globe, volcanoes and volcanic terrains present one of the most complex geological systems on Earth that, depending on magma type, viscosity, and water and gas content, form a diverse range of products in terms of geomorphology, lithologic suites, structures, and stratigraphy. In broad terms, magmas, with their diagnostic composition, derive from specific tectonic settings, e.g., basalt-dominated oceanic crusts, acidic magma from continental plates, and andesitic convergent-plate margins. In addition to magma composition and volcanic rock types, there is a wide range of volcanic products, manifest at all scales, dependent on how magma interacts with the Earth’s surface, varying, for instance, from lava flows such as vesicular lava beds and flow-banded to flow-laminated lava beds, to breccias, tephra (ejecta) deposits, and bombs, amongst others, each commonly with their diagnostic small-scale lithological/structural features. This wealth of rock types, stratigraphy, and structures linked to geologic setting, potentially has geoheritage significance, and we provide here methods tailored for volcanoes and volcanic rocks of identifying, classifying and evaluating the complex and heterogeneous nature of volcanoes so that the full complement of their geology for a given region can be appreciated and incorporated into thematic geoparks, Nature Reserves and protected areas. For sites of geoheritage significance, we present (1) a globally-applicable Geoheritage Tool-kit to systematically identify volcanic geoheritage sites, (2) a technique to classify/categorise geoheritage sites, and (3) a semi-quantitative method to evaluate the geoheritage significance of volcanic sites.
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Ordóñez, Diego, Carlos Dafonte, Bernardino Arcay e Minia Manteiga. "Connectionist Systems and Signal Processing Techniques Applied to the Parameterization of Stellar Spectra". In Soft Computing Methods for Practical Environment Solutions, 187–203. IGI Global, 2010. http://dx.doi.org/10.4018/978-1-61520-893-7.ch012.
Testo completoAbstract (sommario):
A stellar spectrum is the finger-print identification of a particular star, the result of the radiation transport through its atmosphere. The physical conditions in the stellar atmosphere, its effective temperature, surface gravity, and the presence and abundance of chemical elements explain the observed features in the stellar spectra, such as the shape of the overall continuum and the presence and strength of particular lines and bands. The derivation of the atmospheric stellar parameters from a representative sample of stellar spectra collected by ground-based and spatial telescopes is essential when a realistic view of the Galaxy and its components is to be obtained. In the last decade, extensive astronomical surveys recording information of large portions of the sky have become a reality since the development of robotic or semi-automated telescopes. The Gaia satellite is one of the key missions of the European Space Agency (ESA) and its launch is planned for 2011. Gaia will carry out the so-called Galaxy Census by extracting precise information on the nature of its main constituents, including the spectra of objects (Wilkinson, 2005). Traditional methods for the extraction of the fundamental atmospheric stellar parameters (effective temperature (Teff), gravity (log G), metallicity ([Fe/H]), and abundance of alpha elements [a/Fe], elements integer multiples of the mass of the helium nucleus) are time-consuming and unapproachable for a massive survey involving 1 billion objects (about 1% of the Galaxy constituents) such as Gaia. This work presents the results of the authors’ study and shows the feasibility of an automated extraction of the previously mentioned stellar atmospheric parameters from near infrared spectra in the wavelength region of the Gaia Radial Velocity Spectrograph (RVS). The authors’ approach is based on a technique that has already been applied to problems of the non-linear parameterization of signals: artificial neural networks. It breaks ground in the consideration of transformed domains (Fourier and Wavelet Transforms) during the preprocessing stage of the spectral signals in order to select the frequency resolution that is best suited for each atmospheric parameter. The authors have also progressed in estimating the noise (SNR) that blurs the signal on the basis of its power spectrum and the application of noise-dependant algorithms of parameterization. This study has provided additional information that allows them to progress in the development of hybrid systems devoted to the automated classification of stellar spectra.
Atti di convegni sul tema "Semi-conducteur à large bande"
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Partovi, Afshin, Alan Kost, Elsa m. Garmire, George C. Valley e Marvin B. Klein. "Band edge photorefractive effect in GaAs". In OSA Annual Meeting. Washington, D.C.: Optica Publishing Group, 1989. http://dx.doi.org/10.1364/oam.1989.mcc1.
Testo completoAbstract (sommario):
Photorefractive semiconductors have short response times and high sensitivity in the infrared; however, their relatively small electrooptic coefficients are a disadvantage for many devices. One possible solution is to operate at wavelengths closer to the band edge, where the electrooptic coefficient is larger and other nonlinearities may contribute. We have investigated for the first time the photorefractive effect near the band edge of a compound semiconductor. We present measurements of the wavelength dependence of the two-beam coupling gain in semi-insulating GaAs. A large increase in the amount of energy transfer is observed near the band edge, compared to values at longer wavelengths. We discuss possible uses.
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Partovi, A., A. M. Glass, D. H. Olson, G. J. Zydzik, K. T. Short, R. D. Feldman e R. F. Austin. "High sensitivity resonant photorefractive effect in semi-insulating CdZnTe/ZnTe multiple quantum wells". In Photorefractive Materials, Effects, and Devices II. Washington, D.C.: Optica Publishing Group, 1991. http://dx.doi.org/10.1364/pmed.1991.mc1.
Testo completoAbstract (sommario):
Although the photorefractive sensitivity of the semiconductors is many orders of magnitude larger than the oxides, their small Pockels electro-optic coefficient has been a serious drawback. By taking advantage of the quadratic effects near the band-edge, nonlinearity and sensitivity of semiconductor photorefractivcs can be dramatically improved. Recently, two-beam-coupling gain coefficients of 16.3 cm−1 in GaAs [1] and 26.0 cm−1 in InP [2] have been reported near the band-edge. Quantum confinement of excitons in multiple quantum wells (MQWs) provides an additional enhancement of the resonant electro-optic nonlinearities. We have recently demonstrated how enhanced photorefractive sensitivity can be obtained in semi-insulting MQW devices [3]. These devices were made semi-insulting through ion-implantation to provide sufficient density of traps for the photorefractive process as well as relieving the need for any pixelation.
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Chen, Cheng, e James M. Ricles. "Experimental Evaluation of an Adaptive Actuator Control Scheme for Real-Time Tests of Large-Scale Magneto-Rheological Damper Under Variable Current Inputs". In ASME 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. ASMEDC, 2009. http://dx.doi.org/10.1115/smasis2009-1302.
Testo completoAbstract (sommario):
A Magneto-Rheological (MR) fluid damper is a semi-active device for vibration control of engineering structures subjected to dynamic loading. The characteristics of MR dampers vary under different current inputs to achieve optimized vibration control of structural systems. Experimental evaluation of MR dampers under different control laws is necessary before the device can be accepted by the practical design community. Real-time hybrid simulation provides an economical and efficient dynamic testing technique by accounting for the damper rate-dependency and the damper-structure interaction. A successful real-time hybrid simulation requires accurate actuator control to achieve reliable experiment results. A servo-hydraulic actuator usually introduces a time delay due to servo-hydraulic dynamics. The variable current inputs induced by semi-active control laws would pose additional challenges for actuator control by introducing variable delay in a real-time hybrid simulation. In this paper an adaptive compensation technique is experimentally evaluated for real-time hybrid simulation involving an MR damper under variable current inputs. Predefined band-limited white noise is used as the displacement command for the servo-hydraulic actuator and current command for the MR damper. The adaptive compensation scheme is demonstrated to achieve accurate actuator control and therefore shows great potential for real-time hybrid simulation of structural systems with semi-active energy dissipation devices.
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Zhang, Qinqiang, Meng Yang, Ken Suzuki e Hideo Miura. "Highly Sensitive Strain Sensor Using Dumbbell-Shape Graphene Nanoribbon". In ASME 2017 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/imece2017-70318.
Testo completoAbstract (sommario):
A nano-scale strip of graphene is known as graphene nano-ribbon (GNR). Previous studies have shown that the armchair-type GNR (aGNR) can open the electronic band gap at room temperature, and the band gap increases monotonically with the decrease in the width of aGNR. The critical width at which aGNR shows semi-conductive characteristics at room temperature is about 70 nm, when it is passivated by hydrogen on both sides. However, the electronic band structure varies frequently as a function of the number of carbon atoms along its width direction. In order to decrease the large variation of the band gap of aGNR to control the electronic properties of GNR for highly sensitive sensors and high performance devices, the electronic band structure of various dumbbell-shape structure of aGNR was analyzed by first-principles calculations based on the density functional theory using implemented in SIESTA package. It was shown that the width of aGNR had a large effect on the electronic band structure and the amplitude of the fluctuation of the band gap as a function of the number of carbon atoms decreased drastically. The electronic band structure of various GNRs under the application of uniaxial strain was also analyzed by using the first-principles calculations, in this study. It was confirmed that the effective band gap of aGNR thinner than 70 nm varies drastically under the application of uniaxial strain, and this result clearly indicates the possibility of a highly sensitive strain sensor using dumbbell-shape GNR structures.
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Martinazzo, Ana, e Nina Sumiko Tomita Hirata. "Multiband image classification of astronomical objects". In XXXII Conference on Graphics, Patterns and Images. Sociedade Brasileira de Computação - SBC, 2019. http://dx.doi.org/10.5753/sibgrapi.est.2019.8314.
Testo completoAbstract (sommario):
Astronomy has entered the era of large digital sky surveys, transitioning from a relatively data-scarce field of study to a very data-rich one. The images coming from these new surveys are hyperspectral (having up to a few dozen bands) and noisy (due to limitations on telescope resolution and atmospheric conditions), present faint and saturated signals, and can amount to tens of terabytes. This unique set of characteristics make them very attractive for trying out deep learning methods. In this short paper, we present a multiband image classifier for stars, galaxies and quasars, and propose steps towards a semi-supervised scheme that could enable the discovery of new objects.
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Simos, Alexandre N., Joa˜o V. Sparano, Jose´ A. P. Aranha e Vini´cius L. F. Matos. "2nd Order Hydrodynamic Effects on Resonant Heave, Pitch and Roll Motions of a Large-Volume Semi-Submersible Platform". In ASME 2008 27th International Conference on Offshore Mechanics and Arctic Engineering. ASMEDC, 2008. http://dx.doi.org/10.1115/omae2008-57430.
Testo completoAbstract (sommario):
During the last decades, as oil production offshore Brazil moved to deeper waters, technical and economical constraints led to a new generation of floating platforms. Nowadays, in the Brazilian offshore scenario, design trends concerning hull form, size and mooring configurations bring novel characteristics of wave-induced dynamics, including non-linear resonant effects. As part of an extensive study on new semi-submersible configurations for Campos basin, recent model tests have shown that their hulls may be subjected to second-order slow motions in heave, pitch and roll. These resonant motions are directly related to the large dimensions and relatively low natural frequencies of the floating systems. The unexpected effects caused great concern, since, in some cases, the low-frequency motions presented amplitudes comparable to those of the first-order response. This paper discusses the evaluation of the 2nd order wave-induced motions of a large-volume semi-submersible platform using WAMIT® second-order module. It is shown that the hydrodynamic forces induced by the 2nd-order potential represent the prevailing effect in the resonant response. Important aspects concerning the numerical model are addressed, such as the parameters involved in the hull and free-surface panelization. Numerical predictions are directly compared with experimental results obtained with a 1:40 model of the platform. A very good agreement is obtained both for heave and angular (pitch or roll) motions, attesting that the numerical code is able to predict the 2nd order forces accurately. Finally, a simplified procedure for dealing with the slow vertical motions is evaluated, aiming to reduce the substantial computational effort required by the 2nd order calculations. Such procedure takes advantage from the fact that the resonant response spectra of the vertical motions are usually narrow-banded (due to the low damping levels) to propose a “white-noise” approach. According to this approach, 2nd order forces need to be calculated only for one frequency difference, corresponding to the natural frequency of the particular motion. Computational time is, therefore, greatly reduced. It is shown that resonant motions calculated through the simplified approach match those predicted through the “full” analysis perfectly, making it an interesting choice for the evaluation of 2nd order effects, especially in the early stages of the design.
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Watanuki, Keiichi, Tetsuhiro Suzuki, Yusuke Osawa, Kazunori Kaede e Shinsuke Kazama. "Automatic Generation of AI-based Cancer Pathology Data and Highly Accurate Colorectal Cancer Pathology Diagnosis Support". In 13th International Conference on Applied Human Factors and Ergonomics (AHFE 2022). AHFE International, 2022. http://dx.doi.org/10.54941/ahfe1001787.
Testo completoAbstract (sommario):
Owing to the increase in the number of pathological diagnoses and the shortage of pathologists, the burden on pathologists has been increasing. Accordingly, support systems are expected to be utilized for analyzing pathological images using deep learning to reduce the burden on pathologists. However, the deep learning model needs to be trained using a dataset consisting of a large number of cases, to improve its performance. However, the creation of such a dataset is labor-intensive and time-consuming. Thus, an efficient method for creating datasets is required to create large datasets for future practical use. In this paper, we propose a method for creating datasets using image segmentation based on deep learning. First, we investigated whether the discriminative performance of the deep learning model can be improved by using a narrow-band light source for photographing pathological specimens. Consequently, the correct response rate was 0.93 when a white LED was used as the light source and the image was used as the input, and 0.95 when two narrow-band light sources of wavelengths 500 nm and 570 nm were used as the light sources and the image was used as the input. This indicates that using a specific narrow-band light source can improve the discrimination performance of the deep learning model compared with using white LEDs as the light source. In addition, we efficiently constructed a large and precise dataset consisting of 1018 colorectal pathology images (2028 images) and pixel-by-pixel annotation information using a dataset creation method based on image segmentation via deep learning. In contrast to the conventional handwritten annotation process, which takes an average of 520 s, the proposed method takes an average of 137 s; thus, the creation of the database is accelerated. We trained a deep learning model using the dataset of colorectal pathology specimen images created in this study. The deep learning model was trained to classify images obtained by segmenting the large-sized pathological specimen images into those containing malignant tumors and those not containing malignant tumors. The results of the diagnostic accuracy of the model were as follows: a sensitivity of 95.2%, a specificity of 97.1%, a positive predictive value of 95.29%, and a negative predictive value of 97.06%. The percentage of correct classification was 0.97, and the AUC was 0.99. In this study, we constructed a pathological specimen imaging system using narrow-band light sources of two specific wavelengths as the imaging light source, and created a dataset with high accuracy semi-automatically via image segmentation using deep learning. In addition, we constructed a system that can efficiently and semi-automatically create a large and precise dataset consisting of colorectal pathology images and pixel-by-pixel annotation information. We evaluated these systems and confirmed that they could classify colorectal pathology specimen images as accurately and quickly as or more accurately than pathologists. Thus, we demonstrated their usefulness as a pathology image analysis support system.
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Han, Sang-Kook, Ramakant Srivastava e Ramu V. Ramaswamy. "Determination of refractive index of MBE grown InGaAlAs from composition". In Integrated Photonics Research. Washington, D.C.: Optica Publishing Group, 1991. http://dx.doi.org/10.1364/ipr.1991.tud14.
Testo completoAbstract (sommario):
Recently the InGaAlAs system has attracted considerable attention since its band-gap can be tuned over a relatively large wavelength-range (0.85-1.60μm) by varying the ratio of Ga to Al. Moreover, this material system is a promising alternative to InGaAsP for the fabrication of laser diodes emitting at 1.55μm. Inspite of the growing interest in this material system, very little work has been reported on its refractive index and material dispersion.1,2 In this paper, we develop semi-empirical relations to predict the refractive index and material dispersion of bulk In0.53Ga0.47-yAlyAs in the transparent wavelength region. Our approach is based on the modified single effective oscillator model (MSEOM)3 which has been used successfully in several other III-V compound semiconductors.4
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Ziari, Mehrdad, William H. Steier, Marvin B. Klein e Sudhir Trivedi. "Photorefractive properties and alternating electric field gain enhancement of vanadium-doped cadmium telluride and related compounds". In Photorefractive Materials, Effects, and Devices II. Washington, D.C.: Optica Publishing Group, 1991. http://dx.doi.org/10.1364/pmed.1991.tua6.
Testo completoAbstract (sommario):
Recent studies of the photorefractive response of CdTe have demonstrated it to be a highly sensitive material with a broad near infrared sensitivity extending to beyond 1.5 μm[1,2]. Desirable parameters, such as large electrooptic coefficient, small dielectric constant, large carrier mobility and availability in semi-insulating form, makes CdTe a potential material of choice for many applications[3]. This study focuses on photorefractive characterization of Bridgeman grown crystals with the goal of providing the information that can lead to the optimization of key photorefractive parameters. We have observed gain and broad sensitivity (1-1.5 μm) in vanadium doped CdTe and Cd0.96Zn0.4Te samples. Mixed alloys such as CdZnTe and CdMnTe allow band gap engineering such that the sensitivity range could be tuned toward the visible and be matched to the wavelength of interest. An applied alternating (AC) field gain enhancement technique is used to demonstrate net gain which is a prerequisite for coherent amplification and self-pumped phase conjugation.
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Zhao, Qichao, Ran Yang, Xiaoqing Zhu, Chunxiao Li, Xin Gao, Yanan Li, Haiyang Yu et al. "Systematic Comparison of Signal Quality in Portable and Wearable Wireless EEG Devices: Methods and Standards". In 15th International Conference on Applied Human Factors and Ergonomics (AHFE 2024). AHFE International, 2024. http://dx.doi.org/10.54941/ahfe1004813.
Testo completoAbstract (sommario):
Electroencephalography (EEG) is a technique that captures the electrical activity of the cerebral cortex, effectively reflecting various rapid cognitive processes. Traditional EEG devices, often large and bulky, are impractical for real-world applications. To address this, wearable and wireless EEG systems have been developed as cutting-edge technology. However, there remains uncertainty about their signal quality .In this study, we introduce a systematic comparison method customized for portable wireless EEG devices. This experiment includes three tasks: α-suppression, Biofeedback, and the Stroop Colour and Word Test, evaluating signal quality in both time and frequency domains. In the α-suppression task, we analyzed spectral power in the α band, Signal-to-Noise Ratio (SNR), Root Mean Square(RMS),and artifact rejection. For the biofeedback task, we evaluated Engagement (β/(α+θ)) and Relaxation (α/β) indices. We also measured the N400 amplitude during the Stroop Colour and Word Test. These indicators represent varying levels of signal quality requirements, from low to high. Additionally, to assess whether semi-dry EEG devices can match the performance of medical-grade gel electrodes, further significant difference and correlation analysis between the two types of devices were conducted. The frequency domain analysis revealed alpha suppression during eyes-open states and increased relaxation and engagement states during the biofeedback task compared to resting state. In the time domain analysis, although no statistical significance was observed in the N400 component, a clear trend of a larger N400 under inconsistent conditions was evident in waveform and topographic maps. Furthermore, difference tests and correlation analysis between the two devices demonstrated a strong positive correlation in signals and consistent performance across all tasks. These findings suggest that portable EEG devices provide reliable signal accuracy in real-world settings, the signal quality of the semi-dry electrodes used in this study is comparable to that of medical-grade gel electrodes.