38 research outputs found

    High speed characteristics of strained-induced Pockels effect in silicon

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    International audienceWith the fast growing demand of data, current chip-scale communication systems based on electrical links suffer rate limitations and high power consumptions to address these new requirements. In this context, Silicon Photonics has proven to be a viable alternative by replacing electronic links with optical ones while taking advantage of the well-established CMOS foundries techniques to reduce fabrication costs. However, silicon, in spite of being an excellent material to guide light, its centrosymmetry prevents second order nonlinear effects to exist, such as Pockels effect an electro-optic effect extensively used in high speed and low power consumption data transmission. Nevertheless, straining silicon by means of stressed thin films allows breaking the crystal symmetry and eventually enhancing Pockels effect. However the semiconductor nature of silicon makes the analysis of Pockels effect a challenging task because free carriers have a direct impact, through plasma dispersion effect, on its efficiency, which in turn complicates the estimation of the second order susceptibility necessary for further optimizations. However, this analysis is more relaxed working in high-speed regime because of the frequency limitation of free carriers-based modulation. In this work, we report experimental results on the modulation characteristics based on Mach-Zehnder interferometers strained by silicon nitride. We demonstrated high speed Pockels-based optical modulation up to 25 GHz in the C-band

    Low-energy electronic interactions in ferrimagnetic Sr2CrReO6 thin films

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    We reveal in this study the fundamental low-energy landscape in the ferrimagnetic Sr2CrReO6 double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both Sr2CrReO6 powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund's coupling, and the strain-induced tetragonal crystal field. We also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV (±\pm25 meV) through X-L-W-U high-symmetry points and is dispersive toward Γ\GammaComment: 6 figure

    DMTs and Covid-19 severity in MS: a pooled analysis from Italy and France

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    We evaluated the effect of DMTs on Covid-19 severity in patients with MS, with a pooled-analysis of two large cohorts from Italy and France. The association of baseline characteristics and DMTs with Covid-19 severity was assessed by multivariate ordinal-logistic models and pooled by a fixed-effect meta-analysis. 1066 patients with MS from Italy and 721 from France were included. In the multivariate model, anti-CD20 therapies were significantly associated (OR = 2.05, 95%CI = 1.39–3.02, p < 0.001) with Covid-19 severity, whereas interferon indicated a decreased risk (OR = 0.42, 95%CI = 0.18–0.99, p = 0.047). This pooled-analysis confirms an increased risk of severe Covid-19 in patients on anti-CD20 therapies and supports the protective role of interferon

    Intégration d'oxydes fonctionnels pour applications en photonique

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    21st century stands out as a new numeric area, particularly due to the development of Internet of Things. The exceptional growth of produced, transmitted and stored data all around the world led to new emerging technologies such as silicon photonics. However, silicon has intrinsic limitations for photonic applications including indirect bandgap and centrosymmetry of its lattice.In parallel, functional oxides family exhibits an impressive panel of properties such as ferroelectricity or piezoelectricity. The epitaxial growth is the main limitation for their integration in silicon photonics. Indeed, the lattice mismatch between most of the oxides and silicon induces high defects density which strongly degrades their properties. Nevertheless, one of these oxides, Yttria-Stabilized Zirconia (YSZ), can be used for the lattice transition. The objectives of the PhD work was then to study the YSZ growth mechanisms and to determine its properties in integrated optics.First, the epitaxial growth of YSZ, using Pulsed-Laser Deposition (PLD), as well as the fabrication and characterization of photonics structures have been developed on sapphire substrate. We reported the role of substrate annealing before deposition, on the orientation and quality of YSZ thin film. The optimization of the epitaxial growth, and the development of a fabrication process, allowed the demonstration of YSZ-based waveguides with propagation losses as low as 2 dB/cm. We also characterized more complex passives structures, in particular ring resonators, micro-disks and Bragg filters. Furthermore, third-order optical nonlinear properties in YSZ waveguides were experimentally determined for the first time. The n2 value, obtained experimentally, is in agreement with theoretical calculation and is in the same order of magnitude than the value reported for silicon nitride SiN, a well-known nonlinear material.Due to lattice mismatch and thermal expansion coefficient difference, the growth of YSZ is expected to induce large strain in silicon, breaking its crystal centrosymmetry. Recent experimental and theoretical studies have demonstrated the possibility to exploit second-order optical nonlinear effects in strained silicon, usually vanishing in this material. To probe the strain distribution, seen by an optical mode propagating in a silicon waveguide, we developed an innovative Raman-based experimental technique for integrated optics. Even if typical phonons behaviors have been revealed, theoretical modeling of experimental data did not lead to strain values comparable to the simulation predictions and complementary studies are required.Finally, three approaches have been tested for the integration of YSZ in silicon photonics. The first and second one include the growth of YSZ on silicon waveguides, buried or not in a silica layer. The third one involves the fabrication of waveguides in a YSZ thin film, deposited on a flat silicon substrate. No strains in the silicon have been observed, justified by the silica interfacial layer between YSZ and silicon. Propagation losses of hybrid YSZ/Si waveguides, initially more than 250 dB/cm, have been reduced to 7.5 dB/cm thanks to YSZ growth optimization and an adapted waveguides geometry.Le 21Ăšme siĂšcle est marquĂ© par une nouvelle Ăšre du numĂ©rique, notamment due au dĂ©veloppement d’objets connectĂ©s toujours plus nombreux et variĂ©s. L’incroyable croissance, du flux de donnĂ©es produites, Ă©changĂ©es et stockĂ©es au niveau mondial, a permis l’émergence de nouvelles technologies comme la photonique silicium. Cette derniĂšre est cependant limitĂ©e par les propriĂ©tĂ©s intrinsĂšques du silicium, comme son gap indirect et sa structure cristalline centro-symĂ©trique.En parallĂšle, la famille des oxydes fonctionnels prĂ©sente une incroyable diversitĂ© de propriĂ©tĂ©s, comme la ferroĂ©lectricitĂ© ou la piĂ©zoĂ©lectricitĂ©. Leur intĂ©gration en photonique est principalement limitĂ©e par l’épitaxie sur silicium. En effet, la diffĂ©rence de paramĂštre de maille entre la plupart des oxydes et le silicium engendre une grande quantitĂ© de dĂ©fauts et donc une forte dĂ©gradation de leurs propriĂ©tĂ©s. L’oxyde de zirconium stabilisĂ© Ă  l’yttrium (YSZ), qui prĂ©sente un paramĂštre de maille intermĂ©diaire, assure la transition entre les rĂ©seaux cristallins. Ce travail de thĂšse s’articule ainsi autour de la croissance d’YSZ et la caractĂ©risation de ses propriĂ©tĂ©s en optique intĂ©grĂ©e.Dans un premier temps, nous avons Ă©tudiĂ© la croissance d’YSZ par ablation laser pulsĂ© (PLD), ainsi que la fabrication et caractĂ©risation de structures photoniques sur substrat de saphir. Nous avons mis en Ă©vidence le rĂŽle du recuit du substrat avant dĂ©pĂŽt, sur l’orientation et la qualitĂ© du film. L’optimisation du dĂ©pĂŽt et le dĂ©veloppement d’un procĂ©dĂ© de fabrication, a permis la dĂ©monstration de guides d’onde Ă  faibles pertes, d’environs 2 dB/cm, et de composants passifs plus complexes comme des structures rĂ©sonantes en anneau, micro-disques et filtres de Bragg. Nous avons Ă©galement caractĂ©risĂ© les propriĂ©tĂ©s optiques non-linĂ©aires du troisiĂšme ordre de l’YSZ dont les rĂ©sultats expĂ©rimentaux ont Ă©tĂ© confirmĂ©s par des calculs thĂ©oriques. La valeur de l’indice de rĂ©fraction non-linĂ©aire n2, de l’YSZ, est comparable Ă  celle du nitrure de silicium (SiN), dĂ©jĂ  utilisĂ© comme matĂ©riau non-linĂ©aire.En raison de la diffĂ©rence de paramĂštre de maille et du coefficient d’expansion thermique, l’intĂ©gration d’YSZ est susceptible d’induire de larges contraintes dans le silicium, et de briser sa centro-symĂ©trie. De rĂ©centes Ă©tudes, expĂ©rimentales et thĂ©oriques, ont dĂ©montrĂ© que les contraintes permettent d’exploiter des propriĂ©tĂ©s optiques non-linĂ©aires d’ordre deux dans le silicium, normalement inexistantes dans ce matĂ©riau. Pour caractĂ©riser la distribution des contraintes, vues par un mode optique se propageant dans un guide d’onde silicium, nous avons mis en place une nouvelle technique expĂ©rimentale basĂ©e sur l’effet Raman en optique intĂ©grĂ©e. Des signatures d’évolutions de phonons trĂšs intĂ©ressantes ont Ă©tĂ© mesurĂ©es. Cependant, les modĂšles thĂ©oriques n’ont pas permis de calculer des valeurs de contraintes comparables Ă  celles prĂ©vues par les simulations et des Ă©tudes complĂ©mentaires sont nĂ©cessaires.Finalement l’intĂ©gration d’YSZ en photonique silicium a Ă©tĂ© Ă©tudiĂ©e selon trois approches. La premiĂšre et la deuxiĂšme consistent au dĂ©pĂŽt d’YSZ sur des guides d’onde silicium, encapsulĂ©s ou non par une couche de silice. La troisiĂšme comprend la fabrication de guides d’onde dans une couche d’YSZ, dĂ©posĂ©e sur un substrat de silicium non structurĂ©. Nous avons mis en Ă©vidence l’absence de contrainte dans chacune des configurations, justifiĂ©e par la prĂ©sence de silice Ă  l’interface entre l’YSZ et le silicium. Les pertes de propagation dans de tels guides hybrides YSZ/Si, initialement supĂ©rieures Ă  250 dB/cm ont Ă©tĂ© rĂ©duites Ă  7,5 dB/cm par l’optimisation de la croissance et de la gĂ©omĂ©trie des guides

    Integration of functional oxides for photonic applications

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    Le 21Ăšme siĂšcle est marquĂ© par une nouvelle Ăšre du numĂ©rique, notamment due au dĂ©veloppement d’objets connectĂ©s toujours plus nombreux et variĂ©s. L’incroyable croissance, du flux de donnĂ©es produites, Ă©changĂ©es et stockĂ©es au niveau mondial, a permis l’émergence de nouvelles technologies comme la photonique silicium. Cette derniĂšre est cependant limitĂ©e par les propriĂ©tĂ©s intrinsĂšques du silicium, comme son gap indirect et sa structure cristalline centro-symĂ©trique.En parallĂšle, la famille des oxydes fonctionnels prĂ©sente une incroyable diversitĂ© de propriĂ©tĂ©s, comme la ferroĂ©lectricitĂ© ou la piĂ©zoĂ©lectricitĂ©. Leur intĂ©gration en photonique est principalement limitĂ©e par l’épitaxie sur silicium. En effet, la diffĂ©rence de paramĂštre de maille entre la plupart des oxydes et le silicium engendre une grande quantitĂ© de dĂ©fauts et donc une forte dĂ©gradation de leurs propriĂ©tĂ©s. L’oxyde de zirconium stabilisĂ© Ă  l’yttrium (YSZ), qui prĂ©sente un paramĂštre de maille intermĂ©diaire, assure la transition entre les rĂ©seaux cristallins. Ce travail de thĂšse s’articule ainsi autour de la croissance d’YSZ et la caractĂ©risation de ses propriĂ©tĂ©s en optique intĂ©grĂ©e.Dans un premier temps, nous avons Ă©tudiĂ© la croissance d’YSZ par ablation laser pulsĂ© (PLD), ainsi que la fabrication et caractĂ©risation de structures photoniques sur substrat de saphir. Nous avons mis en Ă©vidence le rĂŽle du recuit du substrat avant dĂ©pĂŽt, sur l’orientation et la qualitĂ© du film. L’optimisation du dĂ©pĂŽt et le dĂ©veloppement d’un procĂ©dĂ© de fabrication, a permis la dĂ©monstration de guides d’onde Ă  faibles pertes, d’environs 2 dB/cm, et de composants passifs plus complexes comme des structures rĂ©sonantes en anneau, micro-disques et filtres de Bragg. Nous avons Ă©galement caractĂ©risĂ© les propriĂ©tĂ©s optiques non-linĂ©aires du troisiĂšme ordre de l’YSZ dont les rĂ©sultats expĂ©rimentaux ont Ă©tĂ© confirmĂ©s par des calculs thĂ©oriques. La valeur de l’indice de rĂ©fraction non-linĂ©aire n2, de l’YSZ, est comparable Ă  celle du nitrure de silicium (SiN), dĂ©jĂ  utilisĂ© comme matĂ©riau non-linĂ©aire.En raison de la diffĂ©rence de paramĂštre de maille et du coefficient d’expansion thermique, l’intĂ©gration d’YSZ est susceptible d’induire de larges contraintes dans le silicium, et de briser sa centro-symĂ©trie. De rĂ©centes Ă©tudes, expĂ©rimentales et thĂ©oriques, ont dĂ©montrĂ© que les contraintes permettent d’exploiter des propriĂ©tĂ©s optiques non-linĂ©aires d’ordre deux dans le silicium, normalement inexistantes dans ce matĂ©riau. Pour caractĂ©riser la distribution des contraintes, vues par un mode optique se propageant dans un guide d’onde silicium, nous avons mis en place une nouvelle technique expĂ©rimentale basĂ©e sur l’effet Raman en optique intĂ©grĂ©e. Des signatures d’évolutions de phonons trĂšs intĂ©ressantes ont Ă©tĂ© mesurĂ©es. Cependant, les modĂšles thĂ©oriques n’ont pas permis de calculer des valeurs de contraintes comparables Ă  celles prĂ©vues par les simulations et des Ă©tudes complĂ©mentaires sont nĂ©cessaires.Finalement l’intĂ©gration d’YSZ en photonique silicium a Ă©tĂ© Ă©tudiĂ©e selon trois approches. La premiĂšre et la deuxiĂšme consistent au dĂ©pĂŽt d’YSZ sur des guides d’onde silicium, encapsulĂ©s ou non par une couche de silice. La troisiĂšme comprend la fabrication de guides d’onde dans une couche d’YSZ, dĂ©posĂ©e sur un substrat de silicium non structurĂ©. Nous avons mis en Ă©vidence l’absence de contrainte dans chacune des configurations, justifiĂ©e par la prĂ©sence de silice Ă  l’interface entre l’YSZ et le silicium. Les pertes de propagation dans de tels guides hybrides YSZ/Si, initialement supĂ©rieures Ă  250 dB/cm ont Ă©tĂ© rĂ©duites Ă  7,5 dB/cm par l’optimisation de la croissance et de la gĂ©omĂ©trie des guides.21st century stands out as a new numeric area, particularly due to the development of Internet of Things. The exceptional growth of produced, transmitted and stored data all around the world led to new emerging technologies such as silicon photonics. However, silicon has intrinsic limitations for photonic applications including indirect bandgap and centrosymmetry of its lattice.In parallel, functional oxides family exhibits an impressive panel of properties such as ferroelectricity or piezoelectricity. The epitaxial growth is the main limitation for their integration in silicon photonics. Indeed, the lattice mismatch between most of the oxides and silicon induces high defects density which strongly degrades their properties. Nevertheless, one of these oxides, Yttria-Stabilized Zirconia (YSZ), can be used for the lattice transition. The objectives of the PhD work was then to study the YSZ growth mechanisms and to determine its properties in integrated optics.First, the epitaxial growth of YSZ, using Pulsed-Laser Deposition (PLD), as well as the fabrication and characterization of photonics structures have been developed on sapphire substrate. We reported the role of substrate annealing before deposition, on the orientation and quality of YSZ thin film. The optimization of the epitaxial growth, and the development of a fabrication process, allowed the demonstration of YSZ-based waveguides with propagation losses as low as 2 dB/cm. We also characterized more complex passives structures, in particular ring resonators, micro-disks and Bragg filters. Furthermore, third-order optical nonlinear properties in YSZ waveguides were experimentally determined for the first time. The n2 value, obtained experimentally, is in agreement with theoretical calculation and is in the same order of magnitude than the value reported for silicon nitride SiN, a well-known nonlinear material.Due to lattice mismatch and thermal expansion coefficient difference, the growth of YSZ is expected to induce large strain in silicon, breaking its crystal centrosymmetry. Recent experimental and theoretical studies have demonstrated the possibility to exploit second-order optical nonlinear effects in strained silicon, usually vanishing in this material. To probe the strain distribution, seen by an optical mode propagating in a silicon waveguide, we developed an innovative Raman-based experimental technique for integrated optics. Even if typical phonons behaviors have been revealed, theoretical modeling of experimental data did not lead to strain values comparable to the simulation predictions and complementary studies are required.Finally, three approaches have been tested for the integration of YSZ in silicon photonics. The first and second one include the growth of YSZ on silicon waveguides, buried or not in a silica layer. The third one involves the fabrication of waveguides in a YSZ thin film, deposited on a flat silicon substrate. No strains in the silicon have been observed, justified by the silica interfacial layer between YSZ and silicon. Propagation losses of hybrid YSZ/Si waveguides, initially more than 250 dB/cm, have been reduced to 7.5 dB/cm thanks to YSZ growth optimization and an adapted waveguides geometry

    Phthalocyanine reactivity and interaction on the 6H-SiC(0001)-(3 × 3) surface investigated by core-level experiments and simulations

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    International audienceThe adsorption of phthalocyanine (H2_2 Pc) on the 6H-SiC(0001)-(3 x 3) surface is investigated using X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and density functional theory (DFT) calculations. Spectral features are tracked from the submonolayer to the multilayer growth regime, observing a significant modification of spectroscopic signals at low coverage with respect to the multilayer films, where molecules are weakly interacting. Molecules stay nearly flat on the surface at the mono and submonolayers. Previously proposed adsorption models, where the molecule binds by two N atoms to corresponding Si adatoms, do not reproduce the experimental spectra at the submonolayer coverage. We find instead that another adsorption model where the molecule replaces the two central H atoms by a Si adatom, effectively forming Si-phthalocyanine (SiPc), is both energetically more stable and yields in combination a better agreement between the experimental and simulated spectra. This suggests that the 6H-SiC(0001)-(3 Â 3) surface may be a candidate substrate for the on-surface synthesis of SiPc molecules
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